A significant vulnerability, identified as CVE-2026-31431, has been disclosed in the Linux kernel’s authencesn cryptographic template. This flaw enables unprivileged local users to execute a controlled 4-byte write into the page cache of any readable file, potentially granting root access on various Linux distributions.
What is Copy Fail?
The Copy Fail vulnerability allows an attacker to use a simple 732-byte Python script to modify a setuid binary, thereby obtaining root privileges. This issue affects major distributions including Ubuntu, Amazon Linux, RHEL, and SUSE, all of which have been shipping vulnerable kernels since 2017. Notably, the kernel does not mark the modified page as dirty, meaning standard file integrity checks will not detect the alteration.
Mechanism of the Exploit
The vulnerability arises from a logic flaw in how the Linux kernel handles page cache pages in the writable scatterlist. Specifically, the splice() function allows data to be transferred between file descriptors without copying, which can be exploited to write into the page cache of a target file. The flaw is particularly concerning because it allows for privilege escalation without requiring any race conditions or complex timing attacks.
Technical Details and Impact
The root cause of the vulnerability lies in the authencesn AEAD (Authenticated Encryption with Associated Data) wrapper used by IPsec. This implementation inadvertently allows writes beyond the intended output area, enabling an attacker to overwrite specific bytes in the page cache. The exploit can be executed without any special privileges, making it accessible to any local user.
Next Steps and Mitigation
While the exact timeline for patches or mitigations is not detailed, it is crucial for system administrators to monitor updates from their respective Linux distributions. The vulnerability’s potential for cross-container exploitation further emphasizes the need for immediate attention to security practices in environments utilizing containerization.
This article was produced by NeonPulse.today using human and AI-assisted editorial processes, based on publicly available information. Content may be edited for clarity and style.
