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CVE-2025-39964Linux Kernel Race Condition Vulnerability

Patch nowMedium5.5Exploited, CISA due date 2026-09-21EPSS 1.6%, higher than 76% of scored CVEs

CVE-2025-39964 is a race condition in Linux Kernel, rated medium (5.5 on CVSS 3.1).

Why this priority: Exploited in the wild: listed in CISA's KEV catalog.

Published · Updated · NVD status: Analyzed

What it is

Two operations race on shared state, and an attacker who wins the race bypasses a check or corrupts data.

How it is reached

An attacker can exploit it by someone who can already run code or sign in on the machine, with an ordinary user account, and nobody has to click anything.

What an attacker gains

A successful attack can take it offline. In practice, this kind of flaw lets an attacker win a timing window to bypass a check or corrupt shared state.

Is it being exploited?

CISA added it to the Known Exploited Vulnerabilities catalog on 2026-09-18: it is being exploited in the wild. EPSS estimates a 1.6% chance of exploitation activity in the next 30 days, higher than 76% of scored CVEs.

What to do

  1. Treat it as urgent: it is on CISA's KEV list. CISA's deadline for US federal agencies: 2026-09-21.
  2. No fixed version is listed for Linux Kernel itself; fixed versions for the other affected products are listed on this page.
  3. Limit who can run code or sign in on the hosts that run it.
  4. Check exposed instances for signs of compromise, not only for the version.

Description from NVD

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.

What this means for your architecture.

A CVE names a product. Whether it matters depends on where that product sits in your systems: what reaches it, which trust boundary it guards and what it can reach once compromised. Veriom maps your components, their relationships and your trust boundaries from your code, cloud and delivery evidence, so you can see where a component like this sits. The radar itself does not look at your systems.

Components it touches

  • Network edge devices: VPNs, firewalls, routers and gateways. They sit between the internet and internal networks, so a flaw in one is often the first step into everything behind it.
  • Operating systems on servers, workstations or devices. Everything on the host inherits the flaw.
  • Request handlers and jobs that share state. The flaw sits between concurrent requests or processes.

Trust boundaries in play

  • Host boundary. The attacker needs a foothold on the machine first; the flaw turns a small foothold into a bigger one.
  • Between concurrent requests or processes. An attacker who crosses it can win a timing window to bypass a check or corrupt shared state.

Questions to answer about your own systems

  • Where does Linux Kernel run in your systems, and at which version?
  • Which of those hosts also run code or users you do not fully trust?
  • What can it reach once compromised: which data stores, secrets and services?
See how Veriom maps an architectureTake the interactive tour

Read more

Sources

  • CVE records and CVSS scores from the NVD CVE API. This product uses the NVD API but is not endorsed or certified by the NVD.
  • Exploitation status from CISA’s Known Exploited Vulnerabilities catalog.
  • Exploit likelihood from FIRST’s Exploit Prediction Scoring System (EPSS), refreshed daily.
  • Explanations are written by Veriom from fixed templates over each record’s data, not by a person reviewing each CVE. Descriptions are quoted from the records as published.