Open WebUI: Server-side fetches reach blocked and internal hosts via unvalidated HTTP redirect targets

Summary

Open WebUI protects server-side web fetches with two controls: the operator's list of excluded hosts, and a check that refuses private and internal addresses. Neither control was applied to the destination of an HTTP redirect. On a deployment where redirect following is enabled, any authenticated user who can cause the server to fetch a URL could submit a page that redirects, and the server would fetch the redirect destination without either control being applied to it. The server therefore connects to hosts the operator excluded, and to internal addresses including loopback, private networks and cloud metadata endpoints.

Preconditions

AIOHTTP_CLIENT_ALLOW_REDIRECTS must be set to true. Its default is false, and on the default the affected fetch paths do not follow redirects at all, so a deployment that has not changed this setting is not affected.

The attacker needs an authenticated account of any role, with access to any feature that causes the server to fetch a URL. Web search, ingesting a URL into a collection, the built-in page fetch tool and image URLs in chat all reach it. No internal hostnames need to be known, because the loopback and cloud metadata addresses are fixed and identical on every deployment.

Neither ENABLE_LOCAL_WEB_FETCH at its default of false nor any set of entries in WEB_FETCH_FILTER_LIST prevents this.

Impact

An authenticated user can make the server issue requests to hosts the operator deliberately excluded, and to addresses on the internal network including loopback, private ranges and the cloud metadata endpoints of the major providers.

What reaches the attacker depends on which HTTP client the fetch path uses, and is stated here as verified rather than assumed. On the paths built on requests, the fetched body is returned to the caller, so the response of an excluded host is readable: the built-in page fetch tool hands it to the model, and the URL ingestion endpoint returns it in its response. Those paths kept a working private-address check at connection time, so what they reach is excluded public hosts and internal names, not private IP addresses. On the paths built on aiohttp, which are the ones that reach private addresses written as IP literals, fetched content reaches the web search response, a retrievable collection, or model input as base64 through chat image URLs. A path on that client returning an attacker-chosen response verbatim to the requester was not demonstrated.