Bug 2540880 (CVE-2026-92288)

Summary: CVE-2026-92288 lemonldap-ng: lemonldap-ng: Information disclosure via unauthenticated OAuth 2.0 token introspection
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in lemonldap-ng. The OAuth 2.0 (Open Authorization) token introspection endpoint fails to properly verify client credentials when handling requests for public relying parties. A remote attacker possessing an access token and a public client identifier can exploit this vulnerability to perform unauthorized token introspection, resulting in information disclosure. This exposure allows an attacker to inspect token metadata, confirm token activity, and map user identity claims across different relying parties.
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Bug Depends On: 2540974, 2540975    
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Description OSIDB Bzimport 2026-09-25 01:01:19 UTC 
Lemonldap::NG::Portal versions from 2.20.0 before 2.21.6, from 2.22.0 before 2.23.4 for Perl allow unauthenticated OAuth2 token introspection because checkEndPointAuthenticationCredentials does not verify the client secret of a public Relying Party.

checkEndPointAuthenticationCredentials() skips the secret comparison for a Relying Party marked public and still returns the authentication method deduced from the request, client_secret_basic or client_secret_post. introspection() rejects a caller only when that method is missing or none, so a request carrying a public client_id and an arbitrary or empty secret passes the endpoint's authentication check.

An attacker who holds an access token and knows the client_id of any public Relying Party can confirm the token is active and read its metadata, including scope, audience, expiry and the sub claim. The sub claim is computed with the calling Relying Party's user identifier attribute, so an attacker can translate a user identifier from one Relying Party to another, defeating per-client and pseudonymous identifiers.