Fresh triage: this is a sign-in error-reporting bug, with no reported exploitable vulnerability. No duplicate found among 948 open issues using Antigravity, Google sign-in, redirect URL, and the reported error strings. Related account-verification reports fail after successful sign-in rather than at authenticate.
Source inspection of cached main b5a226e49c still shows the generic fallback in safeAuthFailure and callback validation returning before publishing auth state. This is not a runtime reproduction: no rejected Google account was supplied, and a fresh main fetch has not completed. The triage rules require maintainer handling for changes to authentication. Leaving open for Julius; no code changes or PR.