Python CVE-2026-40594

Summary

The set_session_cookie_secure before_request handler in src/pyload/webui/app/__init__.py reads the X-Forwarded-Proto header from any HTTP request without validating that the request originates from a trusted proxy, then mutates the global Flask configuration SESSION_COOKIE_SECURE on every request. Because pyLoad uses the multi-threaded Cheroot WSGI server (request_queue_size=512), this creates a race condition where an attacker's request can influence the Secure flag on other users' session cookies - either downgrading cookie security behind a TLS proxy or causing a session denial-of-service on plain HTTP deployments.

Details

The vulnerable code is in src/pyload/webui/app/__init__.py:75-84:

# Dynamically set SESSION_COOKIE_SECURE according to the value of X-Forwarded-Proto
# TODO: Add trusted proxy check
@app.before_request
def set_session_cookie_secure():
    x_forwarded_proto = flask.request.headers.get("X-Forwarded-Proto", "")
    is_secure = (
        x_forwarded_proto.split(',')[0].strip() == "https" or
        app.config["PYLOAD_API"].get_config_value("webui", "use_ssl")
    )
    flask.current_app.config['SESSION_COOKIE_SECURE'] = is_secure

The root cause has two components:

1. No origin validation (CWE-346): The X-Forwarded-Proto header is read from any client request. This header is only trustworthy when set by a known reverse proxy. Without ProxyFix middleware or a trusted proxy allowlist, any client can spoof it. The code itself acknowledges this with the TODO on line 76.
2. Global state mutation in a multi-threaded server: flask.current_app.config['SESSION_COOKIE_SECURE'] is application-wide shared state. When Thread A (attacker) writes False to this config, Thread B (victim) may read False when Flask's save_session() runs in the after_request phase, producing a Set-Cookie response without the Secure flag.

The Cheroot WSGI server is configured with request_queue_size=512 in src/pyload/webui/webserver_thread.py:46, confirming concurrent multi-threaded request processing.

No ProxyFix or equivalent middleware is configured anywhere in the codebase (confirmed via codebase-wide search).

PoC

Attack Path 1 - Cookie Security Downgrade (behind TLS-terminating proxy, use_ssl=False):

An attacker with direct access to the backend (e.g., in a containerized/Kubernetes deployment) sends concurrent requests to keep SESSION_COOKIE_SECURE set to False:

# Attacker floods backend directly, bypassing TLS proxy
for i in $(seq 1 200); do
  curl -s -H 'X-Forwarded-Proto: http' http://pyload-backend:8000/ &
done
# Meanwhile, a legitimate user behind the TLS proxy receives a session cookie
# During the race window, their Set-Cookie header lacks the Secure flag
# The cookie is then vulnerable to interception over plain HTTP

Attack Path 2 - Session Denial of Service (default plain HTTP deployment):

# Attacker causes SESSION_COOKIE_SECURE=True on a plain HTTP server
for i in $(seq 1 200); do
  curl -s -H 'X-Forwarded-Proto: https' http://localhost:8000/ &
done
# Concurrent legitimate users receive Set-Cookie with Secure flag
# Browser refuses to send Secure cookies over HTTP
# Users' sessions silently break - they appear logged out

The second attack path works against the default configuration (use_ssl=False) and requires no special network position.

Impact

• Session cookie exposure (Attack Path 1): When deployed behind a TLS-terminating proxy, an attacker can cause session cookies to be issued without the Secure flag. If the victim's browser subsequently makes an HTTP request (e.g., via a mixed-content link or downgrade attack), the session cookie is transmitted in cleartext, enabling session hijacking.
• Session denial of service (Attack Path 2): On default plain HTTP deployments, an attacker can continuously set SESSION_COOKIE_SECURE=True, causing browsers to refuse sending session cookies back to the server. This silently breaks all concurrent users' sessions with no user-visible error message, only a redirect to login.
• No authentication required: Both attack paths are fully unauthenticated - the before_request handler fires before any auth checks.

Recommended Fix

Replace the global config mutation with per-response cookie handling, and add proxy validation:

# Option A: Set Secure flag per-response instead of mutating global config
@app.after_request
def set_session_cookie_secure(response):
# Only trust X-Forwarded-Proto if ProxyFix is configured
    is_secure = app.config["PYLOAD_API"].get_config_value("webui", "use_ssl")
    if 'Set-Cookie' in response.headers:
# Modify cookie flags per-response, not global config
        cookies = response.headers.getlist('Set-Cookie')
        response.headers.remove('Set-Cookie')
        for cookie in cookies:
            if is_secure and 'Secure' not in cookie:
                cookie += '; Secure'
            response.headers.add('Set-Cookie', cookie)
    return response
# Option B (preferred): Use Werkzeug's ProxyFix with explicit trust
from werkzeug.middleware.proxy_fix import ProxyFix
# In App.__new__, before returning:
if trusted_proxy_count:
# from config
    app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=trusted_proxy_count)
# Then set SESSION_COOKIE_SECURE once at startup based on use_ssl config,
# and let ProxyFix handle X-Forwarded-Proto transparently

At minimum, remove the before_request handler entirely and set SESSION_COOKIE_SECURE once at startup (line 130 already does this in _configure_session). The dynamic per-request adjustment is the root cause of both the spoofing and the race condition.