Handle when aud OIDC claim is an Array (#4584)

* Handle when `aud` OIDC claim is an Array

The `aud` claim of OIDC id_tokens [can be an array](ce6d694639/src/Claims.ts (L92)) but the existing logic
incorrectly assumes `aud` is always a string.

This PR adds the necessary check.

* Clarify `aud` OIDC claim check

* Fix for prettier

---------

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

spec/unit/oidc/validate.spec.ts

@@ -170,6 +170,23 @@ describe("validateIdToken()", () => {
         expect(logger.error).toHaveBeenCalledWith("Invalid ID token", new Error("Invalid audience"));
     });
 
+    it("should not throw when audience is an array that includes clientId", () => {
+        mocked(jwtDecode).mockReturnValue({
+            ...validDecodedIdToken,
+            aud: [clientId],
+        });
+        expect(() => validateIdToken(idToken, issuer, clientId, nonce)).not.toThrow();
+    });
+
+    it("should throw when audience is an array that does not include clientId", () => {
+        mocked(jwtDecode).mockReturnValue({
+            ...validDecodedIdToken,
+            aud: [`${clientId},uiop`, "asdf"],
+        });
+        expect(() => validateIdToken(idToken, issuer, clientId, nonce)).toThrow(new Error(OidcError.InvalidIdToken));
+        expect(logger.error).toHaveBeenCalledWith("Invalid ID token", new Error("Invalid audience"));
+    });
+
     it("should throw when nonce does not match", () => {
         mocked(jwtDecode).mockReturnValue({
             ...validDecodedIdToken,

src/oidc/validate.ts

@@ -179,7 +179,8 @@ export const validateIdToken = (
          * The ID Token MUST be rejected if the ID Token does not list the Client as a valid audience, or if it contains additional audiences not trusted by the Client.
          * EW: Don't accept tokens with other untrusted audiences
          * */
-        if (claims.aud !== clientId) {
+        const sanitisedAuds = typeof claims.aud === "string" ? [claims.aud] : claims.aud;
+        if (!sanitisedAuds.includes(clientId)) {
             throw new Error("Invalid audience");
         }