What is CVE-2025-39964?

CVE-2025-39964 is crypto: af_alg - Disallow concurrent writes in af_alg_sendmsg. The supplied enrichment data classifies it as Race Condition mapped to CWE-362 Race Condition.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Yes: CISA KEV signal present. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

Is there a public exploit?

6 public PoC repositories reported; 1 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with Not reported where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2025-39964 using CVSS 5.5, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Treat as emergency remediation because CISA KEV status is present. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

CISA KEVmediumRace Conditionpublic exploit

CVE-2025-39964

CVE-2025-39964 — crypto: af_alg - Disallow concurrent writes in af_alg_sendmsg

Q: Is it in CISA KEV?

Yes. The supplied enrichment data lists CVE-2025-39964 in CISA KEV with 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

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.

Published Oct 13, 2025Updated Sep 18, 2026Sources: NVD, Linux (CNA), CISA, FIRST EPSS, GitHub, Microsoft MSRC, Red Hat (VEX), SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Confirmed

CISA KEV, listed Sep 18, 2026

EPSS

ahead of 26% of scored CVEs

Affects

Linux

18 products listed

CVSS base

5.5

medium · 2 sources

CISA SSVC assessment

Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.

CISA

Exploitation

Active

none · proof-of-concept · active

Automatable

can an attacker script all four kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Remediation

The vendor's own words where we have them.

Upgrade Linux to a fixed release (0 to < 2.6.38). Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

CISA 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

Federal deadline Sep 21, 2026 — in 3 days

Red Hat VEX · CVE-2025-39964

fixed0affected260not affected14

workaround: To mitigate this issue, prevent module af_alg from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loadi…

none_available: Affected

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Treat as emergency remediation because CISA KEV status is present.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to < 0f28c4adbc4a97437874c9b669fd7958a8c6d6ce	Vulnerable
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2	Vulnerable
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to < 1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8	Vulnerable
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to < 7c4491b5644e3a3708f3dbd7591be0a570135b84	Vulnerable
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to < 9aee87da5572b3a14075f501752e209801160d3d	Vulnerable
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to < 45bcf60fe49b37daab1acee57b27211ad1574042	Vulnerable
Linux	Linux	8ff590903d5fc7f5a0a988c38267a3d08e6393a2 to < 1b34cbbf4f011a121ef7b2d7d6e6920a036d5285	Vulnerable
Linux	Linux	2.6.38	Vulnerable
Linux	Linux	0 to < 2.6.38	Patched
Linux	Linux	5.10.245 to <= 5.10.*	Patched
Linux	Linux	5.15.194 to <= 5.15.*	Patched
Linux	Linux	6.1.154 to <= 6.1.*	Patched

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

High

Confidentiality

None

Integrity

None

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Local

Privileges Req

Low

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
7.8	CVSS 3.1	high	1.8	5.9	CNA, Linux (CNA)
5.5	CVSS 3.1	medium	1.8	3.6	NVD, Microsoft, Red Hat (VEX)

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

Weakness & attack patterns

CWE-362Race Condition

Inferred from the weakness class — not observed against this CVE.

CAPEC-26 · Leveraging Race Conditions

CISA KEV details

CISA lists a CVE here once it has evidence of exploitation against real targets.

CISAThat month →

Listed

Sep 18, 2026

Federal deadline

Sep 21, 2026

in 3 days

Ransomware use

Unknown

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a date to argue against.

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

1 of the 5 sampled are weaponized

Social mentions

Posts naming this CVE. Attention, not evidence of exploitation — it spikes on disclosure and decays whether or not anything is exploited.

CVE-2025-39964 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_… https://t.co/SiXeg0E0nM
@CVEnew· Oct 13, 2025

EPSS over time

FIRST rescores it daily; this is its history for as long as we have been tracking this CVE.

FIRST

Aug 18, 2026 → Sep 16, 2026 · 30 days

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Monitor for scanner or exploit-pattern traffic after 6 public PoC repositories were reported.

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added to CISA KEV catalog
Sep 18, 2026 · CISA
Added · CVSS 3.1 5.5 (AV:L)
Sep 18, 2026 · NVD
2025
Bulletin released on NIST.
Oct 16, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 16, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 15, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 15, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 15, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 15, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 15, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 14, 2025 · SOCRadar CTI
Bulletin released on NIST.
Oct 14, 2025 · SOCRadar CTI
Initial · CVE published
Oct 13, 2025 · NVD
CVE published by MITRE.
Oct 13, 2025 · SOCRadar CTI

Source activity

Readings a source repeats on a schedule, counted rather than listed.

4×ModifiedFeb 3, 2026 – Sep 18, 2026 · SOCRadar CTI
3×Tweets · 1 time tweeted.Oct 14, 2025 · SOCRadar CTI
1×SVRS is calculatedNov 14, 2025 · SOCRadar CTI
1×Github_Repos · Github repository created, n1k0oowang/CVE-2025-39964_EXPNov 13, 2025 · SOCRadar CTI

References

17 on the record

git.kernel.org/stable/c/0f28c4adbc4a97437874c9b669fd7958a8c6d6ce
Patch, NIST, Vendor Advisory
git.kernel.org/stable/c/1b34cbbf4f011a121ef7b2d7d6e6920a036d5285
Patch, NIST, Vendor Advisory
git.kernel.org/stable/c/1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8
Patch, NIST, Vendor Advisory
git.kernel.org/stable/c/45bcf60fe49b37daab1acee57b27211ad1574042
Patch, NIST, Vendor Advisory
git.kernel.org/stable/c/7c4491b5644e3a3708f3dbd7591be0a570135b84
Patch, NIST, Vendor Advisory
git.kernel.org/stable/c/9aee87da5572b3a14075f501752e209801160d3d
Patch, NIST, Vendor Advisory
git.kernel.org/stable/c/e4c1ec11132ec466f7362a95f36a506ce4dc08c9
Patch, NIST, Vendor Advisory
cert-portal.siemens.com/productcert/html/ssa-019113.html
Third Party Advisory, NIST, GITHUB

Coverage

20 stories from the feeds we poll

15 more reports on this CVEShowing 5 of 20 · Vendor research and security press coverageUnlock free

Elsewhere on this site

Linuxevery CVE for this vendor
Microsoftevery CVE for this vendor
September 2026what else CISA listed
Race Conditionsame class
CWE-362other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2025-39964 is crypto: af_alg - Disallow concurrent writes in af_alg_sendmsg, a medium vulnerability affecting Linux, azl3 kernel 6.6.96.2-2 on Azure Linux 3.0, azl3 kernel 6.6.104.2-4 on Azure Linux 3.0 from Linux, Microsoft. The current evidence lists it in CISA KEV, and the exploit status is: Yes: CISA KEV signal present. Public exploit evidence is: 6 public PoC repositories reported; 1 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 5.5 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.