PYSEC-2026-2751

Summary

The terminal-server reverse proxy in backend/open_webui/routers/terminals.py does not fully confine the user-controlled path segment before forwarding it to an admin-configured terminal server. An authenticated user who has been granted access to a terminal server can craft path values containing encoded ../ traversal sequences that escape the intended path (or policy) scope on that server, reaching unintended endpoints and files on the terminal-server host. Where the terminal server fans requests out to internal services, this also gives SSRF-style reach into those services.

This is a separate code path from the /api/v1/retrieval/process/web SSRF (GHSA-c6xv-rcvw-v685), with its own input. Two distinct vectors are consolidated here:

1. Raw path forwarding / single-encoded traversal (original report).
2. A bypass of the subsequently-added _sanitize_proxy_path mitigation using double-encoded dots (%252e%252e).

The attacker-controlled input is the request path, supplied by the non-admin user, not anything an administrator configures, so this is not an admin-trust / Rule-9 situation.

Affected code

The proxy route forwards an arbitrary trailing path to the configured terminal server:

# routers/terminals.py
@router.api_route('/{server_id}/{path:path}', methods=PROXY_METHODS)
async def proxy_terminal(server_id, path, request, user=Depends(get_verified_user)):
    ...
    safe_path = _sanitize_proxy_path(path)
    if safe_path is None:
        return JSONResponse({'error': 'Invalid path'}, status_code=400)
    target_url = f'{base_url}/{safe_path}'
    policy_id = connection.get('policy_id')
    if policy_id:
        target_url = f'{base_url}/p/{policy_id}/{safe_path}'

Access requires has_connection_access(user, connection, ...), i.e. a non-admin user the administrator has granted to that terminal server.

Vector 1 — single-encoded traversal (original)

The path was originally concatenated to the base URL with no sanitization (target_url = f"{base_url}/{path}"), so single-encoded traversal escaped the intended scope:

GET /api/v1/terminals/server1/..%2F..%2F..%2Finternal-api/secrets
# proxied to: {base_url}/../../../internal-api/secrets

This vector is closed at HEAD: _sanitize_proxy_path now URL-decodes once, runs posixpath.normpath, strips leading slashes, and rejects results beginning with .. (unquote('..%2F..%2F') -> '../../' -> normpath -> '../..' -> rejected).

Vector 2 — double-encoded bypass of _sanitize_proxy_path

_sanitize_proxy_path decodes the path only once before the .. check, so a double-encoded payload survives:

def _sanitize_proxy_path(path: str) -> str | None:
    decoded = unquote(path)                 # single decode pass only
    normalized = posixpath.normpath(decoded)
    cleaned = normalized.lstrip('/')
    if cleaned.startswith('..') or cleaned == '.':
        return None
    ...

unquote('%252e%252e/secret') yields %2e%2e/secret (not ..), which normpath leaves unchanged and which does not start with .., so it passes the check. The proxy then forwards {base_url}/%2e%2e/secret, and the upstream terminal server decodes %2e%2e into .. and resolves the traversal the check was meant to prevent.

GET /api/v1/terminals/server1/%252e%252e/%252e%252e/sensitive-file
# passes _sanitize_proxy_path as %2e%2e/%2e%2e/sensitive-file
# upstream decodes -> ../../sensitive-file

The policy_id form ({base_url}/p/{policy_id}/{safe_path}) is the higher-impact target: traversal escapes the policy namespace and reaches other policies or the terminal-server root.

Impact

An authenticated user with access to a terminal server can escape the intended path/policy scope on that server, reaching unintended endpoints and files, and, where the terminal server routes onward to internal services, reach those services. CWE-22 (Path Traversal) and CWE-918 (SSRF).

Fix

Decode the proxy path until it is stable before normalising and checking, so no depth of encoding can smuggle a traversal sequence past the check to be re-decoded upstream:

decoded = path
for _ in range(8):
    once = unquote(decoded)
    if once == decoded:
        break
    decoded = once
normalized = posixpath.normpath(decoded)
cleaned = normalized.lstrip('/')
if cleaned.startswith('..') or cleaned == '.':
    return None

This rejects %2e%2e, %252e%252e, %25252e%25252e, ..%2f..%2f, etc., while leaving legitimate paths (including singly-encoded characters such as %20) intact.

Credits

• Tulgaaaaaaaa — original report (terminal-proxy path SSRF / single-encoded traversal).
• sermikr0 — double-encoded (%252e%252e) bypass of the _sanitize_proxy_path mitigation.