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Known exploited vulnerability

CVE-2025-39964

mediumCVSS 3.1 5.5In CISA KEVEPSS 1.6%Published · Linux

The short answer

CVE-2025-39964 affects Linux Linux and is rated medium (CVSS 5.5). CISA added it to the Known Exploited Vulnerabilities catalog on 2026-09-18: attackers are using it, so patch it first. The vendor has linked a patch.

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.

Weakness: CWE-362.

Severity

5.5 medium

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Exploited (CISA KEV)

Yes, since Sep 18, 2026

Federal due date Sep 21, 2026

EPSS, next 30 days

1.6%

Higher than 76% of scored CVEs

How it can be exploited

Attack vector
Local
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
None
Availability impact
High

What CISA says to do

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…

Affected products

Linux, SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0), SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0), SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0), SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0), SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0). NVD's record lists the exact affected versions.

References

Questions

Is CVE-2025-39964 being exploited?

Yes. CISA added CVE-2025-39964 to its Known Exploited Vulnerabilities catalog on 2026-09-18, with a federal remediation due date of 2026-09-21.

How do I fix CVE-2025-39964?

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). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for…

From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Analyzed. The description is NVD's text, quoted as published.

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