Pydantic AI: SSRF cloud-metadata blocklist bypass via IPv6 zone identifiers

Summary

When an application using Pydantic AI opts a URL into local network access — either a FileUrl with force_download='allow-local', or web_fetch_tool(allow_local_urls=True) — the cloud-metadata blocklist could be bypassed by appending an IPv6 zone identifier to a metadata address (for example fd00:ec2::254%251). The host ignores the zone identifier on a destination that is not link-local and delivers the request to the metadata endpoint anyway, exposing cloud IAM short-term credentials.

This is an incomplete fix of GHSA-cqp8-fcvh-x7r3 / CVE-2026-46678 and GHSA-cg7w-rg45-pc59 / CVE-2026-48782, themselves follow-ups to CVE-2026-25580. The parent advisory's remediation guaranteed that cloud metadata endpoints are always blocked, even with local access allowed. That guarantee did not hold for zone-scoped spellings of the IPv6 metadata endpoints.

Details

The cloud-metadata guard compared IPv6 addresses against its blocklist by set membership. Python includes the zone identifier in IPv6Address equality and hashing, so a zone-scoped spelling of a blocked address did not match, while the network stack ignores the zone identifier for a destination that is not link-local. The private-range checks, and the IPv4 and transition-form metadata checks, were already unaffected, because they compare by network containment and by packed bytes respectively.

Only the IPv6 cloud metadata endpoints were reachable this way, so the issue requires an IPv6-enabled environment — for example AWS EC2 or EKS with IPv6, GCP IPv6-only instances, or Scaleway.

Who Is Affected

You are affected only if your application opts a URL that is, or could be, influenced by untrusted input into local network access, through either:

• a FileUrl (ImageUrl, AudioUrl, VideoUrl, DocumentUrl) with force_download='allow-local'; or
• web_fetch_tool(allow_local_urls=True), where the model chooses the URL.

Both are off by default.

You are not affected through the FileUrl path if you use any of the bundled integrations to ingest user input, because they do not propagate force_download from external data:

• Agent.to_web / clai web
• VercelAIAdapter
• AGUIAdapter / Agent.to_ag_ui

web_fetch_tool is configured by your own application, so a client cannot turn on allow_local_urls.

Applications that only download from developer-controlled URLs are not affected.

Remediation

Upgrade to a patched version. The cloud-metadata and private-IP checks now drop an IPv6 zone identifier before evaluating the address, so every blocklist comparison is made on the address itself. A zone identifier is still carried on the connection, so legitimate link-local fetches under local network access continue to work.

Workaround for Unpatched Versions

Avoid opting into local network access — force_download='allow-local' or web_fetch_tool(allow_local_urls=True) — on any URL that could be influenced by untrusted input. If you must, reject URL hosts containing % before constructing the FileUrl or configuring the tool.

Credits

Reported by @euriconicacio.