Exploited Recently addedHunt for compromise (CISA)

CVE-2025-39964

Linux Kernel Race Condition Vulnerability

At a glance

Exploited. Added to CISA’s catalog of exploited vulnerabilities on September 18, 2026.

CISA asks you to hunt for signs of compromise: patching is not enough if the device has already been breached.

Affected product: Linux Kernel (category Operating systems, indicative classification). Name in CISA’s catalog: Linux Kernel.

What should I do?

Apply the patches or mitigations from Linux; if none are available, stop using the product. Start with internet-facing devices.

CISA also asks you to hunt for signs of compromise: patching is not enough if the device has already been breached.

Deadline set by CISA for US federal agencies: 3 days (due September 21, 2026). It only binds those agencies, but it is a useful measure of urgency.

Linux security advisories page (general page, not the advisory for this vulnerability)
The vendor’s specific advisory is listed in the NVD references: CVE-2025-39964 on the NVD website.

Original text of the required action (CISA)

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.

Timeline

October 13, 2025 Vulnerability published Publication date (NVD)
September 18, 2026 Exploitation confirmed Added to CISA’s KEV catalog
September 21, 2026 CISA deadline Remediation deadline set for US federal agencies

The facts, with their sources

Exploitation confirmed: Yes, on September 18, 2026 Date added to the catalog of exploited vulnerabilities. Exploitation itself may have started earlier. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Ransomware: No known use to date That does not guarantee it is not being used: the information is not public. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Hunt for compromise: Requested by CISA Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA required action: Apply the patches or mitigations from Linux; if none are available, stop using the product. Start with internet-facing devices. Rewritten by this site from CISA’s original text. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA deadline: September 21, 2026, 3 days Set for US federal agencies: a measure of urgency, not an obligation elsewhere. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Vulnerability published: October 13, 2025 Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC
Product according to CISA: Linux Kernel Filed by this site under “Operating systems” (indicative classification). Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CVSS vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H Technical detail of the score: attack vector, complexity, impact. Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC

Severity and activity

5.5 / 10 CVSS severity: medium

Score from the NVD. Collected Sep 29, 2026.

— Probability of exploitation in the next 30 days (EPSS)

FIRST’s EPSS model. Since exploitation is already confirmed, this figure is mostly useful to compare activity. Collected Sep 29, 2026.

Description

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.

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.

Summary from CISA (KEV catalog).

CERT-FR advisories

CERTFR-2026-AVI-1202: Multiples vulnérabilités dans le noyau Linux de SUSE (updated September 18, 2026)
CERTFR-2026-AVI-1230: Multiples vulnérabilités dans le noyau Linux de Red Hat (updated September 25, 2026)
CERTFR-2026-AVI-1231: Multiples vulnérabilités dans le noyau Linux de SUSE (updated September 25, 2026)

Information reused under the Open Licence 2.0 (Etalab); the date of last update is shown for each advisory. This site is neither affiliated with nor endorsed by ANSSI.

Sources for this page

CISA KEV catalog: exploitation, date added, required action, deadline, ransomware.
NVD (NIST): CVSS severity, publication date, description, vendor references.
EPSS (FIRST): probability of exploitation.
CERT-FR: advisories and alerts (in French).
Brand and category: indicative classification by this site (method).