CVE-2026-31431 is a logic flaw in the Linux kernel's cryptographic template involving AF_ALG and splice() that permits an unprivileged local user to perform a deterministic 4-byte write into the system page cache of any readable file. Because the page cache is shared across the host, this can be exploited to escalate privileges to root or escape container boundaries by corrupting in-memory setuid binaries or other sensitive files. This vulnerability affects GKE Standard and Autopilot clusters running Container-Optimized OS, but not containers using GKE Sandbox. Upstream kernel patches have been released and are being incorporated into GKE releases. Mitigations include upgrading to fixed GKE Container-Optimized OS versions or using GKE Sandbox to prevent exploitation.

An unprivileged local attacker can exploit this vulnerability to write controlled data into the system page cache, potentially leading to local privilege escalation and container escape. This undermines container isolation and can compromise host system security. The vulnerability affects GKE Standard and Autopilot clusters, impacting the security boundary of containers. Containers running with GKE Sandbox are not affected. The impact is rated high due to the potential for root privilege escalation and container breakout.

Upstream kernel patches have been released and are being integrated into GKE releases. Users should upgrade their Container-Optimized OS node pools to the following fixed versions or later: 1.36.0-gke.1555000, 1.35.3-gke.1943000, 1.34.7-gke.1292000, 1.33.11-gke.1132000, 1.32.13-gke.1446000, 1.31.14-gke.1846000, and 1.30.14-gke.2439000. If these versions are not visible in the Google Cloud console, use the gcloud CLI to upgrade node pools and control planes accordingly. In the interim, it is recommended to use GKE Sandbox as a mitigation since containers should not be relied upon as a security boundary due to the prevalence of container breakout vulnerabilities.