PYSEC-2026-3645 
## Summary

Open WebUI fetches user-supplied URLs on the server for RAG URL ingestion, URL-to-markdown conversion and web-search content retrieval, and decides whether a destination is allowed by asking whether its IP address is globally routable. That test operates on the literal IPv6 address and does not look at the IPv4 address embedded inside it. On a deployment whose network has a NAT64 gateway, any verified user can wrap an internal or cloud-metadata IPv4 address in the NAT64 well-known prefix, pass the filter, and receive the internal response body back through the API.

## Preconditions

- Any verified (authenticated) user account. No admin role, no elevated permission.
- Default configuration: `ENABLE_LOCAL_WEB_FETCH` off, the default `WEB_FETCH_FILTER_LIST` metadata blocklist in place. Neither prevents this, because the blocklist matches hostname strings and the NAT64 literal is not one of them.
- The deployment's network must provide NAT64 translation for the well-known `64:ff9b::/96` prefix, which is the common default on IPv6-only and dual-stack cloud and Kubernetes networks.
- Deployments on IPv4-only networks, or on any network without a NAT64 gateway, are not affected: the address has nowhere to route.

## Impact

On an affected network a low-privilege user can read GET responses from services the server can reach but the internet cannot: cloud instance metadata including IAM role credentials, loopback-bound admin surfaces, and internal APIs in the same VPC or cluster. The response body is returned to the caller, so this is full-read, not blind. Exploitation is not universal, it depends entirely on the deployment's network providing NAT64 translation, which is why the score carries high attack complexity. Deployments without NAT64 lose nothing here.

## Fix

Fixed in v0.11.0 by commit `1717b493d`. Address classification now unwraps the IPv4 embedded in IPv6 transition encodings before deciding whether a destination is global, and applies that at all three checkpoints. NAT64-wrapped public destinations continue to work. Upgrading to v0.11.0 fully resolves the issue with no configuration change.

## Root cause

- `backend/open_webui/retrieval/web/utils.py` — `validate_url()`, the pre-fetch check on the submitted URL.
- `backend/open_webui/retrieval/web/utils.py` — `_ssrf_safe_new_conn()` and `_SSRFSafeResolver`, the connect-time re-checks that defeat DNS rebinding.

All three decided reachability from `ipaddress.ip_address(ip).is_global` applied to the literal address. That predicate answers whether an IPv6 address sits in globally-routable space, which is a different question from where the packet actually ends up once a transition gateway translates it. The NAT64 well-known prefix is by design a global prefix carrying an arbitrary IPv4 destination, so an internal target wrapped in it satisfies the check while reaching exactly what the check exists to prevent. Because the same predicate backed the connect-time re-checks, no later layer caught it either. The fix inspects every standardized transition encoding rather than only the NAT64 prefix, since the same reasoning error applies to each of them.

## Proof of concept

Against the real `POST /api/v1/retrieval/process/web` flow on v0.10.2 as an authenticated user, with internal HTTP services returning a marker string. The plain forms are rejected with HTTP 400:

```
http://169.254.169.254/latest/meta-data/          -> 400
http://127.0.0.1/                                 -> 400
http://[::ffff:169.254.169.254]/                  -> 400
http://metadata.google.internal/                  -> 400
```

The NAT64 encodings of the same targets are accepted, and the response body is returned in the `content` field:

```
http://[64:ff9b::a9fe:a9fe]/latest/meta-data/iam/security-credentials/   -> 200, marker returned
http://[64:ff9b::7f00:1]/admin/internal-status                          -> 200, marker returned
```

NAT64 translation was modelled by binding the translated addresses locally rather than by routing through a real NAT64 gateway; everything else, including the request flow and the validation code, is the unmodified v0.10.2 path. After the fix both URLs return 400 while `http://[64:ff9b::808:808]/` (8.8.8.8, public) still returns 200, confirming no over-blocking.

## Credits

- tonghuaroot — reported the transition-form gap in the address classification and supplied the fix approach.
CVSS v3.1 score: 7.1