CriticalCVE-2025-39964

    Linux Kernel Race Condition Vulnerability (CVE-2025-39964)

    Published Last modified In CISA KEV since Sources: SUSE, Siemens ProductCERT, pwn.watch, Red Hat, CVE.org, EUVD, Debian LTS, Msrc Update Guide, BSI, Debian

    What is affected, what to do

    A race condition was found in the Linux kernel cryptographic user API, AF_ALG in the way concurrent writes to the same AF_ALG socket are handled. Interleaved data and inconsistent internal socket state may result.

    The vulnerability is being actively exploited; treat it as a priority. Update Linux Kernel to version 5.10.245, Linux Kernel to version 5.15.194, Linux Kernel to version 6.1.154. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Linux Kernel lt 2.6.38..5.10.245
    • Linux Kernel lt 5.11..5.15.194
    • Linux Kernel lt 5.16..6.1.154
    • Linux Kernel lt 6.2..6.6.108
    • Linux Kernel lt 6.7..6.12.49
    • Linux Kernel lt 6.13..6.16.9
    Fixed in
    • Linux Kernel 5.10.245
    • Linux Kernel 5.15.194
    • Linux Kernel 6.1.154
    • Linux Kernel 6.6.108
    • Linux Kernel 6.12.49
    • Linux Kernel 6.16.9
    • Linux Kernel 6.17

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    7.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    7.8

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: SUSE, Siemens ProductCERT, pwn.watch, Red Hat, CVE.org, EUVD, Debian LTS, Msrc Update Guide, BSI, Debian

    1.6 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 75.4

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    195.85 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    7.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    1.6 %
    Moderate — model estimates 1-10% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    75.4
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 18 September 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-362

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is only exploitable locally with low privileges and primarily impacts availability, but its KEV listing shows that AF_ALG race conditions are being used in practice as a post-exploitation building block — typically after initial access through a web application or from inside a container, to crash hosts or leverage inconsistent kernel state as a stepping stone for further attacks. CISA does not flag ransomware use, and exploitation likelihood is assessed as moderate. Treat multi-tenant and container platforms, CI runners and anything executing third-party code as top priority; single-purpose appliances without interactive users can follow the regular patch cycle provided AF_ALG is blocked on them.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory kernel versions: Collect uname -r across all Linux hosts via config management or EDR inventory. Branches from 2.6.38 onward are affected. Fixed upstream in 5.10.245, 5.15.194, 6.1.154, 6.6.108, 6.12.49, 6.16.9 and 6.17. Distribution kernels (RHEL, SUSE, Debian, Ubuntu) use their own versioning with backports — patch available from the vendor, look up the exact version in the distribution advisory.
      • Patch and reboot multi-user systems first: Container hosts (Kubernetes nodes, Docker hosts), CI/CD runners, jump hosts, shell servers and hosting platforms come first, since exploitation requires a low-privileged local account. The fix is not active until reboot (or livepatch).
      • Use livepatching where no maintenance window exists: If your distributor ships a livepatch for this fix (kpatch, Canonical Livepatch, SUSE Live Patching), deploy it now and verify status.
      • Determine AF_ALG usage: Run lsmod | grep algif to see whether algif_hash, algif_skcipher, algif_aead or algif_rng are loaded and in use. Hosts with no legitimate need are candidates for an immediate module block (see below).
      • Review unknown local accounts and workloads: Because the attacker already needs code execution, hunt on exposed hosts for unauthorised accounts, webshells and unexpected containers.

    2. Runbook step 2

      Mitigation layers

      • Configuration – block kernel modules: Where AF_ALG is not required, add install af_alg /bin/false, install algif_hash /bin/false, install algif_skcipher /bin/false, install algif_aead /bin/false, install algif_rng /bin/false to /etc/modprobe.d/disable-af-alg.conf and rebuild the initramfs. This has no effect if CONFIG_CRYPTO_USER_API is built in. Check dependencies first (cryptsetup benchmarks, iwd, strongSwan af-alg plugin, OpenSSL afalg engine).
      • Endpoint hardening – seccomp: Apply a seccomp profile to containers and services that denies socket() with domain AF_ALG (value 38). In Kubernetes, enforce a Localhost profile via securityContext.seccompProfile and reject Unconfined pods with an admission policy (Kyverno/Gatekeeper).
      • Mandatory access control: Use SELinux (alg_socket class) or AppArmor (deny network alg,) to deny AF_ALG socket access for untrusted domains/profiles.
      • IAM / least privilege: Restrict interactive server logins to admin groups, disable unused service accounts, and strip shells from application accounts (/usr/sbin/nologin).
      • Tenant isolation: Move untrusted workloads (customer builds, third-party code) to dedicated nodes or VM-isolated sandboxes (gVisor, Kata Containers) until patched.

    3. Runbook step 3

      Detection rules

      • auditd – AF_ALG socket creation: -a always,exit -F arch=b64 -S socket -F a0=38 -k af_alg_socket (repeat for arch=b32). Alert when processes outside an allowlist (e.g. cryptsetup, iwd, charon) open AF_ALG sockets.
      • Kernel log / journald: Hunt for oops, BUG:, KASAN or general protection fault entries with stack frames such as af_alg_sendmsg, af_alg_pull_tsgl, skcipher_recvmsg or aead_recvmsg. SPL: index=linux sourcetype=syslog ("af_alg" AND ("BUG" OR "Oops" OR "KASAN")).
      • EDR process ancestry: Binaries running from /tmp, /dev/shm, /var/tmp or home directories that spawn many threads and use AF_ALG sockets, especially when parented by web-server workers (www-data, nginx) or container runtimes (containerd-shim, runc).
      • Availability signals: Correlate unexpected kernel panics, watchdog resets or Kubernetes node NotReady events with preceding AF_ALG audit events on the same host.
      • Sigma shape: logsource: product: linux, service: auditd / detection: selection: type: SYSCALL, syscall: socket, a0: '26' (hex for 38) filter: exe|endswith: [allowlist] / condition: selection and not filter.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A race condition was found in the Linux kernel cryptographic user API, AF_ALG in the way concurrent writes to the same AF_ALG socket are handled. Interleaved data and inconsistent internal socket state may result. A local user could use this flaw to crash the system or corrupt cryptographic operation results, causing a denial of service or data integrity issues.

    Source: suse_csaf, siemens_advisory, pwn.watch, redhat_csaf, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE), ENISA EUVD, Debian LTS Advisories, Microsoft Security Update Guide, BSI WID Portal, BSI CSAF, Debian Security Advisories

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    LinuxLinux
    • 2.6.38 – < 5.10.245
    • 5.16 – < 6.1.154
    • 6.7 – < 6.12.49
    • 6.13 – < 6.16.9
    • 5.11 – < 5.15.194
    • —
    • 6.2 – < 6.6.108
    • 5.10.245
    • 6.1.154
    • 6.12.49
    • 6.16.9
    • 5.15.194
    • 6.17
    • 6.6.108

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianlinuxbookworm
    Linuxdebianlinuxbullseye
    Linuxdebianlinux-6.1bullseye
    Linuxlinuxlinux_kernel6.17
    Linuxlinuxlinux_kernel—
    Othersiemenssimatic_s7-1500_cpu_1518-4_pn\/dp_mfp_firmware—
    Othersiemenssimatic_s7-1500_cpu_1518f-4_pn\/dp_mfp_firmware—

    Weakness classes (CWE)

    • CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

      Class

      The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

      Entry at MITRE CWE-362

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    PoC-in-GitHub (nomi-sec)CVE-2025-39964 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)linux-kernel-zero-day-mitigation-zero-downtime-kernel-defense- (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. NVD analysis modifiedNVD

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Reference typeadded
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      siemens-SADP: https://cert-portal.siemens.com/productcert/html/ssa-457936.html Types: Third Party Advisory

      Source identifier: nvd@nist.gov

    2. Record modifiedSiemens ADP

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2025/39xxx/CVE-2025-39964.json">CVE-2025-39964</a>
      Referenceadded
      —
      https://cert-portal.siemens.com/productcert/html/ssa-457936.html

      Source identifier: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e

    3. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2025-39964","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-18T14:40:05.718549Z"}
      {"id":"CVE-2025-39964","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-18T00:00:00+00:00"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    4. NVD analysis modifiedNVD

      CVSS V3.1: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

      CVSS V3.1: before AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L, after —

      Weakness type: CWE-362

      Show 8 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      CVSSadded
      —
      AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
      CVSSremoved
      AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
      —
      Weakness type (CWE)added
      —
      CWE-362
      Weakness type (CWE)removed
      NVD-CWE-noinfo
      —
      Product configurationadded
      —
      AND OR *cpe:2.3:o:siemens:simatic_s7-1500_cpu_1518-4_pn/dp_mfp_firmware:*:*:*:*:*:*:*:* versions from (including) 3.1.6 OR cpe:2.3:h:siemens:simatic_s7-1500_cpu_1518-4_pn/dp_mfp:*:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:siemens:simatic_s7-1500_cpu_1518f-4_pn/dp_mfp_firmware:*:*:*:*:*:*:*:* versions from (including) 3.1.6 OR cpe:2.3:h:siemens:simatic_s7-1500_cpu_1518f-4_pn/dp_mfp:*:*:*:*:*:*:*:*
      Reference typeadded
      —
      siemens-SADP: https://cert-portal.siemens.com/productcert/html/ssa-019113.html Types: Third Party Advisory
      Reference typeadded
      —
      CISA-ADP: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-39964 Types: US Government Resource

      Source identifier: nvd@nist.gov

    5. CISA KEV entry updated9119a7d8…

      4 individual changes: Date Added, Due Date, Required Action, Vulnerability Name

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Date Addedadded
      —
      2026-09-18
      Due Dateadded
      —
      2026-09-18
      Required Actionadded
      —
      2026-09-18
      Vulnerability Nameadded
      —
      2026-09-18

      Source identifier: 9119a7d8-5eab-497f-8521-727c672e3725

    Show 5 older changes

    1. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-39964
      SSVC assessmentchanged
      {"id":"CVE-2025-39964","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-18T13:00:36.411880Z"}
      {"id":"CVE-2025-39964","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-18T14:40:05.718549Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    2. Record modifiedLinux

      Show 21 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://git.kernel.org/stable/c/0f28c4adbc4a97437874c9b669fd7958a8c6d6ce
      Referenceadded
      —
      https://git.kernel.org/stable/c/1b34cbbf4f011a121ef7b2d7d6e6920a036d5285
      Referenceadded
      —
      https://git.kernel.org/stable/c/1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8
      Referenceadded
      —
      https://git.kernel.org/stable/c/45bcf60fe49b37daab1acee57b27211ad1574042
      Referenceadded
      —
      https://git.kernel.org/stable/c/7c4491b5644e3a3708f3dbd7591be0a570135b84
      Referenceadded
      —
      https://git.kernel.org/stable/c/9aee87da5572b3a14075f501752e209801160d3d
      Referenceadded
      —
      https://git.kernel.org/stable/c/e4c1ec11132ec466f7362a95f36a506ce4dc08c9
      Referenceremoved
      https://git.kernel.org/stable/c/0f28c4adbc4a97437874c9b669fd7958a8c6d6ce
      —
      Referenceremoved
      https://git.kernel.org/stable/c/1b34cbbf4f011a121ef7b2d7d6e6920a036d5285
      —
      Referenceremoved
      https://git.kernel.org/stable/c/1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8
      —
      Referenceremoved
      https://git.kernel.org/stable/c/45bcf60fe49b37daab1acee57b27211ad1574042
      —
      Referenceremoved
      https://git.kernel.org/stable/c/7c4491b5644e3a3708f3dbd7591be0a570135b84
      —
      Referenceremoved
      https://git.kernel.org/stable/c/9aee87da5572b3a14075f501752e209801160d3d
      —
      Referenceremoved
      https://git.kernel.org/stable/c/e4c1ec11132ec466f7362a95f36a506ce4dc08c9
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/0f28c4adbc4a97437874c9b669fd7958a8c6d6ce Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/1b34cbbf4f011a121ef7b2d7d6e6920a036d5285 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/45bcf60fe49b37daab1acee57b27211ad1574042 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/7c4491b5644e3a3708f3dbd7591be0a570135b84 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/9aee87da5572b3a14075f501752e209801160d3d Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/e4c1ec11132ec466f7362a95f36a506ce4dc08c9 Types: Patch
      —

      Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

    3. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: none known, automatable: no

      Weakness type: CWE-362

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Weakness type (CWE)added
      —
      CWE-362
      SSVC assessmentchanged
      {"id":"CVE-2025-39964","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-06-10T20:40:19.988750Z"}
      {"id":"CVE-2025-39964","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-18T13:00:36.411880Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    4. Record modifiedSiemens ADP

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://cert-portal.siemens.com/productcert/html/ssa-019113.html
      Referenceremoved
      https://cert-portal.siemens.com/productcert/html/ssa-019113.html
      —

      Source identifier: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e

    5. Record modifiedLinux

      CVSS V3.1: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      CVSSadded
      —
      AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

      Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

    One NVD translation into another language is hidden.

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