Copy Fail: Critical Linux Kernel Privilege Escalation Vulnerability - CVE-2026-31431

On April 29, 2026, security researchers publicly disclosed a critical Linux kernel vulnerability known as CVE-2026-31431, also called Copy Fail. This vulnerability is caused by a logic flaw in the kernel, which allows any unprivileged local user to gain root access. Copy Fail has a CVSS score of 7.8 (High) and a publicly available exploit that works reliably across multiple Linux environments. The same exploit can compromise Ubuntu, Amazon Linux, RHEL, and SUSE without any modification. It can also be used as a container escape technique in shared kernel environments, including Kubernetes clusters, CI pipelines, and AI code execution platforms.

In this blog, we explain how the Copy Fail CVE-2026-31431 vulnerability works and provide practical steps for validation and remediation.

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Copy Fail CVE-2026-31431 Vulnerability Explained

The Linux kernel includes a cryptographic subsystem that provides essential security functions such as encryption, hashing, and authentication. These capabilities are used across the operating system to support disk encryption, secure network communication, and kernel-level key management.

Copy Fail CVE-2026-31431 vulnerability is caused by a logic flaw in the authencesn algorithm, which is part of the kernel cryptographic subsystem. This algorithm is designed to support IPsec Extended Sequence Numbers and works by rearranging sequence-related data during cryptographic operations. To do this, it temporarily uses the destination buffer as working memory. This behavior was originally safe. However, a later design change introduced in 2017 altered how memory is handled during certain operations. As a result, page cache memory from files can be unintentionally exposed to write operations. When these two behaviors interact, they create a situation where an attacker can perform a controlled four-byte write directly into kernel memory. The attacker can choose which file is affected, where the modification occurs, and what value is written, making the vulnerability both precise and highly reliable.

Researchers have confirmed that CVE-2026-31431 affects all major Linux distributions, including Ubuntu, Amazon Linux, RHEL, and SUSE. Other distributions that use kernels built within the affected time frame are also vulnerable, such as Debian, Arch, Fedora, Rocky, Alma, Oracle Linux, and many embedded systems. This broad impact is due to the shared kernel design across these platforms. Organizations should take immediate action by updating their kernel to a patched version.

How Copy Fail CVE-2026-31431 Exploit Works

Copy Fail exploit works by combining multiple kernel features into a single reliable attack path. AF_ALG is a Linux socket interface that allows user-space applications to access kernel-level cryptographic functions without requiring special privileges. The process begins when an attacker opens an AF_ALG socket and binds it to a cryptographic function. The attacker then prepares a valid key and establishes a request socket. Importantly, none of these steps requires special privileges, which makes the exploit accessible to any local user on the system.

Next, the attacker uses the splice system call to interact with a target file, usually a setuid binary such as a system utility. This step is critical because splice passes file data by reference instead of copying it, which means the kernel’s in-memory representation of the file becomes part of the cryptographic operation. The attacker then triggers a decryption request. During this process, the kernel unintentionally places page cache memory into a writable structure, allowing it to be modified.

The key moment occurs when the cryptographic algorithm processes the request. At this stage, it performs a small internal operation that writes four bytes of attacker-controlled data into memory. Because of the way memory is structured in this path, this write crosses into the page cache of the target file. As a result, the attacker can directly modify the kernel’s in-memory copy of a privileged binary. Even though the operation technically fails and returns an error, the memory modification remains in place.

To fully exploit this behavior, the attacker repeats the process several times. This allows them to inject a small payload into a chosen location within the target binary. Once the modification is complete, the attacker executes the file. Since the system loads binaries from memory, the modified version is executed instead of the original file on disk. If the binary has elevated privileges, the attacker gains root access. While the exploit often targets common binaries, any readable setuid root binary can be used.

One of the most concerning aspects of Copy Fail is its stealth. The kernel does not mark the modified memory as changed, so nothing is written back to disk. This means traditional detection methods that rely on file integrity checks will not see any difference. Even forensic analysis of the disk will show the original, untouched file. The malicious changes exist only in memory and disappear after a reboot or cache refresh, making detection extremely difficult unless the system is monitored in real time.

How Picus Helps Simulate Copy Fail CVE-2026-31431 Attacks?

We also strongly suggest simulating the Copy Fail CVE-2026-31431 vulnerability to test the effectiveness of your security controls against sophisticated cyber attacks using the Picus Security Validation Platform. You can also test your defenses against other vulnerability exploitation attacks, such as regreSSHion, Citrix Bleed, and Follina, within minutes with a 14-day free trial of the Picus Platform.

Picus Threat Library includes the following threats for Copy Fail CVE-2026-31431 exploitation attacks:

Picus also provides actionable mitigation content. Picus Mitigation Library includes prevention signatures to address Copy Fail attacks in preventive security controls. Currently, Picus Labs has validated the following signatures for Copy Fail attacks:

Start simulating emerging threats today and get actionable mitigation insights with a  14-day free trial  of the Picus Security Validation Platform.