CVE-2025-39964
Linux Kernel Race Condition Vulnerability
Linux Kernel
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.
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 18 September 2026. US federal agencies were required to remediate it by 21 September 2026.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 0.8%, which is higher than 54.7% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Frequently asked questions
Is CVE-2025-39964 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2025-39964.
What does CVE-2025-39964 affect?
CVE-2025-39964 affects Linux Kernel. 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.
How severe is CVE-2025-39964?
CVE-2025-39964 is rated HIGH with a CVSS base score of 7.8. EPSS puts the probability of exploitation in the next 30 days at 0.8%, higher than 54.7% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 21 September 2026.
How should organisations respond to CVE-2025-39964?
Apply the vendor patch for Linux Kernel as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.