From ddf109a683ca38439249cadcece10cace17ca812 Mon Sep 17 00:00:00 2001
From: OWConnoi <123777754+OWConnoi@users.noreply.github.com>
Date: Mon, 11 May 2026 22:57:24 +0100
Subject: [PATCH] Validate FLAGS_SECRET length for serialization

---
 packages/flags/src/lib/serialization.test.ts | 18 ++++++++++++++++++
 packages/flags/src/lib/serialization.ts      | 18 ++++++++++++++++--
 2 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 packages/flags/src/lib/serialization.test.ts

diff --git a/packages/flags/src/lib/serialization.test.ts b/packages/flags/src/lib/serialization.test.ts
new file mode 100644
index 00000000..3e091026
--- /dev/null
+++ b/packages/flags/src/lib/serialization.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest';
+import { deserialize, serialize } from './serialization';
+
+const invalidSecret = 'short';
+
+describe('serialization secret validation', () => {
+  it('rejects signing with a secret that is not 32 bytes', async () => {
+    await expect(
+      serialize({ feature: true }, [{ key: 'feature' }], invalidSecret),
+    ).rejects.toThrow('flags: Invalid secret');
+  });
+
+  it('rejects verification with a secret that is not 32 bytes', async () => {
+    await expect(
+      deserialize('invalid.code', [{ key: 'feature' }], invalidSecret),
+    ).rejects.toThrow('flags: Invalid secret');
+  });
+});
diff --git a/packages/flags/src/lib/serialization.ts b/packages/flags/src/lib/serialization.ts
index 8dc9ec78..b47b810a 100644
--- a/packages/flags/src/lib/serialization.ts
+++ b/packages/flags/src/lib/serialization.ts
@@ -3,6 +3,8 @@ import type { JsonValue } from '..';
 import type { FlagOption } from '../types';
 import { memoizeOne } from './async-memoize-one';
 
+const SECRET_BYTE_LENGTH = 32;
+
 // 252 max options length allows storing index  0 to 251,
 // so 252 is the first SPECIAL_INTEGER
 export const MAX_OPTION_LENGTH = 252;
@@ -15,9 +17,21 @@ enum SPECIAL_INTEGERS {
   UNLISTED_VALUE = 255,
 }
 
+function getSecretKey(secret: string): Uint8Array {
+  const encodedSecret = base64url.decode(secret);
+
+  if (encodedSecret.length !== SECRET_BYTE_LENGTH) {
+    throw new Error(
+      'flags: Invalid secret, it must be a 256-bit key (32 bytes)',
+    );
+  }
+
+  return encodedSecret;
+}
+
 const memoizedVerify = memoizeOne(
   (code: string, secret: string) =>
-    compactVerify(code, base64url.decode(secret), {
+    compactVerify(code, getSecretKey(secret), {
       algorithms: ['HS256'],
     }),
   (a, b) => a[0] === b[0] && a[1] === b[1], // only first two args matter
@@ -28,7 +42,7 @@ const memoizedSign = memoizeOne(
   (uint8Array: Uint8Array, secret) =>
     new CompactSign(uint8Array)
       .setProtectedHeader({ alg: 'HS256' })
-      .sign(base64url.decode(secret)),
+      .sign(getSecretKey(secret)),
   (a, b) =>
     // matchedIndices array must be equal
     a[0].length === b[0].length &&