New “Copy Fail” bug puts Linux systems at serious risk

Cybersecurity researchers have disclosed details of a Linux local privilege escalation (LPE) vulnerability that could allow an unprivileged local user to obtain root access.

The high-severity vulnerability, tracked as CVE-2026-31431 (CVSS score: 7.8), has been codenamed “Copy Fail” by Xint.io and Theori, News.Az reports, citing foreign media.

“An unprivileged local user can write four controlled bytes into the page cache of any readable file on a Linux system, and use that to gain root,” the vulnerability research team at Xint.io and Theori said.

At its core, the vulnerability originates from a logic flaw in the Linux kernel’s cryptographic subsystem, specifically within the algif_aead module. According to the researchers, the issue was introduced in a source code commit made in August 2017.

Successful exploitation of the shortcoming could allow a simple 732-byte Python script to edit a setuid binary and obtain root on essentially all Linux distributions shipped since 2017, including Amazon Linux, RHEL, SUSE, and Ubuntu. The Python exploit involves four steps -

- Open an AF_ALG socket and bind to authencesn(hmac(sha256),cbc(aes))

- Construct the shellcode payload

- Trigger the write operation to the kernel's cached copy of "/usr/bin/su"

- Call execve("/usr/bin/su") to load the injected shellcode and run it as root

While the vulnerability is not remotely exploitable in isolation, a local unprivileged user can get root simply by corrupting the page cache of a setuid binary. The same primitive also has cross-container impacts as the page cache is shared across all processes on a system.

In response to the disclosure, Linux distributions have released their own advisories -

- Amazon Linux

- Debian

- Red Hat Enterprise Linux

- SUSE

- Ubuntu

Copy Fail has its echoes in Dirty Pipe (CVE-2022-0847), another Linux kernel LPE vulnerability that could permit unprivileged users to splice data into the page cache of read-only files and ultimately overwrite sensitive files on the system to achieve code execution.

"Copy Fail is the same class of primitive, in a different subsystem," Bugcrowd's David Brumley said. "The 2017 in-place optimization in algif_aead allows a page-cache page to end up in the kernel’s writable destination scatterlist for an AEAD operation submitted over an AF_ALG socket. An unprivileged process can then drive splice() into that socket and complete a small, targeted write into the page cache of a file it doesn't own."

What makes the vulnerability dangerous is that it can be reliably triggered and does not require any race condition or kernel offset. On top of that, the same exploit works across distributions.

"This vulnerability is unique because it has four properties that almost never appear together: it's portable, tiny, stealthy, and cross-container," a Xint.io spokesperson told The Hacker News in a statement. "It allows any user account, no matter how low-level, to increase their privilege to full admin access. It also allows them to bypass sandboxing and works across all Linux versions and distributions."

News.Az 

By Nijat Babayev