CVE-2026-40594: Pyload ng PyLoad Has A Session Cookie Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mp82-fmj6-f22v",
  "modified" : "2026-06-08T19:47:41Z",
  "published" : "2026-04-16T01:20:49Z",
  "aliases" : [ "CVE-2026-40594" ],
  "summary" : "pyLoad has a Session Cookie Security Downgrade via Untrusted X-Forwarded-Proto Header Spoofing (Global State Race Condition)",
  "details" : "## Summary\n\nThe `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.\n\n## Details\n\nThe vulnerable code is in `src/pyload/webui/app/__init__.py:75-84`:\n\n```python\n# Dynamically set SESSION_COOKIE_SECURE according to the value of X-Forwarded-Proto\n# TODO: Add trusted proxy check\n@app.before_request\ndef set_session_cookie_secure():\n    x_forwarded_proto = flask.request.headers.get(\"X-Forwarded-Proto\", \"\")\n    is_secure = (\n        x_forwarded_proto.split(',')[0].strip() == \"https\" or\n        app.config[\"PYLOAD_API\"].get_config_value(\"webui\", \"use_ssl\")\n    )\n    flask.current_app.config['SESSION_COOKIE_SECURE'] = is_secure\n```\n\nThe root cause has two components:\n\n1. **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.\n\n2. **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.\n\nThe Cheroot WSGI server is configured with `request_queue_size=512` in `src/pyload/webui/webserver_thread.py:46`, confirming concurrent multi-threaded request processing.\n\nNo `ProxyFix` or equivalent middleware is configured anywhere in the codebase (confirmed via codebase-wide search).\n\n## PoC\n\n**Attack Path 1 — Cookie Security Downgrade (behind TLS-terminating proxy, `use_ssl=False`):**\n\nAn 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`:\n\n```bash\n# Attacker floods backend directly, bypassing TLS proxy\nfor i in $(seq 1 200); do\n  curl -s -H 'X-Forwarded-Proto: http' http://pyload-backend:8000/ &\ndone\n\n# Meanwhile, a legitimate user behind the TLS proxy receives a session cookie\n# During the race window, their Set-Cookie header lacks the Secure flag\n# The cookie is then vulnerable to interception over plain HTTP\n```\n\n**Attack Path 2 — Session Denial of Service (default plain HTTP deployment):**\n\n```bash\n# Attacker causes SESSION_COOKIE_SECURE=True on a plain HTTP server\nfor i in $(seq 1 200); do\n  curl -s -H 'X-Forwarded-Proto: https' http://localhost:8000/ &\ndone\n\n# Concurrent legitimate users receive Set-Cookie with Secure flag\n# Browser refuses to send Secure cookies over HTTP\n# Users' sessions silently break — they appear logged out\n```\n\nThe second attack path works against the default configuration (`use_ssl=False`) and requires no special network position.\n\n## Impact\n\n- **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.\n\n- **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.\n\n- **No authentication required:** Both attack paths are fully unauthenticated — the `before_request` handler fires before any auth checks.\n\n## Recommended Fix\n\nReplace the global config mutation with per-response cookie handling, and add proxy validation:\n\n```python\n# Option A: Set Secure flag per-response instead of mutating global config\n@app.after_request\ndef set_session_cookie_secure(response):\n    # Only trust X-Forwarded-Proto if ProxyFix is configured\n    is_secure = app.config[\"PYLOAD_API\"].get_config_value(\"webui\", \"use_ssl\")\n    if 'Set-Cookie' in response.headers:\n        # Modify cookie flags per-response, not global config\n        cookies = response.headers.getlist('Set-Cookie')\n        response.headers.remove('Set-Cookie')\n        for cookie in cookies:\n            if is_secure and 'Secure' not in cookie:\n                cookie += '; Secure'\n            response.headers.add('Set-Cookie', cookie)\n    return response\n\n# Option B (preferred): Use Werkzeug's ProxyFix with explicit trust\nfrom werkzeug.middleware.proxy_fix import ProxyFix\n\n# In App.__new__, before returning:\nif trusted_proxy_count:  # from config\n    app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=trusted_proxy_count)\n# Then set SESSION_COOKIE_SECURE once at startup based on use_ssl config,\n# and let ProxyFix handle X-Forwarded-Proto transparently\n```\n\nAt 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "pyload-ng"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.5.0b3.dev98"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.5.0b3.dev97"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/pyload/pyload/security/advisories/GHSA-mp82-fmj6-f22v"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-40594"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/pyload/pyload"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pyload-ng/PYSEC-2026-125.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-346" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-16T01:20:49Z",
    "nvd_published_at" : "2026-04-21T18:16:51Z"
  }
}