harden: verify the signed transaction against what the popup displayed (closes #216)

Verification compared the signed artifact against the dApp's request object.
For every field the dApp omitted -- normally nonce, gas limit and all the fee
fields, since the popup filled them in -- the number the user actually read on
screen was verified by nothing, and only absolute ceilings stood behind it.

The transaction is now populated in the background before the approval window
opens, and that populated object is both what the popup displays and what the
signed artifact is verified against. Every consequential field becomes an
equality comparison; the ceilings remain as a backstop. Population failing
means no approval and no window, and the error goes to the requesting page --
earlier than before, where the same estimate failed after the password had been
typed.

The account is pinned too: `from` is compared against the address named at
approval time rather than whichever address is active at signing, so switching
accounts mid-flow refuses instead of signing from an account the approval did
not name. The message-signing path had the same defect and gets the same fix.

Nonce selection moves earlier as a consequence; the concurrent-approval case
that follows from it is tracked at #271.

src/background/index.js

@@ -18,11 +18,14 @@ const {
     verifySignature,
     failureIsRetryable,
     describeTxFailure,
+    sameAddress,
+    ApprovalMismatchError,
     TX_STAGE_SIGN,
     TX_STAGE_VERIFY,
     TX_STAGE_BROADCAST,
     TX_STAGE_INFLIGHT,
 } = require("../shared/approvalVerify");
+const { prepareApprovalTx } = require("../shared/approvalTx");
 const {
     isPhishingDomain,
     refreshPhishingListOnSchedule,
@@ -77,6 +80,14 @@ async function getActiveAddress() {
     return null;
 }
 
+// Whether a request names a signing address other than the active one. Such a
+// request is refused rather than quietly signed as whichever address happens
+// to be active: the page asked for account A and would otherwise be handed
+// something from account B.
+function namesAnotherAddress(requested, activeAddress) {
+    return !!requested && !sameAddress(requested, activeAddress);
+}
+
 async function getRpcUrl() {
     const s = await getState();
     return s.rpcUrl || DEFAULT_RPC_URL;
@@ -225,13 +236,21 @@ function requestApproval(origin, hostname) {
 // Uses windows.create() directly because tx approvals are triggered programmatically
 // (from a dApp RPC call), not from a user gesture, so action.openPopup() is
 // unreliable in this context.
-function requestTxApproval(origin, hostname, txParams) {
+//
+// `approvedTx` is the fully populated transaction (see approvalTx.js): the
+// object the popup displays, the object it signs, and the object the artifact
+// is verified against. `approvedFrom` is the address that is active now, and
+// it is pinned here rather than read again at signing time — an address switch
+// between approval and signing must refuse, not sign from an account this
+// screen never named.
+function requestTxApproval(origin, hostname, approvedTx, approvedFrom) {
     return new Promise((resolve) => {
         const id = crypto.randomUUID();
         pendingApprovals[id] = {
             origin,
             hostname,
-            txParams,
+            approvedTx,
+            approvedFrom,
             resolve,
             type: "tx",
         };
@@ -244,13 +263,14 @@ function requestTxApproval(origin, hostname, txParams) {
 // Uses windows.create() directly because sign approvals are triggered programmatically
 // (from a dApp RPC call), not from a user gesture, so action.openPopup() is
 // unreliable in this context.
-function requestSignApproval(origin, hostname, signParams) {
+function requestSignApproval(origin, hostname, signParams, approvedFrom) {
     return new Promise((resolve) => {
         const id = crypto.randomUUID();
         pendingApprovals[id] = {
             origin,
             hostname,
             signParams,
+            approvedFrom,
             resolve,
             type: "sign",
         };
@@ -502,6 +522,16 @@ async function handleRpc(method, params, origin) {
                 ? { method, message: params[0], from: params[1] }
                 : { method, message: params[1], from: params[0] };
 
+        if (namesAnotherAddress(signParams.from, activeAddress)) {
+            return {
+                error: {
+                    code: 4100,
+                    message:
+                        "This site asked to sign as an address that is not the active one.",
+                },
+            };
+        }
+
         if (method === "eth_sign") {
             signParams.dangerWarning =
                 "\u26a0\ufe0f DANGER: This site is requesting to sign a raw hash. " +
@@ -513,6 +543,7 @@ async function handleRpc(method, params, origin) {
             origin,
             hostname,
             signParams,
+            activeAddress,
         );
         if (decision.error) return { error: decision.error };
         return { result: decision.signature };
@@ -534,10 +565,20 @@ async function handleRpc(method, params, origin) {
         }
 
         const signParams = { method, typedData: params[1], from: params[0] };
+        if (namesAnotherAddress(signParams.from, activeAddress)) {
+            return {
+                error: {
+                    code: 4100,
+                    message:
+                        "This site asked to sign as an address that is not the active one.",
+                },
+            };
+        }
         const decision = await requestSignApproval(
             origin,
             hostname,
             signParams,
+            activeAddress,
         );
         if (decision.error) return { error: decision.error };
         return { result: decision.signature };
@@ -559,7 +600,51 @@ async function handleRpc(method, params, origin) {
         }
 
         const txParams = params?.[0] || {};
-        const decision = await requestTxApproval(origin, hostname, txParams);
+        if (namesAnotherAddress(txParams.from, activeAddress)) {
+            return {
+                error: {
+                    code: 4100,
+                    message:
+                        "This site asked to send from an address that is not the active one.",
+                },
+            };
+        }
+
+        // Populate here, before any window opens, so that the transaction the
+        // user is shown is a complete one and is the same object the signed
+        // artifact is checked against. A failure raises no approval at all and
+        // is reported to the requesting page; see approvalTx.js.
+        let approvedTx;
+        try {
+            approvedTx = await prepareApprovalTx(
+                getProvider(await getRpcUrl()),
+                activeAddress,
+                txParams,
+            );
+        } catch (e) {
+            return { error: { message: e.message } };
+        }
+
+        // Population is a network round trip, and the user can switch address
+        // during it. Raising the approval anyway would put an account on the
+        // screen that the wallet is no longer on, and it could never be signed
+        // — the signing handler refuses exactly that. Refuse it here instead,
+        // while the page is still waiting and nothing has been displayed.
+        if (!sameAddress(await getActiveAddress(), activeAddress)) {
+            return {
+                error: {
+                    message:
+                        "The active address changed while this transaction was being prepared, so it was not sent.",
+                },
+            };
+        }
+
+        const decision = await requestTxApproval(
+            origin,
+            hostname,
+            approvedTx,
+            activeAddress,
+        );
         if (decision.error) return { error: decision.error };
         return { result: decision.txHash };
     }
@@ -810,11 +895,16 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
             };
             if (approval.type === "tx") {
                 resp.type = "tx";
-                resp.txParams = approval.txParams;
+                // The populated transaction, and the address it was raised
+                // for. The popup displays and signs exactly this and does not
+                // populate or re-read anything itself.
+                resp.approvedTx = approval.approvedTx;
+                resp.approvedFrom = approval.approvedFrom;
             }
             if (approval.type === "sign") {
                 resp.type = "sign";
                 resp.signParams = approval.signParams;
+                resp.approvedFrom = approval.approvedFrom;
             }
             // Flag if the requesting domain is on the phishing blocklist.
             resp.isPhishingDomain = isPhishingDomain(approval.hostname);
@@ -888,14 +978,27 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
             try {
                 await loadState();
                 const activeAddress = await getActiveAddress();
+                // An address switch between approval and signing refuses. The
+                // approval named one account; signing from whichever account
+                // is active now would send funds from an account this screen
+                // never showed. A switch normally rejects every pending
+                // approval on its way through broadcastAccountsChanged(), so
+                // this is the case where that did not reach the approval —
+                // and it is a refusal, not a retry, because the transaction
+                // the user saw is no longer the transaction that would go out.
+                if (!sameAddress(activeAddress, approval.approvedFrom)) {
+                    throw new ApprovalMismatchError(
+                        "The active address changed after this transaction was approved, so it was not sent.",
+                    );
+                }
                 // The popup holds the secret, but the background stays the
                 // authority on what is broadcast: the raw transaction must be
-                // the approved one, signed by the approved address, on the
-                // network that is selected.
+                // the transaction that was displayed, signed by the address
+                // the approval named, on the network that is selected.
                 verifySignedTx(
                     msg.rawSignedTx,
-                    approval.txParams,
-                    activeAddress,
+                    approval.approvedTx,
+                    approval.approvedFrom,
                     currentNetwork().chainId,
                 );
             } catch (e) {
@@ -932,10 +1035,10 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
                 sendResponse({ txHash: tx.hash });
             } catch (e) {
                 // Terminal, never retried: the node may have accepted the
-                // transaction and still failed to answer, and the popup's
-                // retry re-signs at a freshly fetched nonce rather than
-                // re-broadcasting these bytes. Retrying would send the
-                // approved transfer a second time.
+                // transaction and still failed to answer, so the wallet cannot
+                // tell a transaction that never left from one already in the
+                // mempool. The page has been given its outcome for this
+                // request; a second attempt would report a second one.
                 const outcome = describeTxFailure(TX_STAGE_BROADCAST, e);
                 settleApproval(
                     msg.id,
@@ -998,12 +1101,24 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
         (async () => {
             try {
                 const activeAddress = await getActiveAddress();
+                // Same as the transaction path: the address the approval named
+                // is the one that must have signed, and a switch since then is
+                // a refusal rather than a signature from another account.
+                if (!sameAddress(activeAddress, approval.approvedFrom)) {
+                    throw new ApprovalMismatchError(
+                        "The active address changed after this request was approved, so it was not signed.",
+                    );
+                }
                 // The popup holds the secret, but the background stays the
                 // authority on what is handed back to the page: the signature
-                // must cover the approved payload and recover to the approved
-                // address.
+                // must cover the approved payload and recover to the address
+                // the approval named.
                 const signature = msg.signature;
-                verifySignature(approval.signParams, signature, activeAddress);
+                verifySignature(
+                    approval.signParams,
+                    signature,
+                    approval.approvedFrom,
+                );
                 settleApproval(msg.id, { signature }, { holdsClaim: true });
                 sendResponse({ signature });
             } catch (e) {

src/popup/index.html

@@ -1496,6 +1496,33 @@
                     <div class="text-xs text-muted mb-1">Value</div>
                     <div id="approve-tx-value" class="text-xs font-bold"></div>
                 </div>
+                <div class="mb-3">
+                    <div class="text-xs text-muted mb-1">Network fee (max)</div>
+                    <div
+                        id="approve-tx-fee"
+                        class="text-xs font-bold min-h-[1rem]"
+                    ></div>
+                    <div
+                        id="approve-tx-fee-detail"
+                        class="text-xs text-muted min-h-[1rem]"
+                    ></div>
+                </div>
+                <div class="mb-3 flex justify-between">
+                    <div>
+                        <div class="text-xs text-muted mb-1">Network</div>
+                        <div
+                            id="approve-tx-network"
+                            class="text-xs min-h-[1rem]"
+                        ></div>
+                    </div>
+                    <div>
+                        <div class="text-xs text-muted mb-1">Nonce</div>
+                        <div
+                            id="approve-tx-nonce"
+                            class="text-xs min-h-[1rem]"
+                        ></div>
+                    </div>
+                </div>
                 <div id="approve-tx-data-section" class="mb-3 hidden">
                     <div class="text-xs text-muted mb-1">Raw data</div>
                     <div id="approve-tx-data" class="text-xs break-all"></div>

src/popup/views/approval.js

@@ -10,6 +10,7 @@ const {
     onViewLeave,
 } = require("./helpers");
 const { state, saveState, currentNetwork } = require("../../shared/state");
+const { networkByChainId } = require("../../shared/networks");
 const {
     formatEther,
     formatUnits,
@@ -23,7 +24,6 @@ const { TOKEN_BY_ADDRESS } = require("../../shared/tokenList");
 const { decryptWithPassword } = require("../../shared/vault");
 const { getSignerForAddress } = require("../../shared/wallet");
 const { walletDefect } = require("../../shared/walletDefects");
-const { getProvider } = require("../../shared/balances");
 const { describeSigningFailure } = require("../../shared/approvalVerify");
 const txStatus = require("./txStatus");
 const uniswap = require("../../shared/uniswap");
@@ -159,21 +159,61 @@ function showPhishingWarning(elementId, isPhishing) {
     }
 }
 
+// The fields of the approved transaction the value and recipient lines do not
+// already carry: network, gas limit, fee per gas, the most the fee can come to,
+// and the nonce. The background compares every one of them against the signed
+// artifact, so every one of them has to be on the screen — a number that is
+// verified but never displayed is verified against nothing the user agreed to.
+function showTxFee(approvedTx, ethPrice) {
+    const network = networkByChainId(approvedTx.chainId);
+    $("approve-tx-network").textContent = network
+        ? network.name
+        : "Unknown network (chain id " + BigInt(approvedTx.chainId) + ")";
+
+    const gasLimit = BigInt(approvedTx.gasLimit);
+    const feePerGas = BigInt(approvedTx.maxFeePerGas || approvedTx.gasPrice);
+    const maxFeeEth = formatTxValue(formatEther(gasLimit * feePerGas));
+    const usdStr = formatUsd(
+        ethPrice ? parseFloat(maxFeeEth) * ethPrice : null,
+    );
+    $("approve-tx-fee").textContent =
+        maxFeeEth + " ETH" + (usdStr ? " (" + usdStr + ")" : "");
+
+    let detail =
+        gasLimit.toString() +
+        " gas at up to " +
+        formatUnits(feePerGas, 9) +
+        " gwei";
+    if (approvedTx.maxPriorityFeePerGas) {
+        detail +=
+            ", " +
+            formatUnits(approvedTx.maxPriorityFeePerGas, 9) +
+            " gwei priority";
+    }
+    $("approve-tx-fee-detail").textContent = detail;
+    $("approve-tx-nonce").textContent = BigInt(approvedTx.nonce).toString();
+}
+
 function showTxApproval(details) {
     showPhishingWarning(
         "approve-tx-phishing-warning",
         details.isPhishingDomain,
     );
 
-    pendingTxParams = details.txParams;
+    // The transaction the background populated. It is displayed as it stands,
+    // signed as it stands, and verified against as it stands — the popup fills
+    // nothing in, so there is no number on this screen that the background
+    // cannot compare with the artifact it gets back.
+    pendingTxParams = details.approvedTx;
+    const approvedTx = details.approvedTx;
 
-    const toAddr = details.txParams.to;
+    const toAddr = approvedTx.to;
     const token = toAddr ? TOKEN_BY_ADDRESS.get(toAddr.toLowerCase()) : null;
-    const ethValue = formatEther(details.txParams.value || "0");
+    const ethValue = formatEther(approvedTx.value || "0");
 
     // Build txInfo for status screens
     pendingTxDetails = {
-        from: state.activeAddress,
+        from: details.approvedFrom,
         to: toAddr || "",
         amount: formatTxValue(ethValue),
         token: "ETH",
@@ -181,7 +221,7 @@ function showTxApproval(details) {
     };
 
     // If this is an ERC-20 call, try to extract the real recipient and amount
-    const decoded = decodeCalldata(details.txParams.data, toAddr || "");
+    const decoded = decodeCalldata(approvedTx.data, toAddr || "");
     if (decoded && decoded.details) {
         let decodedTokenAddr = null;
         let decodedTokenSymbol = null;
@@ -219,7 +259,7 @@ function showTxApproval(details) {
     }
 
     $("approve-tx-hostname").textContent = details.hostname;
-    $("approve-tx-from").innerHTML = approvalAddressHtml(state.activeAddress);
+    $("approve-tx-from").innerHTML = approvalAddressHtml(details.approvedFrom);
 
     // Show token symbol next to contract address if known
     const symbol = toAddr ? tokenLabel(toAddr) : null;
@@ -235,7 +275,7 @@ function showTxApproval(details) {
     }
 
     const ethValueFormatted = formatTxValue(
-        formatEther(details.txParams.value || "0"),
+        formatEther(approvedTx.value || "0"),
     );
     const ethPrice = getPrice("ETH");
     const ethUsd = ethPrice ? parseFloat(ethValueFormatted) * ethPrice : null;
@@ -243,6 +283,8 @@ function showTxApproval(details) {
     $("approve-tx-value").textContent =
         ethValueFormatted + " ETH" + (usdStr ? " (" + usdStr + ")" : "");
 
+    showTxFee(approvedTx, ethPrice);
+
     // Decode calldata (reuse decoded from above)
     const decodedEl = $("approve-tx-decoded");
     if (decoded) {
@@ -271,8 +313,8 @@ function showTxApproval(details) {
     }
 
     // Always show raw data when present
-    if (details.txParams.data && details.txParams.data !== "0x") {
-        $("approve-tx-data").textContent = details.txParams.data;
+    if (approvedTx.data && approvedTx.data !== "0x") {
+        $("approve-tx-data").textContent = approvedTx.data;
         $("approve-tx-data-section").classList.remove("hidden");
     } else {
         $("approve-tx-data-section").classList.add("hidden");
@@ -283,7 +325,11 @@ function showTxApproval(details) {
 
     showView("approve-tx");
     attachCopyHandlers("view-approve-tx");
-    gateOnWalletDefect("approve-tx-error", "btn-approve-tx");
+    gateOnWalletDefect(
+        "approve-tx-error",
+        "btn-approve-tx",
+        details.approvedFrom,
+    );
 }
 
 function decodeHexMessage(hex) {
@@ -342,9 +388,12 @@ function showSignApproval(details) {
 
     const sp = details.signParams;
     pendingSignParams = sp;
+    pendingSignFrom = details.approvedFrom;
 
     $("approve-sign-hostname").textContent = details.hostname;
-    $("approve-sign-from").innerHTML = approvalAddressHtml(sp.from);
+    $("approve-sign-from").innerHTML = approvalAddressHtml(
+        details.approvedFrom,
+    );
 
     const isTyped =
         sp.method === "eth_signTypedData_v4" ||
@@ -383,7 +432,11 @@ function showSignApproval(details) {
 
     showView("approve-sign");
     attachCopyHandlers("view-approve-sign");
-    gateOnWalletDefect("approve-sign-error", "btn-approve-sign");
+    gateOnWalletDefect(
+        "approve-sign-error",
+        "btn-approve-sign",
+        details.approvedFrom,
+    );
 }
 
 function show(id) {
@@ -418,11 +471,15 @@ function show(id) {
 
 let approvalId = null;
 let pendingTxDetails = null;
-// The exact parameters shown to the user, kept so the popup signs what it
-// displayed rather than re-fetching anything at approval time. Both are
-// repopulated by show() when the popup is closed and reopened.
+// The exact objects shown to the user, kept so the popup signs what it
+// displayed rather than re-fetching or re-populating anything at approval
+// time. All are repopulated by show() when the popup is closed and reopened.
 let pendingTxParams = null;
 let pendingSignParams = null;
+// The address the approval was raised for. Signing uses this rather than the
+// active address, so that an address switch since the approval fails here
+// instead of producing a signature from an account the screen never named.
+let pendingSignFrom = null;
 
 // Approve buttons stay disabled and muted while the popup derives the key and
 // signs, which is slow enough (Argon2id) that a double click is likely.
@@ -437,12 +494,13 @@ function setSignButtonBusy(busy) {
 }
 
 // Say so on the approval screen itself, and disable the approve button, when
-// the active address belongs to a wallet whose keys cannot be derived. Without
-// this the screen would take a password and fail after deriving it. Reject
-// stays available; the wallet is not touched. Returns true when it gated.
-function gateOnWalletDefect(errorId, buttonId) {
-    const active = findActiveWallet();
-    const defect = active ? walletDefect(active.wallet) : null;
+// the address the approval was raised for belongs to a wallet whose keys
+// cannot be derived. Without this the screen would take a password and fail
+// after deriving it. Reject stays available; the wallet is not touched.
+// Returns true when it gated.
+function gateOnWalletDefect(errorId, buttonId, address) {
+    const owner = findWalletFor(address);
+    const defect = owner ? walletDefect(owner.wallet) : null;
     if (!defect) return false;
     showError(errorId, defect.shortMessage);
     $(buttonId).disabled = true;
@@ -450,12 +508,14 @@ function gateOnWalletDefect(errorId, buttonId) {
     return true;
 }
 
-// Locate the wallet and the address index owning the currently active
-// address. Returns null when no wallet holds it.
-function findActiveWallet() {
+// Locate the wallet and the address index owning an address. Returns null when
+// no wallet holds it. Approvals look up the address they were raised for, not
+// whichever address is active now: the approval named one account, and signing
+// with another is what verification refuses.
+function findWalletFor(address) {
     for (const wallet of state.wallets) {
         for (let i = 0; i < wallet.addresses.length; i++) {
-            if (wallet.addresses[i].address === state.activeAddress) {
+            if (wallet.addresses[i].address === address) {
                 return { wallet, addrIndex: i };
             }
         }
@@ -517,12 +577,12 @@ function init(ctx) {
         hideError("approve-tx-error");
         setTxButtonBusy(true);
 
-        const active = findActiveWallet();
+        const active = findWalletFor(pendingTxParams.from);
         if (!active) {
             password = null;
             showError(
                 "approve-tx-error",
-                "No wallet was found for the active address.",
+                "No wallet was found for the address this transaction was approved for.",
             );
             setTxButtonBusy(false);
             return;
@@ -569,15 +629,16 @@ function init(ctx) {
                 active.addrIndex,
                 decryptedSecret,
             );
-            const provider = getProvider(state.rpcUrl);
-            const connected = signer.connect(provider);
-            // This is the sequence ethers' own sendTransaction() runs
-            // internally, so nonce, gas, fee and chain id population are
-            // identical to when the background did the signing.
-            const populated =
-                await connected.populateTransaction(pendingTxParams);
-            delete populated.from;
-            payload.rawSignedTx = await connected.signTransaction(populated);
+            // Sign the approved transaction exactly as it was displayed. The
+            // background populated it before this screen was drawn and checks
+            // the artifact against it field for field, so there is nothing to
+            // fill in here and no provider to fill it in from. The copy is
+            // because ethers may strip `from` off what it is handed, and the
+            // approval has to survive a retry intact; keeping `from` on it
+            // makes ethers refuse a key that is not the approved address.
+            payload.rawSignedTx = await signer.signTransaction({
+                ...pendingTxParams,
+            });
         } catch (e) {
             payload.error =
                 e.shortMessage || e.message || "Transaction signing failed.";
@@ -626,12 +687,12 @@ function init(ctx) {
         hideError("approve-sign-error");
         setSignButtonBusy(true);
 
-        const active = findActiveWallet();
+        const active = findWalletFor(pendingSignFrom);
         if (!active) {
             password = null;
             showError(
                 "approve-sign-error",
-                "No wallet was found for the active address.",
+                "No wallet was found for the address this request was approved for.",
             );
             setSignButtonBusy(false);
             return;

src/shared/approvalTx.js

@@ -0,0 +1,213 @@
+// Preparation of the transaction an approval screen displays.
+//
+// A dApp's eth_sendTransaction normally fixes only `to`, `value` and `data`.
+// The nonce, the gas limit and the fees have to be filled in from the network
+// before anything can be signed, and whoever fills them in decides what the
+// user is shown. That work used to happen in the popup, after the user had
+// already approved: the numbers on the approval screen came from the popup and
+// were compared against nothing, so a compromised popup could display one fee
+// and sign another, and the ceilings in approvalVerify.js were all that stood
+// between the user and a fee that hands the validator the balance.
+//
+// So it happens here instead, in the background, before the approval window is
+// opened. The background populates the transaction, shows that object, and
+// verifies the signed artifact against that same object — the popup is handed
+// a finished transaction and signs it as given. Every field the user reads is
+// then a field that is compared.
+//
+// The cost is an RPC round trip before the approval window exists. Nothing is
+// displayed while it is in flight, and a failure — an unreachable node, a
+// reverting gas estimate, a transaction type this wallet does not sign, a fee
+// past the ceilings — means no approval and no window at all: the error goes
+// back to the requesting page, which is where the user's click came from. That
+// is deliberate. The alternative, opening the window first and populating
+// behind a spinner, needs a pending approval that exists before it can be
+// displayed or signed, and a half-initialised approval is exactly the state
+// the settle interlock in the background exists to keep out of that record.
+// The failure also lands earlier than it used to rather than later: the same
+// estimate previously failed after the user had typed their password.
+
+const {
+    VoidSigner,
+    accessListify,
+    getAddress,
+    getBytes,
+    hexlify,
+    toQuantity,
+} = require("ethers");
+const {
+    ALLOWED_TX_TYPES,
+    SERIALIZED_FIELDS,
+    assertWithinCeilings,
+} = require("./approvalVerify");
+
+// How long the population may take before the request is failed back to the
+// page. Without a bound a hung RPC endpoint leaves the dApp's promise pending
+// forever with nothing on screen to explain it; ethers' own request timeout is
+// minutes long, which is not a wait anyone will sit through.
+const POPULATE_TIMEOUT_MS = 20000;
+
+// The request fields taken from the page. Anything else is dropped rather than
+// passed to ethers: the object is page-controlled, and a future ethers that
+// learns to carry a new transaction field must not start picking one up out of
+// it without this module knowing.
+const REQUEST_FIELDS = [
+    "to",
+    "value",
+    "data",
+    "nonce",
+    "gasLimit",
+    "gasPrice",
+    "maxFeePerGas",
+    "maxPriorityFeePerGas",
+    "chainId",
+    "accessList",
+    "type",
+];
+
+class ApprovalPrepareError extends Error {
+    constructor(message) {
+        super(message);
+        this.name = "ApprovalPrepareError";
+    }
+}
+
+function fail(message) {
+    return new ApprovalPrepareError(message);
+}
+
+function present(v) {
+    return v !== null && v !== undefined && v !== "";
+}
+
+// These strings reach the user through the requesting page, so they are full
+// sentences even when the tail of one came from ethers or from the node.
+function sentence(text) {
+    return /[.!?]$/.test(text) ? text : text + ".";
+}
+
+// Reject a promise that has taken too long, and never leave the timer behind.
+async function withTimeout(promise, ms, message) {
+    let timer = null;
+    try {
+        return await Promise.race([
+            promise,
+            new Promise((_resolve, reject) => {
+                timer = setTimeout(() => reject(fail(message)), ms);
+            }),
+        ]);
+    } finally {
+        if (timer !== null) clearTimeout(timer);
+    }
+}
+
+// The page's request, reduced to the fields this wallet acts on.
+function requestFrom(txParams, from) {
+    const request = { from: getAddress(from) };
+    for (const key of REQUEST_FIELDS) {
+        if (present(txParams[key])) request[key] = txParams[key];
+    }
+    if (
+        present(request.type) &&
+        !ALLOWED_TX_TYPES.includes(Number(request.type))
+    ) {
+        throw fail(
+            "The site asked for a transaction of a type this wallet does not sign.",
+        );
+    }
+    return request;
+}
+
+// Turn a populated transaction into the object that crosses to the popup, is
+// displayed, and is compared with the signed artifact. It carries exactly the
+// fields its type serializes, plus the address it is to be signed by, and
+// every quantity as a hex string: extension messaging is JSON, which has no
+// bigint, and a field that did not survive the trip would be a field the user
+// was shown and nothing compared.
+function serializeApprovedTx(populated, from) {
+    const type = Number(populated.type);
+    if (!ALLOWED_TX_TYPES.includes(type)) {
+        throw fail(
+            "This transaction would have to be sent as a type this wallet does not sign.",
+        );
+    }
+    const approved = { type, from: getAddress(from) };
+    for (const key of SERIALIZED_FIELDS[type]) {
+        if (key === "to") {
+            approved.to = present(populated.to)
+                ? getAddress(populated.to)
+                : null;
+        } else if (key === "data") {
+            approved.data = present(populated.data)
+                ? hexlify(getBytes(populated.data))
+                : "0x";
+        } else if (key === "accessList") {
+            approved.accessList = accessListify(populated.accessList || []);
+        } else if (key === "value") {
+            approved.value = toQuantity(populated.value || 0);
+        } else if (!present(populated[key])) {
+            // Unreachable while populateTransaction() fills every quantity of
+            // the type it produced. If it ever does not, the approval must not
+            // be raised: an unfixed quantity is one the artifact cannot be
+            // checked against.
+            throw fail(
+                "The transaction could not be prepared: the network did not supply a " +
+                    key +
+                    ".",
+            );
+        } else {
+            approved[key] = toQuantity(populated[key]);
+        }
+    }
+    return approved;
+}
+
+// Populate the transaction a site asked for, as the address it will be signed
+// by, and return the object to display, sign and verify against. Throws with a
+// full sentence when no approval can be raised.
+async function prepareApprovalTx(provider, from, txParams) {
+    if (!present(from)) {
+        throw fail("There is no active address to send this transaction from.");
+    }
+    const request = requestFrom(txParams || {}, from);
+
+    let populated;
+    try {
+        // The sequence ethers' own sendTransaction() runs internally, so the
+        // nonce, gas, fee and chain id are populated exactly as they were when
+        // the popup did this. VoidSigner cannot sign, which is the point: the
+        // background prepares, the popup signs.
+        populated = await withTimeout(
+            new VoidSigner(getAddress(from), provider).populateTransaction(
+                request,
+            ),
+            POPULATE_TIMEOUT_MS,
+            "The transaction could not be prepared: the network did not answer in time.",
+        );
+    } catch (e) {
+        if (e instanceof ApprovalPrepareError) throw e;
+        throw fail(
+            sentence(
+                "The transaction could not be prepared: " +
+                    (e.shortMessage ||
+                        e.message ||
+                        "the network did not answer"),
+            ),
+        );
+    }
+
+    const approved = serializeApprovedTx(populated, from);
+    // The backstop, applied before the user is shown anything rather than
+    // after they have approved it: what is displayed here is what gets signed,
+    // so an RPC node reporting an absurd fee has to be refused here.
+    assertWithinCeilings(approved);
+    return approved;
+}
+
+module.exports = {
+    prepareApprovalTx,
+    serializeApprovedTx,
+    ApprovalPrepareError,
+    POPULATE_TIMEOUT_MS,
+    REQUEST_FIELDS,
+};

src/shared/approvalVerify.js

@@ -7,6 +7,13 @@
 // the signer from the artifact and checks it against the approval it is
 // holding before acting on it. All recovery is delegated to ethers.
 //
+// What the artifact is checked against is the transaction the background
+// populated and the popup displayed (see approvalTx.js), not the request the
+// dApp made. The two differ in every field a dApp normally leaves out — nonce,
+// gas limit, fees — and those are the fields the user reads off the approval
+// screen, so comparing against the request would leave the numbers on screen
+// vouched for by nothing.
+//
 // The check is an allowlist, in both directions, because a denylist cannot be
 // correct against a transaction format that keeps gaining fields:
 //
@@ -31,14 +38,12 @@
 // never a warning: what the user approved is what gets broadcast, or nothing
 // does.
 //
-// Fields the approval does not carry are not treated as zero. The popup
-// populates nonce, gas limit, fee and chain id through populateTransaction()
-// when the requesting page did not fix them, so there is no approved value to
-// compare against; treating absent as zero would refuse every legitimate
-// transaction. Those fields are instead held to the absolute ceilings below,
-// and the chain id is always checked against the selected network rather than
-// against the approval alone, which is what makes a cross-chain replay
-// impossible.
+// The approved transaction is required to fix every field its type serializes,
+// so there is no "the approval did not say" branch to fall through: a quantity
+// the approval does not carry is a refusal, because an artifact that cannot be
+// compared with what was displayed has not been checked. The chain id is
+// checked against the selected network as well as against the approval, which
+// is what makes a cross-chain replay impossible.
 //
 // Every failure message is a full sentence, because these strings are shown to
 // the user and returned to the dApp.
@@ -113,6 +118,17 @@ const FORBIDDEN_FIELDS = [
     },
 ];
 
+// Absolute ceilings — a BACKSTOP, not the primary control.
+//
+// The primary control is equality: every field of the artifact is compared
+// with the populated transaction the user was shown, so nothing the popup
+// signs can differ from the screen. What equality cannot bound is the
+// populated transaction itself, which is built from what the configured RPC
+// node answered — a node that reports an absurd fee gets that fee displayed,
+// and a user who does not read the fee line would approve it. These ceilings
+// bound that, and they are therefore applied where the transaction is
+// populated (approvalTx.js) as well as here.
+//
 // Above the block gas limit of every supported network (see networks.js), so
 // no transaction that could ever be included is refused by it.
 const MAX_GAS_LIMIT = 100000000n;
@@ -224,40 +240,151 @@ function normalizeData(v) {
     return String(v).toLowerCase();
 }
 
-// Quantity fields the requesting page may fix in the approval. Each is
-// compared exactly when the approval carries it, and left to the ceilings
-// above when it does not.
-const APPROVED_QUANTITIES = [
-    {
-        key: "nonce",
+// How each field of an approved transaction is compared with the artifact.
+// There is an entry here for every field any allowed type serializes — a test
+// pins that against SERIALIZED_FIELDS — so the comparison loop covers the
+// whole of what gets signed and cannot silently skip a field for want of a
+// comparator.
+//
+// `kind` decides how the two sides are made comparable. A `quantity` must be
+// fixed by the approval: it is one of the numbers on the approval screen, and
+// an absent one means the artifact cannot be checked against what was
+// displayed. `to`, `value`, `data` and `accessList` have canonical absent
+// forms — contract creation, zero, "0x" and the empty list — so they are
+// normalized on both sides instead.
+const APPROVED_FIELDS = {
+    chainId: {
+        kind: "quantity",
+        label: "network",
+        message:
+            "The signed transaction is for a different network than the one that was approved.",
+    },
+    nonce: {
+        kind: "quantity",
         label: "nonce",
         message: "The signed transaction does not carry the approved nonce.",
     },
-    {
-        key: "gasLimit",
+    gasLimit: {
+        kind: "quantity",
         label: "gas limit",
         message:
             "The signed transaction does not carry the approved gas limit.",
     },
-    {
-        key: "gasPrice",
+    gasPrice: {
+        kind: "quantity",
         label: "gas price",
         message:
             "The signed transaction does not carry the approved gas price.",
     },
-    {
-        key: "maxFeePerGas",
+    maxFeePerGas: {
+        kind: "quantity",
         label: "maximum fee per gas",
         message:
             "The signed transaction does not carry the approved maximum fee per gas.",
     },
-    {
-        key: "maxPriorityFeePerGas",
+    maxPriorityFeePerGas: {
+        kind: "quantity",
         label: "maximum priority fee per gas",
         message:
             "The signed transaction does not carry the approved maximum priority fee per gas.",
     },
-];
+    to: {
+        kind: "address",
+        label: "recipient",
+        message:
+            "The signed transaction does not go to the approved recipient.",
+    },
+    value: {
+        kind: "value",
+        label: "value",
+        message: "The signed transaction does not carry the approved value.",
+    },
+    data: {
+        kind: "data",
+        label: "call data",
+        message:
+            "The signed transaction does not carry the approved call data.",
+    },
+    accessList: {
+        kind: "accessList",
+        label: "access list",
+        message:
+            "The signed transaction does not carry the approved access list.",
+    },
+};
+
+// Compare one field of the artifact with the approved transaction. A field
+// with no entry in the table above is refused rather than skipped: the loop
+// below runs over the fields the type serializes, so an unmatched key means
+// something that gets signed has no comparator at all.
+function assertFieldMatches(key, parsed, approvedTx) {
+    const field = APPROVED_FIELDS[key];
+    if (!field) {
+        throw refuse(
+            "The signed transaction carries a field this wallet cannot compare with the approval.",
+        );
+    }
+    switch (field.kind) {
+        case "quantity": {
+            if (!present(approvedTx[key])) {
+                throw refuse(
+                    "The approved transaction fixes no " +
+                        field.label +
+                        ", so the signed transaction cannot be checked" +
+                        " against what was shown.",
+                );
+            }
+            const approved = normalizeQuantity(approvedTx[key], field.label);
+            if (normalizeQuantity(parsed[key], field.label) !== approved) {
+                throw refuse(field.message);
+            }
+            return;
+        }
+        case "address":
+            if (!sameAddress(parsed[key], approvedTx[key])) {
+                throw refuse(field.message);
+            }
+            return;
+        case "value":
+            if (normalizeValue(parsed[key]) !== normalizeValue(approvedTx[key]))
+                throw refuse(field.message);
+            return;
+        case "data":
+            if (normalizeData(parsed[key]) !== normalizeData(approvedTx[key]))
+                throw refuse(field.message);
+            return;
+        default:
+            if (
+                normalizeAccessList(parsed[key]) !==
+                normalizeAccessList(approvedTx[key])
+            ) {
+                throw refuse(field.message);
+            }
+    }
+}
+
+// The ceilings, applied to a transaction that is either about to be displayed
+// or about to be broadcast. See MAX_GAS_LIMIT above for what they are for:
+// they bound what the RPC node can talk this wallet into showing the user,
+// which is the one thing comparing the artifact with the screen cannot do.
+function assertWithinCeilings(tx) {
+    if (
+        present(tx.gasLimit) &&
+        normalizeQuantity(tx.gasLimit, "gas limit") > MAX_GAS_LIMIT
+    ) {
+        throw refuse(
+            "The signed transaction sets a gas limit no network this wallet supports can accept.",
+        );
+    }
+    for (const key of ["gasPrice", "maxFeePerGas", "maxPriorityFeePerGas"]) {
+        if (!present(tx[key])) continue;
+        if (normalizeQuantity(tx[key], "fee per gas") > MAX_FEE_PER_GAS) {
+            throw refuse(
+                "The signed transaction sets a fee per gas far above any plausible value.",
+            );
+        }
+    }
+}
 
 // Refuse a field only a transaction type this wallet does not sign can carry.
 // The type allowlist keeps these unreachable in production, which is exactly
@@ -317,10 +444,28 @@ function assertCanonicalBytes(parsed, rawSignedTx) {
 // signed by the address the approval was raised for, on the network that is
 // selected. Returns the parsed ethers Transaction on success, throws
 // otherwise.
-function verifySignedTx(rawSignedTx, txParams, expectedFrom, selectedChainId) {
+//
+// `approvedTx` is the populated transaction the approval screen displayed, and
+// `expectedFrom` is the address that was active when the approval was raised —
+// not whichever address is active now. An address switch between approval and
+// signing therefore refuses here rather than producing a transaction from an
+// account the approval did not name.
+function verifySignedTx(
+    rawSignedTx,
+    approvedTx,
+    expectedFrom,
+    selectedChainId,
+) {
     if (typeof rawSignedTx !== "string" || !rawSignedTx.startsWith("0x")) {
         throw refuse("The signed transaction is missing or malformed.");
     }
+    // Nothing to compare against is a refusal like any other: an approval that
+    // does not carry the transaction it displayed cannot vouch for one.
+    if (!approvedTx || typeof approvedTx !== "object") {
+        throw refuse(
+            "There is no approved transaction to check the signed transaction against.",
+        );
+    }
 
     let parsed;
     try {
@@ -360,46 +505,15 @@ function verifySignedTx(rawSignedTx, txParams, expectedFrom, selectedChainId) {
             "The signed transaction is for a different network than the one that is selected.",
         );
     }
-    if (
-        present(txParams.chainId) &&
-        parsed.chainId !== normalizeQuantity(txParams.chainId, "network")
-    ) {
-        throw refuse(
-            "The signed transaction is for a different network than the one that was approved.",
-        );
-    }
 
-    if (!sameAddress(parsed.to, txParams.to)) {
-        throw refuse(
-            "The signed transaction does not go to the approved recipient.",
-        );
-    }
-    if (normalizeValue(parsed.value) !== normalizeValue(txParams.value)) {
-        throw refuse(
-            "The signed transaction does not carry the approved value.",
-        );
-    }
-    if (normalizeData(parsed.data) !== normalizeData(txParams.data)) {
-        throw refuse(
-            "The signed transaction does not carry the approved call data.",
-        );
-    }
-    if (
-        normalizeAccessList(parsed.accessList) !==
-        normalizeAccessList(txParams.accessList)
-    ) {
-        throw refuse(
-            "The signed transaction does not carry the approved access list.",
-        );
-    }
-
-    // An approval that fixed EIP-1559 fees must not be signed as a legacy
-    // transaction, and vice versa: the fee the user agreed to is only
-    // meaningful under the mechanism it was quoted in.
+    // The approved fee mechanism, named before the type comparison below
+    // subsumes it: the fee the user agreed to is only meaningful under the
+    // mechanism it was quoted in, and saying so is more use than "a different
+    // transaction type".
     const approvedEip1559 =
-        present(txParams.maxFeePerGas) ||
-        present(txParams.maxPriorityFeePerGas);
-    const approvedLegacy = present(txParams.gasPrice);
+        present(approvedTx.maxFeePerGas) ||
+        present(approvedTx.maxPriorityFeePerGas);
+    const approvedLegacy = present(approvedTx.gasPrice);
     const signedEip1559 = parsed.type === 2;
     if (
         (approvedEip1559 && !signedEip1559) ||
@@ -410,28 +524,33 @@ function verifySignedTx(rawSignedTx, txParams, expectedFrom, selectedChainId) {
         );
     }
 
-    for (const field of APPROVED_QUANTITIES) {
-        if (!present(txParams[field.key])) continue;
-        const approved = normalizeQuantity(txParams[field.key], field.label);
-        if (normalizeQuantity(parsed[field.key], field.label) !== approved) {
-            throw refuse(field.message);
-        }
-    }
-
-    if (parsed.gasLimit > MAX_GAS_LIMIT) {
+    // The type decides which fields are compared, so it is compared first and
+    // against the approval, not merely checked for membership of the
+    // allowlist above.
+    if (!present(approvedTx.type)) {
         throw refuse(
-            "The signed transaction sets a gas limit no network this wallet supports can accept.",
+            "The approved transaction fixes no transaction type, so the signed transaction cannot be checked against what was shown.",
         );
     }
-    for (const key of ["gasPrice", "maxFeePerGas", "maxPriorityFeePerGas"]) {
-        const fee = parsed[key];
-        if (fee !== null && fee !== undefined && fee > MAX_FEE_PER_GAS) {
-            throw refuse(
-                "The signed transaction sets a fee per gas far above any plausible value.",
-            );
-        }
+    if (
+        BigInt(parsed.type) !==
+        normalizeQuantity(approvedTx.type, "transaction type")
+    ) {
+        throw refuse(
+            "The signed transaction does not use the approved transaction type.",
+        );
     }
 
+    // Every field this type serializes, compared with the transaction the user
+    // was shown. Driving the loop off SERIALIZED_FIELDS is what keeps this
+    // exhaustive: the same table decides what assertNothingUnchecked() rebuilds
+    // from, so a field that gets signed and is not compared here cannot exist.
+    for (const key of SERIALIZED_FIELDS[parsed.type]) {
+        assertFieldMatches(key, parsed, approvedTx);
+    }
+
+    assertWithinCeilings(parsed);
+
     assertNothingUnchecked(parsed);
     assertCanonicalBytes(parsed, rawSignedTx);
 
@@ -504,10 +623,10 @@ function errorText(err) {
 //    approval. Anything else failed before the check ran and is retryable.
 //  - broadcast: always terminal. A broadcast that throws after the node
 //    accepted the transaction is routine (a timeout, a dropped response, a
-//    node answering "already known"), and the popup's retry does not
-//    re-broadcast these bytes — it re-runs populateTransaction() and signs
-//    again at a freshly fetched pending-tag nonce. Retrying would therefore
-//    put a second transaction on the chain for one approval.
+//    node answering "already known"), so the wallet cannot tell a transaction
+//    that never left from one that is already in the mempool. The approval is
+//    spent and the requesting page has been given its outcome; a second
+//    attempt against it would report a second outcome for one request.
 function describeTxFailure(stage, err) {
     const error = errorText(err);
     const retryable =
@@ -553,6 +672,7 @@ module.exports = {
     assertNoForbiddenFields,
     assertNothingUnchecked,
     assertCanonicalBytes,
+    assertWithinCeilings,
     sameAddress,
     failureIsRetryable,
     describeTxFailure,
@@ -561,6 +681,7 @@ module.exports = {
     ALLOWED_TX_TYPES,
     SERIALIZED_FIELDS,
     FORBIDDEN_FIELDS,
+    APPROVED_FIELDS,
     TX_STAGE_SIGN,
     TX_STAGE_VERIFY,
     TX_STAGE_BROADCAST,