fix: decode terminal proxy path until stable to block multi-encoded traversal (#25157)

_sanitize_proxy_path decoded the proxy path once before the '..' check, so a double-encoded payload (%252e%252e) survived the check as %2e%2e and was then re-decoded into '..' by the upstream terminal server, defeating the traversal guard. Decode until stable so no encoded traversal sequence can reach the upstream. Single-encoded payloads were already rejected; this closes the double (and deeper) encoding bypass.

Co-authored-by: sermikr0 <230672901+sermikr0@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/open_webui/routers/terminals.py

@@ -35,7 +35,14 @@ def _sanitize_proxy_path(path: str) -> str | None:
     Trailing slashes are preserved — many upstream frameworks treat
     ``/path`` and ``/path/`` differently.
     """
-    decoded = unquote(path)
+    # Decode until stable: a single unquote pass leaves %252e%252e as %2e%2e,
+    # which the upstream then re-decodes into '..', bypassing the check below.
+    decoded = path
+    for _ in range(8):
+        once = unquote(decoded)
+        if once == decoded:
+            break
+        decoded = once
     had_trailing_slash = decoded.endswith('/')
     normalized = posixpath.normpath(decoded)
     # Remove any leading slashes that would reset the base