CVE-2026-111639 in bbs-go
Summary
by MITRE • 10/16/2026
bbs-go through 4.4.6 contains an authentication bypass vulnerability in the /api/login/google_one_tap endpoint that allows remote attackers to log in as other users by submitting Google ID tokens issued to any OAuth client. Attackers operating their own Google OAuth client can capture a victim's Google-signed ID token and replay it, since the aud claim is never checked, to obtain an authenticated session as the victim's Google-linked account.
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Analysis
by VulDB Data Team • 10/16/2026
The vulnerability identified in bbs-go versions prior to 4.4.6 represents a critical authentication bypass flaw located within the /api/login/google_one_tap endpoint. This security defect stems from improper validation of OpenID Connect ID tokens issued by Google OAuth services. Specifically, the application fails to verify the audience claim (aud) contained within these tokens. In standard OAuth and OIDC implementations, the aud claim serves as a crucial mechanism for ensuring that an identity token is intended for the specific relying party or service requesting authentication. By neglecting this validation step, bbs-go inadvertently accepts any valid Google-signed ID token regardless of which client originally requested it. This oversight fundamentally breaks the trust model required to establish secure user sessions via third-party providers.
The operational impact of this flaw allows remote attackers to perform account takeover attacks with significant ease. An attacker who controls their own malicious Google OAuth application can intercept or capture a victim's valid ID token, typically through phishing campaigns, cross-site scripting vulnerabilities on other sites using the same client ID, or by exploiting public clients that do not enforce strict redirect URI validations. Once obtained, this token is submitted to the vulnerable bbs-go endpoint. Because the server does not check whether the aud claim matches its own registered OAuth client identifier, it accepts the token as valid for the victim's account linked to Google. Consequently, the attacker gains full authenticated access to the victim's profile without needing the victim's password or multi-factor authentication codes associated with that specific platform session.
This vulnerability aligns closely with CWE-287, which describes Improper Authentication, and specifically relates to issues where identity tokens are not validated against their intended audience. From a threat modeling perspective using the MITRE ATT&CK framework, this behavior facilitates Account Manipulation techniques under Initial Access or Persistence vectors, depending on how long the attacker maintains access after exploiting the flaw. The lack of audience validation is a common misconfiguration in OAuth integrations where developers assume that any token signed by Google's private key is inherently trustworthy for their application, ignoring the necessity of scope and audience restrictions defined during client registration.
To mitigate this vulnerability, immediate upgrade to bbs-go version 4.4.6 or later is required as it addresses the missing validation logic. For systems still running affected versions, a temporary workaround involves implementing strict server-side verification of the aud claim in any custom middleware handling Google One Tap logins before session creation occurs. Security teams should also audit their OAuth client configurations to ensure that only necessary scopes are requested and that redirect URIs are strictly whitelisted to prevent token interception via malicious third-party applications. Regular security assessments focusing on identity provider integration points can help identify similar misconfigurations across other authentication endpoints within the application architecture.