High · CVSS 7.5CVE-2026-87853

Sssd: sssd: idp authentication prefix comparison allows cross-user impersonationCVE-2026-87853

Answer in brief

CVE-2026-87853 records a High severity (CVSS 7.5) vulnerability in Sssd: sssd: idp authentication prefix comparison allows cross-user impersonation. The current sources do not mark it as known exploited. The current feed maps Red Hat/sssd (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 9, 2026Updated Oct 21, 2026Source checked Oct 25, 2026First seen by HOL Sep 9, 2026Material review Oct 21, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.5
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Not reported

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Red Hat/sssd (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
Red Hat/sssdgeneric	*	Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for Red Hat/sssd.
Review the reported fix. Source-reported workaround: Deployments not using the IdP authentication provider (idp_type = keycloak) are not affected. For affected deployments: (1) If using Keycloak LDAP federation, switching to Import Users: enabled mode causes Keycloak to assign random UUIDs as sub claims, making prefix collisions astronomically unlikely. (2) Restrict PAM login to specific users via simple_allow_users or access_provider = simple. (3) Disable Keycloak self-registration to prevent attackers from creating accounts with chosen prefix usernames.

Evidence timeline and material changes

Published upstream
Sep 9, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 21, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 9, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.

Package

Affected range

Fixed version

Red Hat/sssdgeneric

Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for Red Hat/sssd.

Review the reported fix. Source-reported workaround: Deployments not using the IdP authentication provider (idp_type = keycloak) are not affected. For affected deployments: (1) If using Keycloak LDAP federation, switching to Import Users: enabled mode causes Keycloak to assign random UUIDs as sub claims, making prefix collisions astronomically unlikely. (2) Restrict PAM login to specific users via simple_allow_users or access_provider = simple. (3) Disable Keycloak self-registration to prevent attackers from creating accounts with chosen prefix usernames.

Sources and claim methodology

Upstream source description

Quoted source text, attributed separately from HOL analysis.