CVE-2025-39964: Linux Kernel Race Condition Vulnerability

Compare the exact product, branch, package, platform, and vendor status. An upstream version boundary is not proof that a distribution backport or appliance firmware is vulnerable or fixed.

How the vulnerability works

In the Linux kernel, the following vulnerability has been resolved: crypto: af_alg – Disallow concurrent writes in af_alg_sendmsg Issuing two writes to the same af_alg socket is bogus as the data will be interleaved in an unpredictable fashion. Furthermore, concurrent writes may create inconsistencies in the internal socket state. Disallow this by adding a new ctx->write field that indiciates exclusive ownership for writing. CVE CNA: Linux

Linux Kernel may be embedded across servers, containers, appliances, and administration hosts. Package installation alone does not prove that the corrected code is running.

How to check your systems

Compare distribution package versions, identify the loaded kernel or library, plan required service restarts or reboots, and validate workload health after the change.

uname -r
cat /etc/os-release

These read-only commands report the running kernel and distribution. Compare the package build against the distribution advisory, including backports. A newly installed kernel is not active until the host boots into it. For containers and Kubernetes workloads, assess the host or node kernel; replacing a container image does not replace it.

How to fix

Permanent remediation

1. Map the advisory to distribution package versions and confirm the kernel or library currently loaded by each workload.
2. Apply the latest vendor remediation or isolate the vulnerable path when immediate patching is not possible.
3. Review available telemetry for exploitation attempts before restarting, rebuilding, or rotating evidence away.
4. Verify reboot or service-restart state, loaded modules, application health, and monitoring after remediation.

Official remediation direction: CISA due date: 2026-09-21. This deadline applies to covered US federal civilian agencies; other organizations should set a risk-based deadline. Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines. Official action

If you cannot patch yet

No product-specific workaround is established by the source data available for this article. Ask the vendor for supported options. As an operational precaution, restrict access to the affected interface or isolate the component where service dependencies permit; this is not evidence that the underlying flaw is fixed.

Give any temporary control an owner and expiry, verify it covers every affected instance, and retain a rollback path. Remove it only after the permanent remediation is verified.

Detection and post-fix validation

• Confirm the installed and running version of Linux Kernel against the current vendor advisory.
• Check whether the vulnerable interface is reachable from untrusted networks or lower-privileged identities.
• Look for new accounts, privilege changes, crashes, child processes, or unusual outbound traffic associated with the component.
• Run a post-remediation service and security-control health check and retain evidence with the change record.

A clean scan or a version match alone does not rule out earlier compromise. If telemetry suggests exploitation, preserve evidence and follow the incident-response process alongside patching.

Timeline and exploitation status

• October 13, 2025 — CVE record published.
• September 18, 2026 — Source entry published.
• September 18, 2026 — Added to CISA KEV.
• October 15, 2026 — Source record updated.

Record publication and KEV addition dates describe those source events; they do not establish when exploitation first began.

The linked official record reports exploitation in the wild. This does not establish that any particular deployment has been compromised.

Public proof-of-concept availability is not independently established here. Recheck the vendor and government records as evidence changes.

Primary sources and references

• CNA-linked reference: git.kernel.org
• CNA-linked reference: git.kernel.org
• CNA-linked reference: git.kernel.org
• CNA-linked reference: git.kernel.org
• CNA-linked reference: git.kernel.org
• CNA-linked reference: git.kernel.org
• CNA-linked reference: git.kernel.org
• CISA-linked vendor guidance: www.cisa.gov
• CISA-linked vendor guidance: www.cisa.gov
• CISA-linked vendor guidance: nvd.nist.gov
• CVE CNA: Linux: CVE-2025-39964 CNA record
• CISA Known Exploited Vulnerabilities: CVE-2025-39964: Linux Kernel Race Condition Vulnerability