CVE-2026-87853 in Red Hat
Summary
by MITRE • 09/09/2026
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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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified in System Security Services Daemon (SSSD) represents a critical authentication bypass flaw within its Identity Provider integration module, specifically affecting the OpenID Connect protocol implementation. The core technical defect resides in the eval_access_token_buf function, which is responsible for validating the subject identifier contained within an OIDC access token against the local user database. Instead of performing a strict equality check to ensure that the identity presented by the authentication provider matches exactly with the intended target account, the system utilizes the strncmp() library function. This C standard library routine compares only the first n characters of two strings, where n is determined by the length of one of the input strings. In this specific implementation context, the comparison logic inadvertently allows a prefix match rather than requiring an exact string equivalence. Consequently, if an attacker controls or can forge an Identity Provider identifier that constitutes a strict prefix of a legitimate user's identifier, the validation routine will return true, granting access to the account associated with the longer identifier.
This flaw fundamentally undermines the integrity of single sign-on operations relying on SSSD for identity verification. In practical terms, consider a scenario where a valid enterprise user has an identifier such as [email protected] and an attacker can configure their Identity Provider to issue tokens containing johndoe or even just john as the subject claim. Because strncmp() will compare up to the length of the shorter string (the attacker's prefix) and find them identical, SSSD erroneously accepts this token as valid for the full account. This allows unauthorized individuals to impersonate high-privilege users without possessing their actual credentials or multi-factor authentication tokens, provided they can control the claims issued by a compromised or maliciously configured Identity Provider. The severity of this issue is amplified in environments where SSSD serves as the primary mechanism for authenticating against remote directory services like Active Directory or LDAP servers, potentially leading to widespread unauthorized access across networked systems.
From a threat modeling perspective, this vulnerability aligns with CWE-287 Improper Authentication and CWE-697 Incorrect Comparison During Authentication. The root cause is a classic off-by-one or logic error in string handling where the boundary conditions of the comparison are not strictly enforced to prevent partial matches. In terms of adversarial tactics, this exploitation technique corresponds to ATT&CK T1078 Valid Accounts, specifically the sub-technique involving impersonation through token manipulation or identity spoofing. Attackers can leverage this flaw during initial access phases by exploiting trusted relationships between organizations and their Identity Providers. If an organization uses a federated login system where users log in via external providers such as Google, Azure AD, or Okta, an attacker who compromises one of these providers or creates a malicious provider that issues tokens with prefix-matching identifiers can gain persistent foothold within the target infrastructure without triggering typical credential-based alerts.
The operational impact extends beyond simple unauthorized access to include potential data exfiltration and lateral movement within the network. Once authenticated as a legitimate user, an attacker gains all permissions associated with that account, including read/write access to sensitive files, ability to execute commands on managed endpoints if SSSD is integrated with host-based authentication systems like SSH or PAM, and visibility into internal resources restricted by role-based access controls. This can lead to significant confidentiality breaches and integrity violations across the enterprise environment. Furthermore, because this flaw relies on protocol-level validation logic rather than network transmission issues, it may evade standard intrusion detection signatures that focus on anomalous traffic patterns, making it a stealthy vector for persistent threats.
Mitigation strategies must address both immediate remediation and long-term architectural resilience. The primary solution is to apply the vendor-provided patch or update SSSD to a version where this logic error has been corrected by replacing strncmp() with an exact string comparison function such as strcmp(). Administrators should verify that their systems are running patched versions of SSSD immediately upon release of the fix. In addition to software updates, organizations should review their Identity Provider configurations and enforce strict validation rules for subject identifiers. Implementing additional layers of verification, such as requiring multi-factor authentication at the application layer rather than relying solely on identity provider assertions, can mitigate the risk even if this specific vulnerability is present. Security teams should also monitor logs for unusual login patterns or successful authentications from unexpected Identity Providers to detect potential exploitation attempts in real-time. Regular audits of user identifier uniqueness and length policies within directory services can further reduce the attack surface by minimizing the likelihood that an attacker's prefix matches a valid target identifier.