Copy Fail: A Deterministic Local Privilege Escalation Vulnerability in Linux Kernels
On April 29, 2026, the vulnerability CVE-2026-31431, known as Copy Fail, was disclosed by researchers from Palo Alto Networks. It represents a persistent local privilege escalation (LPE) flaw in the Linux kernel that affects all major distributions since 2017.
This vulnerability is particularly concerning due to its deterministic nature, which sets it apart from many other kernel vulnerabilities that depend on race conditions or specific memory offsets. Exploitation requires only a simple 732-byte Python script that can be executed without modification across different Linux distributions. The flaw arises from a logic error in the Linux kernel’s cryptographic subsystem, specifically within the algif_aead module of the AF_ALG interface. This flaw enables an unprivileged local attacker to manipulate the in-memory cache of critical executable files while leaving the original on-disk files untouched, thus circumventing common integrity checks.
The attack vector involves corrupting the page cache of setuid-root binaries, such as su and sudo, during cryptographic operations. An attacker can craft specific inputs to gain superuser privileges when executing these binaries, thereby breaking the kernel’s trust boundaries. The attack is not only stealthy, as the changes are ephemeral and do not persist across reboots, but also poses a significant risk to environments utilizing Kubernetes or CI/CD pipelines, where container escape or compromising multi-tenant hosts could occur.
Defensive Context
Organizations running Linux systems, especially those with distributions released since 2017, should prioritize addressing this vulnerability to avert potential privilege escalation incidents. Entities in cloud computing, container orchestration, and development environments are particularly vulnerable.
Why This Matters
The simplicity and reliability of the Copy Fail exploit make it a serious threat to environments where attackers can gain local access. Given the prevalence of Linux operating systems in enterprise and cloud infrastructures, unpatched systems represent a substantial risk.
Defender Considerations
Immediate patching of affected kernels is imperative, with vendor-issued updates readily available. If patching is not feasible, administrators should implement interim mitigation by disabling the vulnerable algif_aead module. Palo Alto Networks has provided specific commands for this mitigation.
Environment Exposure
This threat is relevant when unprivileged local access exists on systems running vulnerable Linux kernel versions (4.14 to 6.19.12). It may not be relevant in environments where updates have been adequately applied or in systems lacking adequate local access controls.
Indicators of Compromise (IOCs)
- CVE ID: CVE-2026-31431
- Vulnerable Kernel Versions: 4.14 to 6.19.12
