harden: bound the total network fee by gasLimit × fee, on both send paths (closes #399)

The two per-field ceilings in approvalVerify.js were checked independently,
but the fee a validator is paid is gasLimit × fee per gas: a gas limit and a
fee each under their own ceiling still multiply to thousands of ETH, which a
gas-consuming contract really collects. assertWithinCeilings now also bounds
that product against MAX_TOTAL_FEE (1 ETH), so both callers — populating the
dApp transaction and verifying the signed artifact — refuse it with a full
sentence naming the fee and the limit.

The wallet's own send in confirmTx.js pinned no fee fields, so ethers filled
them from the node with no bound; it now populates the transaction and runs the
same check before signing, showing the same error in the confirmation screen's
reserved errors box so nothing on screen moves.

Model: opus-4-8

TODO.md

@@ -45,6 +45,28 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-09-21: The network fee a transaction can commit is bounded by the product
+  of the gas limit and the fee per gas, not by each field alone, and the
+  wallet's own send is bounded the same way
+  ([#399](https://git.eeqj.de/sneak/AutistMask/issues/399)). The two per-field
+  ceilings in `src/shared/approvalVerify.js` were checked independently, so a
+  gas limit and a fee that were each under their own ceiling still multiplied to
+  thousands of ETH — a fee a gas-consuming contract really collects — while the
+  comment claimed the ceiling caught exactly that. `assertWithinCeilings` now
+  also refuses a transaction whose gas limit times its fee per gas
+  (`maxFeePerGas` for a type-2 transaction, `gasPrice` for a legacy or type-1
+  one) exceeds `MAX_TOTAL_FEE`, a new constant of 1 ETH beside the existing
+  ceilings, so both callers — where the dApp transaction is populated and where
+  the signed artifact is verified — reject it with a full sentence naming the
+  fee and the limit. The wallet's own send in `src/popup/views/confirmTx.js`
+  pinned no fee fields, so ethers filled them from whatever the configured node
+  answered with nothing bounding them; it now populates the transaction and runs
+  the same check before signing, showing the same error in the confirmation
+  screen's reserved errors box so nothing on screen moves. Deliberately out of
+  scope: comparing a supplied fee against the node's own suggested fee, which
+  the absolute bound already makes unnecessary for the balance-draining case. 1
+  ETH is a plain constant, one line to change; the owner may prefer another
+  figure.
 - 2026-09-21: The test recovery phrase no longer survives in a release bundle,
   and the committed-key guard matches by content
   ([#351](https://git.eeqj.de/sneak/AutistMask/issues/351)). `DEBUG_MNEMONIC` in

src/popup/views/confirmTx.js

@@ -30,6 +30,7 @@ const {
     displayedDecimals,
     transferAmountUnits,
 } = require("../../shared/transferAmount");
+const { assertWithinCeilings } = require("../../shared/approvalVerify");
 const {
     CODES,
     FEE_PENDING,
@@ -394,6 +395,46 @@ async function estimateGas(txInfo) {
     }
 }
 
+// Populate the transaction this send describes, enforce the fee bound against
+// the fees that were actually filled in, then sign and broadcast it. The send
+// pins no fee fields, so ethers fills maxFeePerGas and the gas limit from what
+// the configured RPC node answers, with nothing otherwise bounding what a
+// hostile node can set — the dApp path's ceilings never reached this one.
+// Populating before the check is what makes assertWithinCeilings() see the
+// same numbers that would be signed; it throws an ApprovalMismatchError when
+// the product gasLimit × maxFeePerGas is over the bound, which the caller
+// shows in the reserved error area rather than sending.
+async function populateVerifyAndSend(connectedSigner, tx) {
+    let request;
+    if (tx.token === "ETH") {
+        request = { to: tx.to, value: parseEther(tx.amount) };
+    } else {
+        const contract = new Contract(tx.token, ERC20_ABI, connectedSigner);
+        // The contract's decimals() is read to be COMPARED with the scale the
+        // screen rendered this amount at, not to encode with: encoding from it
+        // signs whatever the contract answers now, which is not what the user
+        // read. A disagreement throws. See transferAmount.js.
+        const amount = transferAmountUnits(
+            tx.amount,
+            tx.tokenDecimals,
+            await contract.decimals(),
+        );
+        request = await contract.transfer.populateTransaction(tx.to, amount);
+    }
+    const populated = await connectedSigner.populateTransaction(request);
+    assertWithinCeilings(populated);
+    return connectedSigner.sendTransaction(populated);
+}
+
+// Show a full-sentence send failure in the reserved errors box, the same
+// element and markup renderValidation() uses for messages carrying the user's
+// own numbers, so it never moves anything on the screen.
+function showSendError(message) {
+    const el = $("confirm-errors");
+    el.innerHTML = `<div class="text-xs">${escapeHtml(message)}</div>`;
+    el.style.visibility = "visible";
+}
+
 async function checkRecipientHistory(txInfo) {
     try {
         const provider = getProvider(state.rpcUrl, state.networkId);
@@ -467,29 +508,7 @@ function init(_ctx) {
             const provider = getProvider(state.rpcUrl, state.networkId);
             const connectedSigner = signer.connect(provider);
 
-            if (pendingTx.token === "ETH") {
+            tx = await populateVerifyAndSend(connectedSigner, pendingTx);
-                tx = await connectedSigner.sendTransaction({
-                    to: pendingTx.to,
-                    value: parseEther(pendingTx.amount),
-                });
-            } else {
-                const contract = new Contract(
-                    pendingTx.token,
-                    ERC20_ABI,
-                    connectedSigner,
-                );
-                // The contract's decimals() is read to be COMPARED with the
-                // scale the screen rendered this amount at, not to encode with:
-                // encoding from it signs whatever the contract answers now,
-                // which is not what the user read. A disagreement throws and is
-                // reported on the error screen. See transferAmount.js.
-                const amount = transferAmountUnits(
-                    pendingTx.amount,
-                    pendingTx.tokenDecimals,
-                    await contract.decimals(),
-                );
-                tx = await contract.transfer(pendingTx.to, amount);
-            }
 
             // Best-effort: clear decrypted secret after use.
             // Note: JS strings are immutable; this nulls the reference but
@@ -498,6 +517,14 @@ function init(_ctx) {
             txStatus.showWait(pendingTx, tx.hash);
         } catch (e) {
             decryptedSecret = null;
+            // A fee over the bound is refused before anything is broadcast, so
+            // there is no transaction that may have reached the network to warn
+            // about: the message stays on the confirmation screen where the
+            // user can go back, rather than routing to the sent/failed screen.
+            if (e && e.approvalMismatch) {
+                showSendError(e.message);
+                return;
+            }
             const hash = tx ? tx.hash : null;
             txStatus.showError(pendingTx, hash, e.shortMessage || e.message);
         } finally {
@@ -511,4 +538,4 @@ function init(_ctx) {
     });
 }
 
-module.exports = { init, show, restore };
+module.exports = { init, show, restore, populateVerifyAndSend };

src/shared/approvalVerify.js

@@ -51,6 +51,7 @@
 const {
     Transaction,
     accessListify,
+    formatEther,
     getAddress,
     getBytes,
     verifyMessage,
@@ -134,10 +135,19 @@ const FORBIDDEN_FIELDS = [
 const MAX_GAS_LIMIT = 100000000n;
 
 // 100,000 gwei per gas: orders of magnitude above the highest fee either
-// supported network has produced, and low enough to catch a fee that would
+// supported network has produced.
-// hand the validator the balance.
 const MAX_FEE_PER_GAS = 100000000000000n;
 
+// The largest total fee this wallet will sign, in wei. The two ceilings above
+// bound the gas limit and the price per gas each on its own, but the fee a
+// validator is actually paid is their product, and a gas limit and a price
+// that are each under their own ceiling still multiply to thousands of ETH —
+// 30,000,000 gas at 100,000 gwei is about 3,000 ETH. Bounding the product is
+// what catches a fee that would hand the validator the balance; the per-field
+// ceilings alone do not. A full 30,000,000-gas block at 33 gwei reaches this,
+// which no ordinary wallet transaction approaches.
+const MAX_TOTAL_FEE = 1000000000000000000n; // 1 ETH
+
 // A refusal to act on an artifact: it is not the thing that was approved, so
 // the approval it was offered against is spent and must not be retried. Every
 // throw in this module is one of these; the background distinguishes them from
@@ -384,6 +394,28 @@ function assertWithinCeilings(tx) {
             );
         }
     }
+    // The product: gasLimit × the most this transaction could pay per gas —
+    // maxFeePerGas for a type-2 transaction, gasPrice for a legacy or type-1
+    // one. This is the fee a gas-consuming contract can really extract, and it
+    // is the bound the two per-field ceilings above cannot express.
+    if (present(tx.gasLimit)) {
+        const gasLimit = normalizeQuantity(tx.gasLimit, "gas limit");
+        let price = null;
+        if (present(tx.maxFeePerGas)) {
+            price = normalizeQuantity(tx.maxFeePerGas, "maximum fee per gas");
+        } else if (present(tx.gasPrice)) {
+            price = normalizeQuantity(tx.gasPrice, "gas price");
+        }
+        if (price !== null && gasLimit * price > MAX_TOTAL_FEE) {
+            throw refuse(
+                "This transaction would allow a network fee of up to " +
+                    formatEther(gasLimit * price) +
+                    " ETH, which is more than the " +
+                    formatEther(MAX_TOTAL_FEE) +
+                    " ETH this wallet will sign for.",
+            );
+        }
+    }
 }
 
 // Refuse a field only a transaction type this wallet does not sign can carry.
@@ -775,4 +807,5 @@ module.exports = {
     TX_STAGE_NONCE,
     MAX_GAS_LIMIT,
     MAX_FEE_PER_GAS,
+    MAX_TOTAL_FEE,
 };

tests/approvalTx.test.js

@@ -212,6 +212,24 @@ describe("prepareApprovalTx", () => {
         ).rejects.toThrow(/gas limit no network this wallet supports/);
     });
 
+    // The combined bound at population: a gas limit and a fee that are each
+    // under their own ceiling but multiply to thousands of ETH is refused
+    // before the approval window opens, so the user is never shown a
+    // balance-draining fee to click past.
+    test("refuses a fee whose product with the gas limit is over the bound", async () => {
+        const gouging = providerWith({
+            estimateGas: async () => 30000000n,
+            getFeeData: async () => ({
+                gasPrice: MAX_FEE_PER_GAS,
+                maxFeePerGas: MAX_FEE_PER_GAS,
+                maxPriorityFeePerGas: 1000000000n,
+            }),
+        });
+        await expect(
+            prepareApprovalTx(gouging, signer.address, TX_PARAMS),
+        ).rejects.toThrow(/network fee of up to/);
+    });
+
     // No approval and no window: the failure goes back to the page the click
     // came from, in a sentence.
     test("reports a failed estimate as a full sentence", async () => {

tests/approvalVerify.test.js

@@ -28,6 +28,7 @@ const {
     TX_STAGE_NONCE,
     MAX_GAS_LIMIT,
     MAX_FEE_PER_GAS,
+    MAX_TOTAL_FEE,
 } = require("../src/shared/approvalVerify");
 const { prepareApprovalTx } = require("../src/shared/approvalTx");
 const { getSignerForAddress } = require("../src/shared/wallet");
@@ -475,18 +476,131 @@ describe("verifySignedTx field comparison", () => {
                 assertWithinCeilings({ [key]: MAX_FEE_PER_GAS + 1n }),
             ).toThrow(/fee per gas far above any plausible value/);
         }
+        // Each field at its own ceiling multiplies to about 10,000 ETH, which
+        // is exactly the combination the per-field ceilings cannot see and the
+        // product bound is for: it is refused, not accepted.
         expect(() =>
             assertWithinCeilings({
                 gasLimit: MAX_GAS_LIMIT,
                 maxFeePerGas: MAX_FEE_PER_GAS,
                 maxPriorityFeePerGas: MAX_FEE_PER_GAS,
             }),
+        ).toThrow(/network fee of up to/);
+        // An ordinary transaction — a modest gas limit and a modest fee, each
+        // far under its ceiling and their product far under the bound — passes.
+        expect(() =>
+            assertWithinCeilings({
+                gasLimit: 21000n,
+                maxFeePerGas: 2000000000n,
+                maxPriorityFeePerGas: 1000000000n,
+            }),
         ).not.toThrow();
         // Nothing to bound is not a failure: a type 2 approval carries no gas
         // price, and a bare object must not be refused for lacking one.
         expect(() => assertWithinCeilings({})).not.toThrow();
     });
 
+    // The defect this issue closes: gasLimit and maxFeePerGas each under their
+    // own ceiling, but their product — the fee a gas-consuming contract can
+    // really extract — thousands of ETH. The per-field ceilings accept it; the
+    // product bound refuses it, on either side of the screen.
+    describe("the combined fee bound", () => {
+        // A gas limit and a fee that are each comfortably under their own
+        // ceiling but multiply to well over 1 ETH: 30,000,000 gas at 100,000
+        // gwei is about 3,000 ETH.
+        const OVER = { gasLimit: 30000000n, maxFeePerGas: 100000000000000n };
+
+        test("each field is under its own ceiling", () => {
+            expect(OVER.gasLimit).toBeLessThan(MAX_GAS_LIMIT);
+            expect(OVER.maxFeePerGas).toBeLessThanOrEqual(MAX_FEE_PER_GAS);
+            expect(OVER.gasLimit * OVER.maxFeePerGas).toBeGreaterThan(
+                MAX_TOTAL_FEE,
+            );
+        });
+
+        test("assertWithinCeilings refuses the product over the bound", () => {
+            expect(() =>
+                assertWithinCeilings({
+                    ...OVER,
+                    maxPriorityFeePerGas: 1000000000n,
+                }),
+            ).toThrow(/network fee of up to 3000\.0 ETH/);
+        });
+
+        test("assertWithinCeilings bounds a legacy gasPrice the same way", () => {
+            expect(() =>
+                assertWithinCeilings({
+                    gasLimit: OVER.gasLimit,
+                    gasPrice: OVER.maxFeePerGas,
+                }),
+            ).toThrow(/network fee of up to/);
+        });
+
+        // The boundary itself, pinned rather than only some value well past
+        // it. Both fields stay under their own ceilings, so it is the product
+        // and nothing else that decides these two cases: a gas limit of 10,000
+        // at the per-gas ceiling is exactly 1 ETH.
+        test("assertWithinCeilings accepts a product exactly at the bound and refuses one wei over", () => {
+            expect(MAX_FEE_PER_GAS * 10000n).toBe(MAX_TOTAL_FEE);
+            expect(() =>
+                assertWithinCeilings({
+                    gasLimit: 10000n,
+                    maxFeePerGas: MAX_FEE_PER_GAS,
+                }),
+            ).not.toThrow();
+            expect(() =>
+                assertWithinCeilings({
+                    gasLimit: 10001n,
+                    maxFeePerGas: MAX_FEE_PER_GAS,
+                }),
+            ).toThrow(/network fee of up to/);
+        });
+
+        // The dApp path: an artifact whose fee is within each field's ceiling
+        // but over the product bound, both displayed and signed, is refused at
+        // verification just as it is at population.
+        test("verifySignedTx refuses an over-bound product even when displayed", async () => {
+            const raw = await signedWith(OVER);
+            expect(() =>
+                verifySignedTx(
+                    raw,
+                    approvedFor(TX_PARAMS, OVER),
+                    signer.address,
+                    SELECTED,
+                ),
+            ).toThrow(/network fee of up to/);
+        });
+
+        test("verifySignedTx accepts a product just under the bound", async () => {
+            // 21,000 gas at 40 gwei is 0.00084 ETH — an ordinary send.
+            const under = { gasLimit: 21000n, maxFeePerGas: 40000000000n };
+            expect(under.gasLimit * under.maxFeePerGas).toBeLessThan(
+                MAX_TOTAL_FEE,
+            );
+            const raw = await signedWith(under);
+            expect(() =>
+                verifySignedTx(
+                    raw,
+                    approvedFor(TX_PARAMS, under),
+                    signer.address,
+                    SELECTED,
+                ),
+            ).not.toThrow();
+        });
+
+        test("the refusal names the fee and the limit in a full sentence", () => {
+            try {
+                assertWithinCeilings(OVER);
+                throw new Error("expected a rejection");
+            } catch (e) {
+                expect(e.approvalMismatch).toBe(true);
+                expect(e.message).toMatch(/^[A-Z].*\.$/);
+                expect(e.message).toContain("3000.0 ETH");
+                expect(e.message).toContain("1.0 ETH");
+            }
+        });
+    });
+
     test("every field mismatch is a refusal, not a warning", async () => {
         const raw = await signedWith({ nonce: 8 });
         try {

tests/confirmTxFeeBound.test.js

@@ -0,0 +1,100 @@
+// The wallet's OWN send path enforces the same combined fee bound the dApp
+// path does (https://git.eeqj.de/sneak/AutistMask/issues/399).
+//
+// The send in src/popup/views/confirmTx.js pins no fee fields, so ethers fills
+// maxFeePerGas and the gas limit from whatever the configured RPC node
+// answers. Nothing bounded that: a hostile node could report a fee whose
+// product with the gas limit is thousands of ETH, and it would be both
+// displayed and signed. populateVerifyAndSend() populates the transaction and
+// runs assertWithinCeilings() on the populated fees before signing, so an
+// over-bound send is refused before anything is broadcast.
+//
+// The check is driven here with a fake connected signer rather than a real
+// one: populateTransaction() returns the fees the node would have produced,
+// and sendTransaction() records whether the send actually happened. The real
+// DOM path around it — reading the fee error into the reserved errors box — is
+// covered by the Chrome e2e suite.
+
+globalThis.chrome = {
+    storage: { local: { get: async () => ({}), set: async () => {} } },
+};
+
+global.fetch = jest.fn(() => {
+    throw new Error("tests must not perform network requests");
+});
+
+const { populateVerifyAndSend } = require("../src/popup/views/confirmTx");
+const {
+    MAX_FEE_PER_GAS,
+    MAX_TOTAL_FEE,
+} = require("../src/shared/approvalVerify");
+
+const RECIPIENT = "0x66133E8ea0f5D1d612D2502a968757D1048c214a";
+
+// A signer whose populateTransaction() fills in the fees a node quoted and
+// whose sendTransaction() records the call, so a test can assert whether the
+// send was reached at all.
+function fakeSigner(fees) {
+    const sent = [];
+    return {
+        sent,
+        populateTransaction: async (request) => ({
+            ...request,
+            from: RECIPIENT,
+            nonce: 0,
+            type: 2,
+            chainId: 1n,
+            gasLimit: fees.gasLimit,
+            maxFeePerGas: fees.maxFeePerGas,
+            maxPriorityFeePerGas: 1000000000n,
+        }),
+        sendTransaction: async (tx) => {
+            sent.push(tx);
+            return { hash: "0xabc" };
+        },
+    };
+}
+
+const ETH_SEND = { token: "ETH", to: RECIPIENT, amount: "1.0" };
+
+describe("populateVerifyAndSend enforces the combined fee bound", () => {
+    // A gas limit and a fee that are each under their own field ceiling, but
+    // multiply to about 3,000 ETH — the combination the per-field ceilings
+    // cannot see.
+    const OVER = { gasLimit: 30000000n, maxFeePerGas: MAX_FEE_PER_GAS };
+
+    test("each field is under its ceiling but the product is over the bound", () => {
+        expect(OVER.maxFeePerGas).toBeLessThanOrEqual(MAX_FEE_PER_GAS);
+        expect(OVER.gasLimit * OVER.maxFeePerGas).toBeGreaterThan(
+            MAX_TOTAL_FEE,
+        );
+    });
+
+    test("refuses an over-bound send without broadcasting it", async () => {
+        const signer = fakeSigner(OVER);
+        let thrown;
+        try {
+            await populateVerifyAndSend(signer, ETH_SEND);
+        } catch (e) {
+            thrown = e;
+        }
+        expect(thrown).toBeDefined();
+        expect(thrown.approvalMismatch).toBe(true);
+        expect(thrown.message).toMatch(/^[A-Z].*\.$/);
+        expect(thrown.message).toContain("3000.0 ETH");
+        expect(thrown.message).toContain("1.0 ETH");
+        // The one guarantee that matters: nothing was signed or sent.
+        expect(signer.sent).toHaveLength(0);
+    });
+
+    test("broadcasts a send whose product is just under the bound", async () => {
+        // 21,000 gas at 40 gwei is 0.00084 ETH — an ordinary send.
+        const under = { gasLimit: 21000n, maxFeePerGas: 40000000000n };
+        expect(under.gasLimit * under.maxFeePerGas).toBeLessThan(MAX_TOTAL_FEE);
+        const signer = fakeSigner(under);
+        const tx = await populateVerifyAndSend(signer, ETH_SEND);
+        expect(tx.hash).toBe("0xabc");
+        expect(signer.sent).toHaveLength(1);
+        expect(signer.sent[0].gasLimit).toBe(under.gasLimit);
+    });
+});