Allow Advanced Permissions metadata in signtypeddata payload (MetaMask#8603)

## Explanation

MetaMask#8526 adds tighter validation to
signtypeddata v4 payloads, to ensure that no extraneous properties are
added.

This additional validation disallows Advanced Permissions `metadata`
which is used to communicate the origin and justification of the
permission.

This change loosens the validation just enough to allow `metadata: {
justification: string; origin: string }` as a property on the payload
that is not used within the message encoding.

## References

MetaMask#8526
MetaMask/metamask-extension#42181


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## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches security-adjacent request validation for typed-data signing;
while the new `metadata` allowance is tightly constrained, any loosening
here could affect input filtering behavior.
> 
> **Overview**
> Relaxes `signTypedData` (V4) payload validation to permit an
additional top-level `metadata` field used by Advanced Permissions.
> 
> `validateTypedMessageKeys` now explicitly allows `metadata` and
enforces it is exactly `{ justification: string, origin: string }`
(rejecting non-objects, missing/typed fields, or extra keys), with new
unit tests covering the allowed and rejected cases; changelog updated
accordingly.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
17f9432. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: jeffsmale90

packages/eth-json-rpc-middleware/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Allow Advanced Permissions `metadata` in signTypedData V4 requests ([#8603](https://github.com/MetaMask/core/pull/8603))
+
 ## [23.1.2]
 
 ### Changed

packages/eth-json-rpc-middleware/src/utils/validation.test.ts

@@ -160,5 +160,122 @@ describe('Validation Utils', () => {
 
       expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
     });
+
+    describe('metadata', () => {
+      const baseTypedData = {
+        types: {
+          EIP712Domain: [{ name: 'name', type: 'string' }],
+        },
+        primaryType: 'EIP712Domain',
+        domain: {},
+        message: {},
+      };
+
+      it('does not throw when metadata has exactly justification and origin as strings', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {
+            justification: 'Permission to spend tokens',
+            origin: 'https://example.com',
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).not.toThrow();
+      });
+
+      it('does not throw when metadata is the only top-level key', () => {
+        const data = JSON.stringify({
+          metadata: {
+            justification: 'Permission to spend tokens',
+            origin: 'https://example.com',
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).not.toThrow();
+      });
+
+      it.each([
+        ['null', null],
+        ['a string', 'not-an-object'],
+        ['a number', 42],
+        ['a boolean', true],
+        ['an array', ['justification', 'origin']],
+      ])('throws when metadata is %s', (_label, value) => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: value,
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+
+      it('throws when metadata is missing justification', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {
+            origin: 'https://example.com',
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+
+      it('throws when metadata is missing origin', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {
+            justification: 'Permission to spend tokens',
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+
+      it('throws when metadata.justification is not a string', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {
+            justification: 123,
+            origin: 'https://example.com',
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+
+      it('throws when metadata.origin is not a string', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {
+            justification: 'Permission to spend tokens',
+            origin: 123,
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+
+      it('throws when metadata has an extraneous third key', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {
+            justification: 'Permission to spend tokens',
+            origin: 'https://example.com',
+            extra: 'unexpected',
+          },
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+
+      it('throws when metadata is an empty object', () => {
+        const data = JSON.stringify({
+          ...baseTypedData,
+          metadata: {},
+        });
+
+        expect(() => validateTypedMessageKeys(data)).toThrow('Invalid input.');
+      });
+    });
   });
 });

packages/eth-json-rpc-middleware/src/utils/validation.ts

@@ -199,12 +199,38 @@ export function validateTypedDataForPrototypePollution(data: string): void {
  */
 export function validateTypedMessageKeys(data: string): void {
   const parsedData = parseTypedMessage(data);
-  const allowedKeys = new Set(Object.keys(TYPED_MESSAGE_SCHEMA.properties));
+  const allowedKeys = new Set([
+    ...Object.keys(TYPED_MESSAGE_SCHEMA.properties),
+    'metadata',
+  ]);
   const hasExtraneousKey = Object.keys(parsedData).some(
     (key) => !allowedKeys.has(key),
   );
 
   if (hasExtraneousKey) {
     throw rpcErrors.invalidInput();
   }
+
+  // Advanced Permissions adds `metadata: { justification: string, origin: string }` to eth_signTypedData requests.
+  // see GatorPermissionsController.decodePermissionFromPermissionContextForOrigin for more details.
+  const { metadata } = parsedData as { metadata?: unknown };
+  if (metadata !== undefined) {
+    if (typeof metadata !== 'object' || metadata === null) {
+      throw rpcErrors.invalidInput();
+    }
+
+    const { justification, origin } = metadata as {
+      justification?: unknown;
+      origin?: unknown;
+    };
+
+    if (typeof justification !== 'string' || typeof origin !== 'string') {
+      throw rpcErrors.invalidInput();
+    }
+
+    // we only need to check the keys length, because we already checked the known keys (justification and origin).
+    if (Object.keys(metadata).length !== 2) {
+      throw rpcErrors.invalidInput();
+    }
+  }
 }