From 311da56779cb26f544a1c17b758542a3ad439a6a Mon Sep 17 00:00:00 2001
From: rissrice2105-agent
 <289161642+rissrice2105-agent@users.noreply.github.com>
Date: Fri, 17 Jul 2026 15:42:52 -0600
Subject: [PATCH] fix(auth): compare webhook secrets by byte length

---
 src/app/api/auth/backfill-dids/route.ts  | 12 +++++-------
 src/app/api/auth/confirmed/route.test.ts |  5 +++++
 src/app/api/auth/confirmed/route.ts      | 12 +++++-------
 src/lib/security/constant-time.test.ts   | 13 +++++++++++++
 src/lib/security/constant-time.ts        |  7 +++++++
 5 files changed, 35 insertions(+), 14 deletions(-)
 create mode 100644 src/lib/security/constant-time.test.ts
 create mode 100644 src/lib/security/constant-time.ts

diff --git a/src/app/api/auth/backfill-dids/route.ts b/src/app/api/auth/backfill-dids/route.ts
index 589d4574..3d11b7cb 100644
--- a/src/app/api/auth/backfill-dids/route.ts
+++ b/src/app/api/auth/backfill-dids/route.ts
@@ -6,14 +6,9 @@
  */
 
 import { NextRequest, NextResponse } from "next/server";
-import { timingSafeEqual } from "crypto";
 import { createServiceClient } from "@/lib/supabase/service";
 import { generateAndStoreDid } from "@/lib/auth/did";
-
-function safeCompare(a: string, b: string): boolean {
-  if (a.length !== b.length) return false;
-  return timingSafeEqual(Buffer.from(a), Buffer.from(b));
-}
+import { timingSafeStringEqual } from "@/lib/security/constant-time";
 
 export async function POST(request: NextRequest) {
   const authHeader = request.headers.get("authorization") || "";
@@ -24,7 +19,10 @@ export async function POST(request: NextRequest) {
     return NextResponse.json({ error: "Server misconfiguration" }, { status: 500 });
   }
 
-  if (!safeCompare(authHeader, `Bearer ${secret}`) && !safeCompare(authHeader, secret)) {
+  if (
+    !timingSafeStringEqual(authHeader, `Bearer ${secret}`) &&
+    !timingSafeStringEqual(authHeader, secret)
+  ) {
     return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
   }
 
diff --git a/src/app/api/auth/confirmed/route.test.ts b/src/app/api/auth/confirmed/route.test.ts
index 3f438fce..c7dae08d 100644
--- a/src/app/api/auth/confirmed/route.test.ts
+++ b/src/app/api/auth/confirmed/route.test.ts
@@ -162,6 +162,11 @@ describe("POST /api/auth/confirmed", () => {
       expect(res.status).toBe(401);
     });
 
+    it("rejects Unicode secrets with mismatched byte lengths", async () => {
+      const res = await POST(makeRequest(confirmationPayload(), "éest-webhook-secret"));
+      expect(res.status).toBe(401);
+    });
+
     it("returns 500 when AUTH_WEBHOOK_SECRET is not configured", async () => {
       delete process.env.AUTH_WEBHOOK_SECRET;
       const res = await POST(makeRequest(confirmationPayload(), null));
diff --git a/src/app/api/auth/confirmed/route.ts b/src/app/api/auth/confirmed/route.ts
index 4e94aa66..89b9a10f 100644
--- a/src/app/api/auth/confirmed/route.ts
+++ b/src/app/api/auth/confirmed/route.ts
@@ -13,13 +13,13 @@
  */
 
 import { NextRequest, NextResponse } from "next/server";
-import { timingSafeEqual } from "crypto";
 import { createClient } from "@/lib/supabase/server";
 import { createServiceClient } from "@/lib/supabase/service";
 import { sendEmail, welcomeEmail } from "@/lib/email";
 import { generateAndStoreDid } from "@/lib/auth/did";
 import { createUserLnWallet } from "@/lib/lightning/create-wallet";
 import { safeParseBody } from "@/lib/sanitize";
+import { timingSafeStringEqual } from "@/lib/security/constant-time";
 
 type AuthWebhookPayload = {
   type?: string;
@@ -33,11 +33,6 @@ type AuthWebhookPayload = {
   };
 };
 
-function safeCompare(a: string, b: string): boolean {
-  if (a.length !== b.length) return false;
-  return timingSafeEqual(Buffer.from(a), Buffer.from(b));
-}
-
 export async function POST(request: NextRequest) {
   try {
     // Verify the webhook secret to prevent unauthorized calls
@@ -49,7 +44,10 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: "Server misconfiguration" }, { status: 500 });
     }
 
-    if (!safeCompare(authHeader, `Bearer ${webhookSecret}`) && !safeCompare(authHeader, webhookSecret)) {
+    if (
+      !timingSafeStringEqual(authHeader, `Bearer ${webhookSecret}`) &&
+      !timingSafeStringEqual(authHeader, webhookSecret)
+    ) {
       return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
     }
 
diff --git a/src/lib/security/constant-time.test.ts b/src/lib/security/constant-time.test.ts
new file mode 100644
index 00000000..4cf31f16
--- /dev/null
+++ b/src/lib/security/constant-time.test.ts
@@ -0,0 +1,13 @@
+import { describe, expect, it } from "vitest";
+import { timingSafeStringEqual } from "./constant-time";
+
+describe("timingSafeStringEqual", () => {
+  it("compares equal and unequal ASCII strings", () => {
+    expect(timingSafeStringEqual("secret", "secret")).toBe(true);
+    expect(timingSafeStringEqual("secret", "public")).toBe(false);
+  });
+
+  it("rejects strings with equal character length but unequal byte length", () => {
+    expect(timingSafeStringEqual("é", "a")).toBe(false);
+  });
+});
diff --git a/src/lib/security/constant-time.ts b/src/lib/security/constant-time.ts
new file mode 100644
index 00000000..5b55f81c
--- /dev/null
+++ b/src/lib/security/constant-time.ts
@@ -0,0 +1,7 @@
+import { timingSafeEqual } from "crypto";
+
+export function timingSafeStringEqual(a: string, b: string): boolean {
+  const aBytes = Buffer.from(a);
+  const bBytes = Buffer.from(b);
+  return aBytes.length === bBytes.length && timingSafeEqual(aBytes, bBytes);
+}