Fix crash when EMM emitted non-string errors

CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- Fix a crash when a server error response carries a non-string value under its `error` key. The EMM error handler sent `-hasPrefix:` to whatever value was present, raising an unrecognized selector exception on a number, array or object.
+
 # 10.0.0
 - **BREAKING**: Update to AppAuth 3.0.0 and GTMAppAuth 6.0.0, which raises the minimum deployment targets to iOS 15.0 and macOS 12.0, widens the `GTMSessionFetcher` dependency to allow 4.x and 5.x, and renames the version-specific Swift Package Manager manifest to `Package@swift-5.7.swift`. Projects that must keep supporting earlier OS versions should stay on GoogleSignIn 9.2.0. ([#628](https://github.com/google/GoogleSignIn-iOS/pull/628))
 - Add `GIDSignIn.wrapperIdentifier` so SDKs that embed Google Sign-In can self-identify in Google's diagnostic logs via a new `gidwrapper` parameter. It is opt-in and pre-existing behavior is unchanged. ([#625](https://github.com/google/GoogleSignIn-iOS/pull/625))

GoogleSignIn/Sources/GIDEMMErrorHandler.m

@@ -64,23 +64,25 @@ - (BOOL)handleErrorFromResponse:(NSDictionary<NSString *, id> *)response
     if (!_pendingDialog && [UIAlertController class] &&
         [response isKindOfClass:[NSDictionary class]]) {
       id errorValue = response[kErrorKey];
-      if ([errorValue isEqual:kScreenlockRequiredError]) {
-        errorCode = ErrorCodeScreenlockRequired;
-      } else if ([errorValue hasPrefix:kAppVerificationRequiredErrorPrefix]) {
-        errorCode = ErrorCodeAppVerificationRequired;
-        NSString *appVerificationString =
-            [errorValue substringFromIndex:kAppVerificationRequiredErrorPrefix.length];
-        if ([appVerificationString hasPrefix:kErrorPayloadSeparator]) {
-          appVerificationString =
-              [appVerificationString substringFromIndex:kErrorPayloadSeparator.length];
+      if ([errorValue isKindOfClass:[NSString class]]) {
+        if ([errorValue isEqual:kScreenlockRequiredError]) {
+          errorCode = ErrorCodeScreenlockRequired;
+        } else if ([errorValue hasPrefix:kAppVerificationRequiredErrorPrefix]) {
+          errorCode = ErrorCodeAppVerificationRequired;
+          NSString *appVerificationString =
+              [errorValue substringFromIndex:kAppVerificationRequiredErrorPrefix.length];
+          if ([appVerificationString hasPrefix:kErrorPayloadSeparator]) {
+            appVerificationString =
+                [appVerificationString substringFromIndex:kErrorPayloadSeparator.length];
+          }
+          appVerificationString = [appVerificationString
+              stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
+          if (appVerificationString.length) {
+            appVerificationURL = [NSURL URLWithString:appVerificationString];
+          }
+        } else if ([errorValue hasPrefix:kGeneralErrorPrefix]) {
+          errorCode = ErrorCodeDeviceNotCompliant;
         }
-        appVerificationString = [appVerificationString
-            stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
-        if (appVerificationString.length) {
-          appVerificationURL = [NSURL URLWithString:appVerificationString];
-        }
-      } else if ([errorValue hasPrefix:kGeneralErrorPrefix]) {
-        errorCode = ErrorCodeDeviceNotCompliant;
       }
       if (errorCode) {
         _pendingDialog = YES;

GoogleSignIn/Tests/Unit/GIDEMMErrorHandlerTest.m

@@ -120,6 +120,24 @@ - (void)testNoError {
   XCTAssertNil(_presentedViewController);
 }
 
+// Verifies that a non-string value under the `error` key is ignored rather than crashing.
+// The value comes straight from a server JSON response and is typed `id`, so it can be any
+// plist type. `-hasPrefix:` is an `NSString` method, so before the type check was added it
+// raised an unrecognized selector exception on a number, array or dictionary.
+- (void)testNonStringErrorValue {
+  NSArray *nonStringValues = @[ @123, @[ @"emm_passcode_required" ], @{ @"a" : @"b" } ];
+  for (id nonStringValue in nonStringValues) {
+    __block BOOL completionCalled = NO;
+    NSDictionary<NSString *, id> *response = @{ @"error" : nonStringValue };
+    BOOL result = [[GIDEMMErrorHandler sharedInstance] handleErrorFromResponse:response
+                                                                    completion:^() {
+      completionCalled = YES;
+    }];
+    XCTAssertFalse(result);
+    XCTAssertTrue(completionCalled);
+  }
+}
+
 // Verifies that the handler doesn't handle non-EMM error.
 - (void)testNoEMMError {
   __block BOOL completionCalled = NO;