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/database/migration_entrypoint_test.go

@@ -0,0 +1,85 @@
+package database_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/signature"
+)
+
+// TestEntrypointSignatureColumnsMigrateToUnconfigured pins the
+// upgrade path for a deployment that already has entrypoints.
+//
+// The signature columns arrive through GORM's AutoMigrate, so every
+// row written before they existed acquires them with no value. That
+// has to land on "not configured", because the alternative is an
+// upgrade that rejects the traffic the operator was already
+// receiving — a self-inflicted outage on a receiver whose senders
+// cannot be told to start signing.
+//
+// The legacy schema is reproduced by dropping the columns from a
+// migrated database and writing a row through the old shape, so the
+// row really predates them rather than merely being blank.
+func TestEntrypointSignatureColumnsMigrateToUnconfigured(t *testing.T) {
+	t.Parallel()
+
+	db, lc := setupTestDB(t)
+	lc.RequireStart()
+
+	t.Cleanup(lc.RequireStop)
+
+	for _, column := range []string{
+		"signature_scheme", "signature_secret",
+	} {
+		require.NoError(
+			t,
+			db.DB().Exec(
+				"ALTER TABLE entrypoints DROP COLUMN "+column,
+			).Error,
+			"dropping %s to reproduce the pre-upgrade schema",
+			column,
+		)
+	}
+
+	const legacyID = "legacy-entrypoint"
+
+	require.NoError(
+		t,
+		db.DB().Exec(
+			`INSERT INTO entrypoints
+			 (id, created_at, updated_at, webhook_id, path,
+			  description, active)
+			 VALUES (?, ?, ?, ?, ?, ?, ?)`,
+			legacyID, "2026-01-01 00:00:00", "2026-01-01 00:00:00",
+			"legacy-webhook", "legacy-path", "predates signatures",
+			true,
+		).Error,
+	)
+
+	// The upgrade.
+	require.NoError(t, db.Migrate())
+
+	var ep database.Entrypoint
+
+	require.NoError(
+		t,
+		db.DB().Where("id = ?", legacyID).First(&ep).Error,
+		"the migrated row must still load; a NULL landing in a "+
+			"string column would fail here",
+	)
+
+	assert.Equal(t, database.SignatureSchemeNone, ep.SignatureScheme)
+	assert.Empty(t, ep.SignatureSecret)
+	assert.False(t, ep.SignatureConfigured())
+	assert.True(t, ep.Active, "the row's other columns survive")
+
+	// The behaviour that actually matters: an unsigned request to
+	// this entrypoint is still accepted.
+	assert.NoError(
+		t,
+		signature.Verify(&ep, http.Header{}, []byte(`{"a":1}`)),
+	)
+}

internal/database/model_entrypoint.go

@@ -1,5 +1,22 @@
 package database
 
+// SignatureScheme names the way an entrypoint authenticates inbound
+// requests. A scheme fixes both the header the signature arrives in
+// and the algorithm used to check it, so an operator cannot pair one
+// sender's header with another sender's comparison.
+type SignatureScheme string
+
+// Signature scheme values. The empty scheme means the entrypoint
+// performs no inbound verification: it is the default, and it is the
+// state every entrypoint created before this column existed migrates
+// to, so an existing deployment keeps accepting the requests it
+// accepted before.
+const (
+	SignatureSchemeNone   SignatureScheme = ""
+	SignatureSchemeGitHub SignatureScheme = "github"
+	SignatureSchemeGitLab SignatureScheme = "gitlab"
+)
+
 // Entrypoint represents an inbound URL endpoint that feeds into a webhook
 type Entrypoint struct {
 	BaseModel
@@ -12,6 +29,43 @@ type Entrypoint struct {
 	Description string `json:"description"`
 	Active      bool   `gorm:"default:true" json:"active"`
 
+	// SignatureScheme selects how inbound requests to this
+	// entrypoint are authenticated. Empty means unauthenticated,
+	// which is what a UUID-only entrypoint has always been.
+	SignatureScheme SignatureScheme `gorm:"default:''" json:"signatureScheme"`
+
+	// SignatureSecret is the secret shared with the sender.
+	//
+	// It is stored in the clear because HMAC verification needs the
+	// key itself: a hash of it cannot recompute the sender's digest.
+	// It is therefore a live credential, and json:"-" keeps it out of
+	// any handler that marshals the model, the way APIKey.Key and
+	// Target.Config are kept out. handlers.EntrypointView is the
+	// matching barrier for the HTML path.
+	SignatureSecret string `gorm:"default:''" json:"-"`
+
 	// Relations
 	Webhook Webhook `json:"webhook,omitzero"`
 }
+
+// SignatureConfigured reports whether this entrypoint verifies
+// inbound requests. Both halves must be present: a scheme without a
+// secret, or a secret without a scheme, is a broken configuration
+// rather than a configured one, and signature.Verify fails those
+// closed rather than treating them as "off".
+func (e *Entrypoint) SignatureConfigured() bool {
+	return e.SignatureScheme != SignatureSchemeNone &&
+		e.SignatureSecret != ""
+}
+
+// SignatureHalfConfigured reports whether exactly one half of the
+// scheme/secret pair is present. The receiver refuses such a row on
+// every request, so the UI must not describe it as unverified. It
+// reports the state without exposing the secret, which is why it
+// lives here rather than in the display projection.
+func (e *Entrypoint) SignatureHalfConfigured() bool {
+	hasScheme := e.SignatureScheme != SignatureSchemeNone
+	hasSecret := e.SignatureSecret != ""
+
+	return hasScheme != hasSecret
+}

internal/database/model_secrets_test.go

@@ -34,6 +34,8 @@ func marshalModel(t *testing.T, v any) string {
 //   - APIKey.Key is a bearer token outright.
 //   - Setting.Value holds the session encryption key.
 //   - User.Password holds the Argon2 hash, and was already tagged.
+//   - Entrypoint.SignatureSecret is the secret its senders sign with,
+//     stored in the clear because HMAC verification needs the key.
 func TestModelsDoNotMarshalTheirSecrets(t *testing.T) {
 	t.Parallel()
 
@@ -72,6 +74,14 @@ func TestModelsDoNotMarshalTheirSecrets(t *testing.T) {
 				Password: marker,
 			},
 		},
+		{
+			name: "entrypoint signature secret",
+			model: database.Entrypoint{
+				Description:     keptField,
+				SignatureScheme: database.SignatureSchemeGitHub,
+				SignatureSecret: marker,
+			},
+		},
 	}
 
 	for _, tc := range cases {
@@ -105,3 +115,24 @@ func TestWebhookMarshalsNoTargetConfig(t *testing.T) {
 	assert.NotContains(t, encoded, marker)
 	assert.Contains(t, encoded, keptField)
 }
+
+// TestWebhookMarshalsNoEntrypointSecret covers the same nested case
+// for the entrypoint's inbound signature secret, which reaches a
+// marshalled webhook through the Entrypoints association.
+func TestWebhookMarshalsNoEntrypointSecret(t *testing.T) {
+	t.Parallel()
+
+	const marker = "QQENTRYPOINTMARKERQQ"
+
+	encoded := marshalModel(t, database.Webhook{
+		Name: keptField,
+		Entrypoints: []database.Entrypoint{{
+			Path:            "some-uuid",
+			SignatureScheme: database.SignatureSchemeGitLab,
+			SignatureSecret: marker,
+		}},
+	})
+
+	assert.NotContains(t, encoded, marker)
+	assert.Contains(t, encoded, keptField)
+}