_sanitize_proxy_path decodes the path and then relies on posixpath.normpath plus a leading '..' check. posixpath splits on '/' only, so a backslash run is treated as part of a single path component: 'foo/..\..\etc' normalizes to itself, does not start with '..' and is forwarded unchanged, reaching the upstream as '/foo/..%5C..%5Cetc'. An upstream that treats the backslash as a separator would resolve those '..' sequences.
Reject any path containing a backslash after decoding, matching the existing fail-closed behaviour for paths that are still encoded past the decode cap. A backslash is not meaningful in the upstream API paths this route proxies, so legitimate requests are unaffected.
Co-authored-by: babakizo420 <babakizo420@users.noreply.github.com>
