Copy Fail - Python 3.9 PoC (CVE-2026-31431)

The public Copy Fail exploit wasn't compaitble with python 3.9, this one is.

The original was modifed by replacing the unavailable os.splice() with a lightweight ctypes polyfill.

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🚨 LEGAL DISCLAIMER (CFAA & Related Laws)

THIS SOFTWARE IS PROVIDED FOR EDUCATIONAL AND AUTHORIZED SECURITY RESEARCH PURPOSES ONLY.

By downloading, using, or viewing this code, you agree to the following:

• You will only use this exploit on systems you own or for which you have explicit written authorization from the owner.
• Unauthorized access, modification, or use of this code against any system without permission is illegal and may violate:
  • Computer Fraud and Abuse Act (CFAA) (18 U.S.C. § 1030) — United States
  • Computer Misuse Act — United Kingdom
  • Equivalent computer crime laws in your jurisdiction
• The author(s) and contributors expressly disclaim any and all liability for any damages, legal consequences, or misuse of this software.
• This repository does not condone, encourage, or support any illegal activity.

You are solely responsible for your actions and any consequences that arise from using this code.

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About the Vulnerability

CVE-2026-31431 ("Copy Fail") is a logic flaw in the Linux kernel's authencesn cryptographic template (part of the AF_ALG interface). It allows any unprivileged local user to perform a controlled 4-byte write into the page cache of any readable file.

• Discovered by Xint / Theori
• Affects virtually all Linux distributions shipped since 2017
• Original PoC: theori-io/copy-fail-CVE-2026-31431
• Latest info on systems impacted and mitiagtions: https://github.com/theori-io/copy-fail-CVE-2026-31431/issues

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Tested Environments

• Python 3.9 on RHEL 4.18.0-553.120.1.el8_10.x86_64, but will work on many other distros. See initial

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Usage

1. Clone this repository:
   root@kitploit:~

   git clone https://github.com/w3llr00t3d/CVE-2026-31431-PoC.git
   cd CVE-2026-31431-PoC
   
   

2. Set executable:
   root@kitploit:~

   chmod +x poc.py
   
   

3. Run it:
   root@kitploit:~

   python3 poc.py