Summary

When an application using Pydantic AI opts a URL into forcedownload='allow-local' (which disables the default block on private/internal IPs) and runs on a network that routes the affected IPv6 transition forms (NAT64- or ISATAP-configured networks), the cloud-metadata blocklist could be bypassed by encoding the metadata IP in an IPv6 transition form that the previous fix did not decode — IPv4-compatible IPv6 (::a.b.c.d), the NAT64 RFC 8215 local-use prefix (64:ff9b:1::/48), operator-chosen NAT64 prefixes, or ISATAP. The IPv6 wrapper is then delivered to the underlying IPv4 metadata endpoint, exposing cloud IAM short-term credentials.

The bypass is exploitable only in environments whose network actually routes these forms — NAT64-configured networks (IPv6-only or dual-stack-with-NAT64 deployments, including some Kubernetes setups) for the NAT64 variants, or networks with an ISATAP tunnel for ISATAP. A standard dual-stack cloud VM or container does not route them and is not affected in practice. The IPv4-compatible and Teredo variants are deprecated and addressed as defense-in-depth.

This is an incomplete fix of GHSA-cqp8-fcvh-x7r3 / CVE-2026-46678 (itself a follow-up to CVE-2026-25580). The prior remediation decoded only IPv4-mapped IPv6, 6to4, and the NAT64 well-known prefix; the metadata guarantee did not hold for the remaining transition forms.

Severity

MEDIUM — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N = 6.8

Same impact metrics and narrow attack surface as the parent advisory (AC:H): exploitation requires the application to have opted into allow-local on a URL influenced by untrusted input, and the NAT64/ISATAP variants additionally require the deployment network to route those forms.

CWE-918: Server-Side Request Forgery (SSRF)

Affected Versions

| Package | Vulnerable | Patched | |---|---|---| | pydantic-ai | >= 1.56.0, < 1.102.0; >= 2.0.0b1, < 2.0.0b3 | 1.102.0; 2.0.0b3 | | pydantic-ai-slim | >= 1.56.0, < 1.102.0; >= 2.0.0b1, < 2.0.0b3 | 1.102.0; 2.0.0b3 |

These transition forms have not been decoded since SSRF protection was introduced in 1.56.0.

Who Is Affected

Users are affected only if their application explicitly opts a FileUrl (ImageUrl, AudioUrl, VideoUrl, DocumentUrl) into forcedownload='allow-local' on a URL that is, or could be, influenced by untrusted input.

Beyond that precondition, the affected encodings only reach a metadata endpoint in environments whose network actually routes them. The broadly-routable IPv4-mapped form was addressed in 1.99.0 (CVE-2026-46678); the additional forms addressed here require a NAT64-configured network (IPv6-only or dual-stack-with-NAT64 deployments, including some Kubernetes setups) for the NAT64 variants, or an ISATAP tunnel for the ISATAP variant. The IPv4-compatible and Teredo forms are deprecated and not routed by modern stacks; they are addressed as defense-in-depth. Most deployments on a standard dual-stack cloud VM or container are therefore not exploitable in practice, but the fix restores the "always blocked" guarantee for the environments that are.

Users are not affected if they use any of the bundled integrations to ingest user input, because they do not propagate forcedownload from external data:

- Agent.toweb / clai web - VercelAIAdapter - AGUIAdapter / Agent.toagui

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

Remediation

Upgrade to 1.102.0 or later (or 2.0.0b3 or later on the 2.0 pre-release line). The cloud-metadata and private-IP blocklists now decode the embedded IPv4 of every standardized IPv6 transition form before evaluating it — IPv4-mapped, IPv4-compatible, 6to4, NAT64 across all prefix lengths (including the RFC 8215 local-use prefix and operator-chosen prefixes), ISATAP, and Teredo. The set of always-blocked cloud metadata/credential endpoints has also been expanded across providers.

Workaround for Unpatched Versions

Avoid passing forcedownload='allow-local' on any URL that could be influenced by untrusted input. If developers must, resolve the hostname themselves and validate the result against their own metadata blocklist — including IPv6 transition forms — before constructing the FileUrl.

Credits

Reported by @SnailSploit.

Summary

A client that can submit message history to a Pydantic AI UI adapter can reference arbitrary files in the application's model-provider or cloud-storage account. The server forwards the reference to the model provider, which fetches it using the server's own credentials, allowing the client to read files it should not have access to.

Details

UI adapters reconstruct file parts from client-submitted message history and forward them to the model provider. File URL parts are validated against a scheme allowlist before being forwarded, but UploadedFile references — which point to a file by provider file ID or cloud-storage URI (e.g. s3://…, gs://…) — were forwarded without validation.

Because the provider resolves an UploadedFile using the server-side identity (IAM role, service account, or provider API key) rather than the client's, a client that crafts message history containing an attacker-chosen UploadedFile can cause the server to read objects belonging to its own account or to other tenants, given a referenceable identifier.

Impact

Applications that pass untrusted client-submitted message history to an agent through a UI adapter (such as the Vercel AI adapter). Exploitation requires the attacker to reference a valid file identifier; depending on how the application names objects, such identifiers are not always unguessable.

Patches

Upgrade to 1.106.0 (1.x) or 2.0.0b6 (the 2.x beta line), which validate UploadedFile references on client-submitted messages the same way file URLs are validated.

Workarounds

If users cannot upgrade, do not pass untrusted client-submitted message history to the agent, or strip UploadedFile parts from incoming messages before running the agent.