CVE-2025-39964
CVE-2025-39964 is a security flaw in Linux Kernel, the core of Linux, which runs most servers, Android phones and many home routers. Attackers can corrupt memory in Linux Kernel, which they use to take it over. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Sep 2026.[1]
Linux Kernel is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed by 7 commits to Linux, in the releases that include them.
Description
CISA calls it a race condition in Linux Kernel. In its words: “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.”[1]
The CVE record adds: “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.”[2]
The underlying weakness is race condition (CWE-362), in MITRE’s list of common software weaknesses.
It scores 5.5 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Linux | commit 0f28c4adbc4a |
| Linux | commit e4c1ec11132e |
| Linux | commit 1f323a48e9b5 |
| Linux | commit 7c4491b5644e |
| Linux | commit 9aee87da5572 |
| Linux | commit 45bcf60fe49b |
| Linux | commit 1b34cbbf4f01 |
Most families won’t have Linux Kernel; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 18 Sep 2026, 340 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 21 Sep 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 2%. That’s in the top 25% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Linux has published a fix or advice.[3] CISA’s required action: “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).”[1]
- Check whether you use it.Linux Kernel is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed by 7 commits to Linux, in the releases that include them), or not in use. CISA gave US government agencies until 21 Sep 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 5.5.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 2%.[4] |
See also
Other Linux Kernel flaws
- CVE-2026-46300: missing initialization of a variable ·
- CVE-2026-53266: out-of-bounds write ·
- CVE-2025-39682: improper check for unusual or exceptional conditions ·
- CVE-2026-53362: unspecified ·
- CVE-2022-0995: out-of-bounds write ·
- CVE-2022-0492: improper authentication ·
Other race condition flaws
- CVE-2025-62215, Microsoft Windows: race condition ·
- CVE-2023-36884, Microsoft Windows: search remote code execution ·
- CVE-2021-25395, Samsung Mobile Devices: race condition ·
- CVE-2021-0920, Android Kernel: race condition ·
- CVE-2022-26904, Microsoft Windows: user profile service privilege escalation ·
Added the same day
- CVE-2026-53266, Linux Kernel: out-of-bounds write ·
- CVE-2025-39682, Linux Kernel: improper check for unusual or exceptional conditions ·
Every Linux flaw on CISA’s list · Everything added in September 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Linux Kernel Race Condition Vulnerability”. Known Exploited Vulnerabilities Catalog, added 18 Sep 2026.
- ^ National Institute of Standards and Technology. “CVE-2025-39964 Detail”. National Vulnerability Database, published 13 Oct 2025.
- ^ Linux. Advisories and fixes: git.kernel.org, git.kernel.org, git.kernel.org, git.kernel.org, git.kernel.org, and 2 more on NVD’s record.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.016, percentile 75.4, 31 Oct 2026.
- CVE Program. “CVE-2025-39964”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.