Add optional inbound webhook signature verification (closes #67) (#228)

The receiver had no inbound authentication of any kind: /webhook/{uuid}
was mounted behind a rate limiter alone, so the only thing protecting an
entrypoint was the secrecy of a v4 UUID in a URL path. Inbound headers are
forwarded almost verbatim to the target, so anyone who learned the URL
also chose the headers the downstream service received.

Adds an optional per-entrypoint secret with two schemes: github
(X-Hub-Signature-256, HMAC-SHA256 hex over the raw body) and gitlab
(X-Gitlab-Token, a plain shared token). Comparison is constant-time, the
HMAC is computed over the raw body before any parsing, and rejection
happens before persistence -- an unauthenticated request creates no event
row. An entrypoint with no secret behaves exactly as before, including
every row that predates this change.

The scheme's credential header is stripped from the header map before it
is marshalled into Event.Headers, so the GitLab token reaches neither the
event store nor any delivery target. SchemeInfo.HeaderIsDigest defaults to
false meaning strip, so a scheme added later is protected unless its
header is positively declared a digest.

internal/handlers/webhook.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"encoding/json"
+	"errors"
 	"io"
 	"net/http"
 
@@ -10,6 +11,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/logfield"
+	"sneak.berlin/go/webhooker/internal/signature"
 )
 
 const (
@@ -69,8 +71,8 @@ func (h *Handlers) HandleWebhook() http.HandlerFunc {
 	}
 }
 
-// processWebhookRequest reads the body, serializes headers,
-// loads targets, and delivers the event.
+// processWebhookRequest reads the body, verifies the sender,
+// serializes headers, loads targets, and delivers the event.
 func (h *Handlers) processWebhookRequest(
 	w http.ResponseWriter,
 	r *http.Request,
@@ -81,7 +83,26 @@ func (h *Handlers) processWebhookRequest(
 		return
 	}
 
-	headersJSON, err := json.Marshal(r.Header)
+	// Before anything is written. An unverified request must leave no
+	// event row, no delivery row and no delivery task behind, so this
+	// sits above every write rather than inside the transaction that
+	// performs them. It has to sit below the body read because the
+	// signature is computed over the body; readWebhookBody is what
+	// bounds that read, so an unauthenticated sender still cannot make
+	// the process hold more than the 1 MB cap.
+	if !h.verifyInboundSignature(w, entrypoint, r.Header, body) {
+		return
+	}
+
+	// These headers are about to be stored verbatim and handed to
+	// every delivery target, so the scheme's credential comes out
+	// first. Under GitLab's scheme the header is the shared secret
+	// itself, and leaving it in would hand the ability to forge
+	// signed requests to exactly the parties the signature is meant
+	// to exclude.
+	headersJSON, err := json.Marshal(
+		signature.SanitizeHeaders(&entrypoint, r.Header),
+	)
 	if err != nil {
 		h.serverError(w, "failed to serialize headers", err)
 
@@ -100,6 +121,63 @@ func (h *Handlers) processWebhookRequest(
 	)
 }
 
+// verifyInboundSignature authenticates the request against the
+// entrypoint's configured secret, reporting false once it has written
+// the response.
+//
+// An entrypoint with no secret configured is not checked and this
+// returns true, which is the unchanged behaviour every existing
+// entrypoint keeps.
+//
+// A configuration that cannot be applied — an unknown scheme, or one
+// half of the pair missing — is a 500, not a 401: the request may well
+// be authentic, and calling it unauthorized would tell a legitimate
+// sender to go fix its own signing. Either way it is refused. Failing
+// open here would mean an entrypoint the operator has protected
+// quietly accepting anything.
+func (h *Handlers) verifyInboundSignature(
+	w http.ResponseWriter,
+	entrypoint database.Entrypoint,
+	header http.Header,
+	body []byte,
+) bool {
+	err := signature.Verify(&entrypoint, header, body)
+	if err == nil {
+		return true
+	}
+
+	if errors.Is(err, signature.ErrConfig) {
+		h.log.Error(
+			"entrypoint signature configuration cannot be applied",
+			"entrypoint_id", entrypoint.ID,
+			"webhook_id", entrypoint.WebhookID,
+			"error", err,
+		)
+		http.Error(
+			w, "Internal server error",
+			http.StatusInternalServerError,
+		)
+
+		return false
+	}
+
+	// Every field here is bounded and none is client-chosen: the ids
+	// are ours, the scheme is one of a fixed set, and the error is a
+	// static string carrying no part of the secret or of what the
+	// client presented. Reaching this line also requires a real
+	// entrypoint UUID, so it is not a line a stranger can drive.
+	h.log.Warn(
+		"inbound signature verification failed",
+		"entrypoint_id", entrypoint.ID,
+		"webhook_id", entrypoint.WebhookID,
+		"scheme", string(entrypoint.SignatureScheme),
+		"error", err,
+	)
+	http.Error(w, "Unauthorized", http.StatusUnauthorized)
+
+	return false
+}
+
 // loadActiveTargets returns all active targets for a webhook.
 func (h *Handlers) loadActiveTargets(
 	webhookID string,