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Actively exploited

CVE-2025-39964 Linux Kernel Race Condition Vulnerability

Linux Kernel contains a race condition vulnerability which allows concurrent writes to the same AF_ALG socket causing data to be unpredictably interleaved and creating inconsistencies in the socket's internal state.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch urgentlyCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 18 Sep 2026.
  • Rated High severity (CVSS 7.8, scored by Vendor).
  • It has been in the catalogue for 49 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

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.

Affected Versions

ProductVersions
Linux Linux8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to before 0f28c4adbc4a97437874c9b669fd7958a8c6d6ce
8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to before e4c1ec11132ec466f7362a95f36a506ce4dc08c9
8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to before 1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8
8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to before 7c4491b5644e3a3708f3dbd7591be0a570135b84
8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to before 9aee87da5572b3a14075f501752e209801160d3d
8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to before 45bcf60fe49b37daab1acee57b27211ad1574042
Linux Linux2.6.38

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Linux Kernel. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    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.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 21 Sep 2026 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Linux Flaws

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CVE-2026-46300Linux Kernel Missing Initialization of a Variable VulnerabilityAdded 28 Oct 2026CVE-2026-53266Linux Kernel Out-of-Bounds Write VulnerabilityAdded 18 Sep 2026CVE-2025-39682Linux Kernel Improper Check for Unusual or Exceptional Conditions VulnerabilityAdded 18 Sep 2026CVE-2026-53362Linux Kernel Unspecified VulnerabilityAdded 27 Aug 2026CVE-2022-0995Linux Kernel Out-of-Bounds Write VulnerabilityAdded 26 Aug 2026CVE-2022-0492Linux Kernel Improper Authentication VulnerabilityAdded 2 Jun 2026

All 32 exploited Linux vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2025-39964
  3. CVE record: CVE-2025-39964
  4. FIRST EPSS
  5. git.kernel.org/stable/c/0f28c4adbc4a97437874c9b669fd7958a8c6d6ce
  6. git.kernel.org/stable/c/e4c1ec11132ec466f7362a95f36a506ce4dc08c9
  7. git.kernel.org/stable/c/1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8
  8. git.kernel.org/stable/c/7c4491b5644e3a3708f3dbd7591be0a570135b84
  9. git.kernel.org/stable/c/9aee87da5572b3a14075f501752e209801160d3d
  10. git.kernel.org/stable/c/45bcf60fe49b37daab1acee57b27211ad1574042
  11. git.kernel.org/stable/c/1b34cbbf4f011a121ef7b2d7d6e6920a036d5285