Bug 2540882 (CVE-2026-92289)

Summary:	CVE-2026-92289 lemonldap-ng: lemonldap-ng: Authentication bypass via improper PKCE verification
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	Keywords:	Security
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Hardware:	All
OS:	Linux
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Doc Text:	A flaw was found in lemonldap-ng. When configured to accept either client secrets or Proof Key for Code Exchange (PKCE, a security mechanism designed to protect authorization codes), the portal fails to validate client credentials properly for public clients. A remote attacker who intercepts an authorization code can exploit this flaw by submitting an arbitrary secret to the token endpoint. This allows the attacker to bypass PKCE validation and obtain the victim's access, identity, and refresh tokens, leading to unauthorized account access.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	2540976, 2540977
Bug Blocks:

Description OSIDB Bzimport 2026-09-25 01:01:33 UTC

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.