CVE-2026-104038: SSSD Certificate Processing DoS Vulnerability

CVE-2026-104038 Overview

CVE-2026-104038 is a denial-of-service vulnerability in the System Security Services Daemon (sssd). A remote attacker can crash the service by submitting a certificate that lacks an expected Security Identifier (SID) extension. The flaw affects deployments configured with SID-based certificate mapping rules. The service fails to verify the presence of the extension before processing it, causing the process to crash during authentication or lookup operations. The weakness is classified as [CWE-476] NULL Pointer Dereference.

Critical Impact
Remote unauthenticated attackers can crash sssd on hosts using SID-based certificate mapping, disrupting authentication and identity lookups across affected Linux systems.

Affected Products

sssd (System Security Services Daemon) builds with SID-based certificate mapping enabled
Red Hat Enterprise Linux distributions shipping the affected sssd package
Downstream Linux distributions bundling the vulnerable sssd component

Discovery Timeline

2026-10-06 - CVE-2026-104038 published to NVD
2026-10-07 - Last updated in NVD database

Technical Details for CVE-2026-104038

Vulnerability Analysis

The vulnerability resides in the certificate-processing logic of sssd when SID-based certificate mapping rules are configured. When sssd evaluates a certificate against a mapping rule that expects a Security Identifier extension, it dereferences the extension data without first confirming the extension exists. A certificate missing this extension yields a null reference, and the process terminates.

The attack requires no authentication or user interaction and is reachable over the network wherever sssd processes externally supplied certificates. Successful exploitation produces a service crash, interrupting identity, authentication, and directory lookup operations for all consumers of the affected daemon. Confidentiality and integrity are not impacted; only availability is affected.

The EPSS score for this issue is approximately 0.22%, reflecting a low current probability of observed exploitation. Attack complexity is elevated because exploitation depends on the target configuration using SID-based mapping rules.

Root Cause

The root cause is a missing presence check on the SID certificate extension prior to dereferencing it. The code path assumes the extension is present whenever a SID mapping rule is active and does not validate the extension's existence before accessing its fields.

Attack Vector

An attacker triggers the flaw by presenting a crafted X.509 certificate that omits the SID extension to any sssd-serviced authentication or lookup path. Because sssd runs as a privileged system service that mediates PAM, NSS, and SSH authentication on many Linux hosts, a single malformed certificate submission can halt identity services for the entire host. Repeated submissions can be used to sustain the denial-of-service condition after service restart. See the Red Hat CVE-2026-104038 Advisory and Red Hat Bug Report #2478980 for additional technical detail.

Detection Methods for CVE-2026-104038

Indicators of Compromise

Unexpected sssd process crashes or restarts recorded in systemd journal entries referencing certificate processing.
Core dumps from sssd child processes (sssd_be, sssd_pam, sssd_nss) following external certificate submission.
Authentication failures or NSS lookup timeouts correlated with inbound certificate-based authentication attempts.

Detection Strategies

Monitor /var/log/sssd/ and journalctl -u sssd for repeated service terminations and segmentation faults.
Alert on systemd service restart loops for the sssd unit, which indicate crash-restart cycles consistent with this flaw.
Correlate authentication logs with inbound certificate presentations from untrusted sources to identify probing activity.

Monitoring Recommendations

Enable core dump collection for sssd and forward crash telemetry to a centralized log pipeline.
Track process-lifetime metrics for the sssd service and generate alerts when restart counts exceed baseline thresholds.
Audit active certificate mapping rules in sssd.conf to identify hosts configured with SID-based matching and prioritize them for monitoring.

How to Mitigate CVE-2026-104038

Immediate Actions Required

Apply vendor-supplied sssd updates as soon as patched packages become available from your Linux distribution.
Inventory hosts that use SID-based certificate mapping rules and prioritize them for patching.
Restrict network paths through which untrusted certificates can reach sssd, including PAM, SSH, and smartcard authentication endpoints.

Patch Information

Consult the Red Hat CVE-2026-104038 Advisory for official package versions and errata. Downstream distributions will publish corresponding sssd package updates; apply them through standard package management channels (dnf update sssd or distribution equivalent).

Workarounds

Temporarily disable SID-based certificate mapping rules in sssd.conf and fall back to alternative mapping attributes until patches are deployed.
Limit certificate-based authentication to trusted certificate authorities whose issued certificates always contain the required SID extension.
Place network-layer controls in front of services that forward certificates to sssd to filter malformed or untrusted certificates.

bash

# Configuration example: disable SID-based mapping rule in sssd.conf
# Edit /etc/sssd/sssd.conf and comment out or remove the affected rule
#
# [certmap/example.com/sid_rule]
# matchrule = <ISSUER>CN=Example CA
# maprule = (objectSid={subject_sid})
#
# Then restart the service:
sudo systemctl restart sssd
sudo systemctl status sssd