CVE-2025-39964: Linux kernel AF_ALG concurrent-write race in crypto user API

Exploited in the wild Medium 5.5 EPSS 1.6% Linux kernel

af_alg_sendmsg() in the kernel crypto user-space interface (AF_ALG sockets, crypto/af_alg.c) allowed two writers on the same socket at once, interleaving data and leaving the socket's internal state inconsistent. A local user who can open AF_ALG sockets can trigger it; Red Hat describes crashes or corrupted crypto results, while the kernel CNA scores it 7.8 (high confidentiality, integrity and availability impact). CISA added it to KEV on 18 September 2026 as exploited in the wild.

• CVE-2025-39964CVE identifier
• 5.5CVSS 3.1 base score (medium)
• 2026-09-18added to CISA's exploited list (US federal deadline 2026-09-21)
• 1.6%EPSS: chance of exploitation in the next 30 days (higher than 76% of CVEs)
• 2025-10-13published

What do these terms mean? CVE · CVSS score · exploited (KEV) · EPSS · backported fix · all terms

Affected and fixed versions

Affected:

• Linux 2.6.38 through 5.10.244
• Linux 5.11 through 5.15.193
• Linux 5.16 through 6.1.153
• Linux 6.2 through 6.6.107
• Linux 6.7 through 6.12.48
• Linux 6.13 through 6.16.8

Fixed in:

• Linux 5.10.245
• Linux 5.15.194
• Linux 6.1.154
• Linux 6.6.108
• Linux 6.12.49
• Linux 6.16.9
• Linux 6.17

Am I affected?

uname -r
dpkg -l 'linux-image*' | grep ^ii   # Debian/Ubuntu
rpm -q kernel   # RHEL/Fedora/Amazon/SUSE (SUSE: rpm -q kernel-default)

Patch it

Use the commands for your system. Distribution packages often keep an older version number with the fix backported, so compare with the fixed package versions in the table below, not only with the upstream version.

sudo apt update
sudo apt upgrade
sudo reboot

sudo apt update
sudo apt upgrade
sudo reboot

sudo dnf upgrade kernel
sudo reboot
# RHEL 7: sudo yum update kernel

sudo dnf upgrade --refresh kernel
sudo reboot

sudo zypper refresh
sudo zypper patch --category security
sudo reboot

# Amazon Linux 2023
sudo dnf upgrade --releasever=latest 'kernel*'
# Amazon Linux 2: the default 4.14 kernel and kernel-5.4 extra are 'No Fix Planned';
# switch to the kernel-5.10 or kernel-5.15 extra, then:
sudo yum update kernel
sudo reboot

sudo pacman -Syu
sudo reboot

# as root
apk update
apk upgrade
reboot

not Linux

General update steps for every system are on the Linux and Unix & BSD pages; after updating, see which services to restart.

Fix status in Linux distributions

From the Debian security tracker, Ubuntu CVE tracker and Red Hat Security Data API, refreshed every few hours.

If you cannot patch yet

• Red Hat states no mitigation meets its criteria; install the kernel update.

Related vulnerabilities

• CVE-2026-31431

Sources

• CVE record CVE-2025-39964
• NVD CVE-2025-39964
• CISA KEV entry CVE-2025-39964
• Debian security tracker CVE-2025-39964
• Ubuntu CVE page CVE-2025-39964
• Red Hat CVE page CVE-2025-39964
• SUSE CVE page CVE-2025-39964
• Amazon Linux ALAS CVE page CVE-2025-39964
• Upstream fix (crypto: af_alg - Disallow concurrent writes in af_alg_sendmsg)
• linux-cve-announce: CVE-2025-39964
• https://git.kernel.org/stable/c/0f28c4adbc4a97437874c9b669fd7958a8c6d6ce
• https://git.kernel.org/stable/c/e4c1ec11132ec466f7362a95f36a506ce4dc08c9
• https://git.kernel.org/stable/c/1f323a48e9b5ebfe6dc7d130fdf5c3c0e92a07c8
• https://git.kernel.org/stable/c/7c4491b5644e3a3708f3dbd7591be0a570135b84
• https://git.kernel.org/stable/c/9aee87da5572b3a14075f501752e209801160d3d
• https://git.kernel.org/stable/c/45bcf60fe49b37daab1acee57b27211ad1574042
• CISA Known Exploited Vulnerabilities catalog

Page generated automatically from official sources on 2026-11-03 18:05 UTC and reviewed by an editor. Always confirm with your vendor's advisory.