Bug 2530888 (CVE-2026-87853)

Summary: CVE-2026-87853 sssd: sssd: IdP authentication prefix comparison allows cross-user impersonation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, security-response-team, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in SSSD's IdP authentication provider. The eval_access_token_buf() function compares the OIDC subject identifier using strncmp() with the authenticated user's identifier length, performing a prefix comparison instead of an exact match. An attacker whose IdP identifier is a strict prefix of a target user's identifier can authenticate as the target user.
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Bug Depends On: 2530989    
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Description OSIDB Bzimport 2026-09-09 13:13:03 UTC 
A flaw was found in SSSD's IdP authentication provider. The eval_access_token_buf() function in src/providers/idp/idp_auth_eval.c compares the authenticated user's OIDC subject identifier against the requested user's stored SYSDB_UUID using strncmp() with the authenticated identifier's length as the count. This performs a prefix comparison rather than an exact match. An attacker whose complete IdP identifier is a strict prefix of a target user's identifier can authenticate as the target, gaining full access to the target's account, files, groups, and local authorization. With Keycloak's built-in LDAP provider (Import Users disabled), the identifier format f:<federation_id>:<username> naturally produces prefix relationships for usernames like 'admin' and 'administrator'. No victim credential, interaction, or administrative role is required.
Comment 1 Jon Orris 2026-10-08 11:08:52 UTC 
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 10

Via RHSA-2026:77534 https://access.redhat.com/errata/RHSA-2026:77534