Open WebUI: `WEB_FETCH_FILTER_LIST` host allow/block filter bypassable via URL path and non-label-boundary matching

Summary

The administrator-configured WEB_FETCH_FILTER_LIST (the allow/block list applied to server-side web fetches: RAG URL ingestion, URL-to-markdown, web-search content fetch) matches hostnames incorrectly, so the filter can be bypassed.

Details

is_string_allowed (backend/open_webui/utils/misc.py) matches with str.endswith(...), and the primary web-fetch call site (backend/open_webui/retrieval/web/utils.py) called it with the full URL string, not the hostname:

• Blocklist bypass via path. A blocklist entry !internal.example.com only matches a URL that ends with that string. Any URL with a path (https://internal.example.com/x) ends with /x, so the entry never matches and the fetch proceeds. The blocklist effectively only stopped path-less URLs.
• Allowlist false-reject and bypass. An allowlist company.com rejected the legitimate https://api.company.com/status and admitted https://attacker.example/path/company.com.
• Non-label-boundary matching at the hostname-shaped call site (retrieval/web/main.py): endswith('corp.com') also matched evilcorp.com, and 10.0.0.1 matched 110.0.0.1.

Impact

An authenticated user able to trigger a server-side web fetch can reach hosts the administrator intended to block with WEB_FETCH_FILTER_LIST.

Open WebUI's primary SSRF protection is a separate, always-on guard that rejects any URL resolving to a non-global IP (validate_url and the connection-layer _ssrf_safe_new_conn, active whenever ENABLE_RAG_LOCAL_WEB_FETCH is off, the default). That guard is unaffected by this issue and continues to block loopback, RFC1918 and link-local addresses, including the 169.254.169.254 cloud-metadata endpoint. This bypass therefore does not grant access to those internal targets. What it defeats is the administrator's ability to block specific publicly-resolvable hosts (internal services reachable from the server over a public IP, e.g. split-horizon DNS or internal PaaS endpoints) and to enforce an allowlist. Fetched content is returned to the requester, so for hosts reachable from the server's network position this is a read/content-disclosure SSRF against the admin-blocked host.

Patch

Matching is now performed on the parsed hostname using DNS label boundaries. A dedicated is_host_allowed(host, ...) matches an entry only when host and entry are equal or the entry is a parent domain (host == entry or host.endswith('.' + entry)), so corp.com matches api.corp.com but not evilcorp.com, and IP entries match only the identical address. Both web-fetch call sites pass the parsed hostname rather than the full URL. The generic is_string_allowed is retained unchanged for unrelated non-host filters.

Credit

Reported by @addcontent.