CVE-2026-40594: pyLoad: Session Cookie Security Downgrade via Untrusted X-Forwarded-Proto Header Spoofing (Global State Race Condition)

Severity: medium (CVSS 4.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-21 · updated 2026-06-17
Weakness: CWE-346

Description

pyLoad is a free and open-source download manager written in Python. Prior to 0.5.0b3.dev98, 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. This vulnerability is fixed in 0.5.0b3.dev98.

Affected products

pyload: before 0.5.0b3.dev69 — fixed in 0.5.0b3.dev69

How to fix

Upgrade to a fixed version: pyload 0.5.0b3.dev69. Check the vendor advisory for mitigations.

References

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