Fail closed when the proxy/redirect path decode cap is exceeded (#26050)

The decode-until-stable loops in _sanitize_proxy_path (terminals.py, cap 8) and
_safe_static_redirect_path (models.py, cap 2) proceed with whatever remains after the
cap instead of rejecting it. A path encoded more times than the cap therefore exits the
loop still percent-encoded, passes the literal '..' / prefix checks (the dots are still
%2E, not '..'), and is forwarded to the upstream terminal server, or emitted as a
redirect Location, which then decodes it once more and resolves the traversal. This is
the residual of the decode-until-stable hardening added for CVE-2026-54017: the cap is a
fixed depth, not a true stability guarantee.

Reject when the value is still not stable after the cap (unquote(x) != x), so anything
encoded more deeply than the cap fails closed rather than being forwarded. Legitimate
paths stabilize within a pass or two and are unaffected.

Co-authored-by: DavidCarliez <271374756+DavidCarliez@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/open_webui/routers/models.py

@@ -60,6 +60,9 @@ def _safe_static_redirect_path(url: str) -> str | None:
         if decoded == path:
             break
         path = decoded
+    # Fail closed: a value still encoded after the cap would be decoded further downstream.
+    if unquote(path) != path:
+        return None
     if '\x00' in path or '\\' in path:
         return None
     if not path.startswith('/'):

backend/open_webui/routers/terminals.py

@@ -43,6 +43,9 @@ def _sanitize_proxy_path(path: str) -> str | None:
         if once == decoded:
             break
         decoded = once
+    # Fail closed: still encoded after the cap means the upstream would decode further into traversal.
+    if unquote(decoded) != decoded:
+        return None
     had_trailing_slash = decoded.endswith('/')
     normalized = posixpath.normpath(decoded)
     # Remove any leading slashes that would reset the base