Open WebUI: DNS Rebinding SSRF Bypass

Summary

Open WebUI vetted user-supplied URLs by resolving the hostname once and rejecting private, loopback and link-local addresses, then let the HTTP client resolve that hostname again at connect time. An attacker who controls the authoritative DNS for a hostname they submit can answer with a public address during the check and an internal one at connect, so the fetch reaches an address the check was meant to block. Every user-reachable fetch gated by that check was affected, and most of them hand the internal response back to the attacker.

Preconditions

• An account on the instance. No admin rights and no non-default configuration.
• Control of the authoritative DNS for a hostname the attacker submits, serving a TTL of 0 and alternating answers.
• One of the affected entry points: URL ingest for retrieval, an image_url in a chat completion, image editing, or the OAuth profile-picture fetch.
• The OAuth path additionally needs OAuth login configured and a picture claim (OAUTH_PICTURE_CLAIM, default picture) the user can influence, which is the case on self-service OIDC providers and providers with a user-editable avatar URL. On an existing account it also needs OAUTH_UPDATE_PICTURE_ON_LOGIN, which is off by default. Deployments without OAuth are not affected on that path; the other paths need no configuration at all.

Impact

The server can be made to issue requests to addresses only it can reach: cloud instance metadata such as 169.254.169.254, loopback-bound admin APIs, and internal network services. The response comes back to the attacker on most paths, as document content on the retrieval path, described by the vision model on the chat image path, and base64-encoded into the profile picture on the OAuth path; the image-edit path is blind. On the OAuth path the server also forwards the OAuth access token as a Bearer header to the fetched URL, so a rebind hands that token to the internal target. On a cloud host with IMDSv1 reachable this is enough to take instance IAM credentials.