Linux Kernel Race Condition (CVE-2025-39964): AF_ALG Socket Analysis

The recently detailed Linux Kernel Race Condition Vulnerability identified as CVE-2025-39964 poses a local security risk to multi-tenant Linux systems and container hosts. Originating in the kernel’s AF_ALG socket interface, the flaw allows concurrent write operations to corrupt internal socket state structures without requiring elevated privileges or user interaction.

Mechanism of the AF_ALG Race Condition

The AF_ALG address family provides user-space applications direct access to the Linux Kernel Crypto API. Applications interact with kernel-side cryptographic transforms—such as hash algorithms (algif_hash) or symmetric ciphers (algif_skcipher)—by initializing an AF_ALG socket and issuing standard system calls like sendmsg(), write(), or splice().

CVE-2025-39964 is rooted in improper synchronization (CWE-362) within the socket handling routines when processing concurrent operations. When multiple threads issue simultaneous write operations to the same AF_ALG socket instance, the data streams interleave unpredictably. This race desynchronizes the socket’s internal operational state, damaging buffer offsets and cipher contexts. Under controlled conditions, an attacker can manipulate this state inconsistency to trigger out-of-bounds memory writes or use-after-free conditions in kernel memory.

Assessing Attack Surface and Risk

While the vulnerability carries a CVSS 3.1 score of 7.8 (High), its practical exploitation profile is constrained by its local vector (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H). The EPSS probability sits at 0.32% (25.4th percentile), reflecting that local race conditions are rarely weaponized for broad, automated internet-scale exploitation.

However, the operational impact in specific environments remains severe:

• Container Escapes & Multi-Tenant Hosts: In shared execution environments (such as Kubernetes clusters, web hosting nodes, or CI/CD runners), an attacker who gains unprivileged code execution inside a container can use this vulnerability to destabilize or compromise the underlying host kernel.
• Privilege Escalation: Unprivileged users (PR:L) with basic local access can leverage socket state corruption to escalate privileges to root.
• System Availability: Uncontrolled state desynchronization easily triggers kernel panics, leading to denial of service across the host.

Because the flaw affects long-standing kernel branches stretching back to version 2.6.38, standard enterprise Linux distributions that have not ingested recent point updates remain vulnerable.

Remediation and Mitigation Strategy

To fully resolve CVE-2025-39964, systems must be updated to a patched Linux kernel release. Fixed versions include 5.10.245, 5.15.194, 6.1.154, 6.6.108, 6.12.49, and 6.16.9 (or subsequent point releases in your distribution’s upstream tree).

If immediate kernel reboots are not possible, apply the following subsystem mitigations:

1. Disable AF_ALG Module Loading: Unless your workloads specifically require kernel-level cryptographic offloading, block the socket family module by adding blacklist af_alg and install af_alg /bin/true to /etc/modprobe.d/af_alg.conf.
2. Restrict Socket Creation via Seccomp: Configure container runtime profiles (Docker, containerd) or systemd service units to restrict access to the AF_ALG address family (address family 38) for unprivileged applications.