CVE-2026-87853

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SSSD IdP Authentication Provider Prefix Comparison Flaw

Publication date: 2026-09-09

Last updated on: 2026-09-10

Assigner: redhat-SADP

Description

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.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-09-09

Last Modified

2026-09-10

Generated

2026-09-30

AI Q&A

2026-09-10

EPSS Evaluated

2026-09-29

NVD

CVE-2026-87853

EUVD

EUVD-2026-75084

Affected Vendors & Products

Vendor	Product	Version / Range
redhat	sssd	*
red_hat	sssd	to 2.11.0 (inc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-187	The product performs a comparison that only examines a portion of a factor before determining whether there is a match, such as a substring, leading to resultant weaknesses.

Attack-Flow Graph

Executive Summary

This vulnerability is in SSSD's Identity Provider (IdP) authentication provider. The eval_access_token_buf() function incorrectly uses strncmp() to compare OIDC subject identifiers, performing a prefix match instead of an exact match. An attacker with an IdP identifier that is a strict prefix of a target user's identifier can authenticate as that user.

Detection Guidance

Check SSSD logs for authentication attempts with prefix identifiers. Review Keycloak user federation settings for 'Import Users = false' mode. Inspect SSSD configuration files for 'id_provider = idp' and 'auth_provider = idp' settings.

Impact Analysis

An attacker can impersonate another user, gaining unauthorized access to the target's account, files, groups, and local authorization. Exploitation requires no special privileges, only an ordinary IdP user account with a prefix identifier.

Compliance Impact

This vulnerability allows unauthorized user impersonation, which could lead to unauthorized access to sensitive data. For GDPR, this may result in violations of data protection principles (Article 5) and potential data breaches requiring notification under Article 33. HIPAA compliance could be impacted if protected health information is accessed without authorization, violating the Security Rule's access controls (45 CFR 164.312(a)(1)).

Mitigation Strategies

Switch Keycloak LDAP User Federation to 'Import Users = true' mode. Restrict PAM logins to trusted users. Disable Keycloak self-registration. Update SSSD to a patched version if available.

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