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CVE-2026-92289 – OIDC PKCE Bypass / Authentication Bypass – Lemonldap::NG::Portal 2.23.0–2.23.3

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Summary

CVE-2026-92289 is an authentication bypass vulnerability in Lemonldap::NG::Portal, the identity/access management and web SSO portal component of the Lemonldap::NG suite. It allows an attacker to bypass Proof Key for Code Exchange (PKCE) protections and exchange an intercepted OAuth2/OpenID Connect authorization code for a victim’s tokens by replaying the client_id with an arbitrary secret. The issue is rated CRITICAL (CVSS 9.1) and is remotely exploitable without authentication or user interaction.

Technical details

  • Root cause: the checkEndPointAuthenticationCredentials function in lib/Lemonldap/NG/Portal/Lib/OpenIDConnect.pm / lib/Lemonldap/NG/Portal/Issuer/OpenIDConnect.pm does not verify the client secret for Relying Parties (RPs) marked as "public," yet still returns an authentication method as if credentials were validated.
  • Trigger condition: occurs when a public Relying Party’s RequirePKCE option is set to "PKCE or secret" mode. The authorization endpoint can issue a code even without a code_challenge, and the token endpoint’s validatePKCEChallenge() check passes trivially because neither a challenge nor a verifier is present.
  • Attack vector: network-based, low complexity, no privileges or user interaction required. An attacker who intercepts (or otherwise obtains) an authorization code issued to a public RP can replay the client_id together with an arbitrary/fabricated secret at the token endpoint to complete the code exchange.
  • Impact: successful exploitation lets an attacker obtain a legitimate user’s OIDC access, ID, and refresh tokens, enabling impersonation of the user or application and unauthorized access to protected resources and personal data. Confidentiality and integrity impacts are high; there is no availability impact.

Affected software

  • Lemonldap::NG::Portal (Perl/CPAN module, part of the Lemonldap::NG suite) — versions 2.23.0 through 2.23.3 (i.e., before 2.23.4)
  • A related, separately tracked issue (CVE-2026-92288) affects the same checkEndPointAuthenticationCredentials code path in additional version ranges; environments should confirm they are patched against both.

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: Upgrade Lemonldap::NG::Portal to version 2.23.4 or later, which fixes the credential/PKCE validation logic.
  • If patching is not immediately possible:
    • Enforce strict PKCE requirements ("PKCE required," not "PKCE or secret") on all public Relying Parties so token exchange cannot fall back to secret-based validation without a valid challenge/verifier pair.
    • Disable or restrict dynamic client registration for public clients where not strictly necessary.
    • Review OIDC/OAuth2 authorization and token endpoint logs for anomalous code-exchange requests referencing public client_ids.

References

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