CVE-2026-92289 in LemonLDAP::NG
Summary
by MITRE • 09/25/2026
Lemonldap::NG::Portal versions from 2.23.0 before 2.23.4 for Perl allow a PKCE bypass for public Relying Parties in "PKCE or secret" mode because checkEndPointAuthenticationCredentials does not verify the client secret.
With oidcRPMetaDataOptionsRequirePKCE set to 2, the authorization endpoint issues a code even when the request carries no code_challenge, and token() admits the exchange as long as a challenge was stored or an authentication method was returned for the caller. checkEndPointAuthenticationCredentials() skips the secret comparison for a Relying Party marked public and still returns the method deduced from the request, so any Basic or form credential satisfies the secret branch. validatePKCEChallenge() then passes, because neither a challenge nor a verifier is present.
An attacker who intercepts an authorization code issued to a public Relying Party can exchange it for the user's access, ID and refresh tokens by replaying the client_id with an arbitrary secret, which is the attack PKCE prevents. Dynamic client registration creates every Relying Party in this mode.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability identified in Lemonldap::NG::Portal versions prior to 2.23.4 represents a critical failure in the implementation of OAuth 2.0 and OpenID Connect security controls, specifically concerning Public Key Code Exchange (PKCE) validation for public clients. This flaw allows an attacker to bypass PKCE protections when the authorization server is configured with oidcRPMetaDataOptionsRequirePKCE set to two, a mode intended to enforce PKCE even for public relying parties that typically do not possess client secrets. The core technical deficiency lies within the checkEndPointAuthenticationCredentials function, which fails to properly verify the client secret or validate the presence of required security parameters during the token exchange phase. In this specific configuration, the authorization endpoint issues an authorization code without requiring a code_challenge in the initial request, and subsequently accepts the token exchange as long as some form of challenge storage exists or authentication method is returned for the caller. This logic error effectively nullifies the primary defense mechanism designed to prevent authorization code interception attacks against public clients.
From a technical perspective, the exploitation path begins when an attacker intercepts an authorization code intended for a public relying party. Because the system marks these parties as public, checkEndPointAuthenticationCredentials skips the secret comparison step and instead returns the authentication method deduced from the request context. Consequently, any Basic or form credential satisfies the validation branch that is supposed to enforce PKCE integrity. The subsequent validatePKCEChallenge function then passes its verification because neither a code_challenge nor a code_verifier is present in the token request. This allows an attacker to replay the stolen authorization code along with an arbitrary client_id and secret, successfully exchanging it for access tokens, ID tokens, and refresh tokens. Since dynamic client registration automatically creates relying parties in this vulnerable mode by default, the attack surface is significantly broadened, affecting a large portion of deployments that rely on automatic configuration rather than manual hardening.
The operational impact of this vulnerability is severe, as it directly compromises the confidentiality and integrity of user sessions protected by Lemonldap::NG::Portal. An attacker who successfully exploits this flaw can impersonate legitimate users or applications, gaining unauthorized access to sensitive resources and personal data associated with the compromised accounts. This bypass undermines the fundamental security guarantees provided by PKCE, which is specifically designed to mitigate authorization code interception attacks in public client scenarios where storing a secret securely is not feasible. The ability to exchange intercepted codes for valid tokens means that even if an attacker only has partial visibility into the OAuth flow, they can fully authenticate as the victim or application, leading to potential data breaches, unauthorized transactions, and further lateral movement within protected environments.
This vulnerability maps directly to CWE-287 Improper Authentication, specifically regarding the failure to verify credentials during authentication processes, and CWE-345 Insufficient Verification of Data Authenticity, as the system fails to validate that the token request corresponds to a legitimate PKCE flow initiated by the client. In terms of MITRE ATT&CK framework alignment, this flaw facilitates techniques associated with T1078 Valid Accounts, where attackers use stolen or forged credentials to gain access, and potentially T1528 Steal Application Access Token if the attacker leverages the obtained tokens for further exploitation. The lack of proper PKCE enforcement also relates to CWE-346 Origin Validation Error in some contexts, as the system incorrectly trusts requests that do not meet the expected security parameters for public clients.
To mitigate this vulnerability, organizations must immediately upgrade Lemonldap::NG::Portal to version 2.23.4 or later, where the checkEndPointAuthenticationCredentials function has been corrected to properly enforce PKCE requirements even in modes requiring it for public clients. Administrators should also review their OIDC configurations to ensure that oidcRPMetaDataOptionsRequirePKCE is set appropriately and consider enforcing strict validation of code_challenge and code_verifier parameters during token exchange regardless of client type. Implementing additional monitoring for unusual patterns in authorization code usage, such as rapid exchanges or requests from unexpected IP addresses, can provide early detection indicators while patching efforts are underway. Furthermore, reviewing dynamic client registration policies to ensure that newly created relying parties adhere to strict security standards will help prevent the creation of vulnerable configurations by default.