CVE-2026-48782: pydantic-ai: SSRF blocklist bypass via IPv4-compatible, SIIT/IVI, and local NAT64 IPv6 addresses (incomplete fix of CVE-2026-46678)

Since 1.56.0 Pydantic AI blocks downloads of FileUrl types (ImageUrl, AudioUrl, VideoUrl, DocumentUrl) that point at private or cloud-metadata IPs. Setting force_download='allow-local' turns off the private-IP block, but cloud metadata endpoints were meant to stay blocked. The earlier fix for CVE-2026-46678 decoded IPv4-mapped IPv6, 6to4 and the NAT64 well-known prefix only. An attacker could encode the metadata IP as ::a.b.c.d, in the 64:ff9b:1::/48 prefix, in another NAT64 prefix, or as ISATAP. The wrapped address then reaches the IPv4 metadata endpoint, and the response can expose short-term cloud IAM credentials.

The bar is high, and the advisory says so (CVSS 6.8, medium, attack complexity high). The app must opt into allow-local on an attacker-influenced URL. The network must also route the form: NAT64 for the NAT64 variants, including some IPv6-only or dual-stack Kubernetes setups, or an ISATAP tunnel. A standard dual-stack cloud VM or container does not route them. The IPv4-compatible and Teredo forms are deprecated and fixed as defense in depth.

pydantic-ai and pydantic-ai-slim from 1.56.0 up to but not including 1.102.0, and 2.0.0b1 up to but not including 2.0.0b3. Not affected: apps that download only from developer-controlled URLs, and the bundled Agent.to_web / clai web, VercelAIAdapter and AGUIAdapter / Agent.to_ag_ui, which do not take force_download from external data.

pip show pydantic-ai-slim pydantic-ai

Then search your code for force_download='allow-local'.

Upgrade to 1.102.0, or to 2.0.0b3 on the 2.0 pre-release line.

pip install --upgrade "pydantic-ai-slim>=1.102.0"

The blocklists now decode the embedded IPv4 in every standard IPv6 transition form before checking it: IPv4-mapped, IPv4-compatible, 6to4, NAT64 at all prefix lengths, ISATAP and Teredo. The always-blocked list of cloud metadata endpoints also grew across providers (pull request 5596).

Workaround on older versions: do not use force_download='allow-local' on URLs untrusted input can influence. If you must, resolve the hostname yourself and check the result against your own metadata blocklist, transition forms included, before you build the FileUrl.

Search code for force_download='allow-local', then trace whether the URL value comes from users, documents or tool output. In egress and DNS logs, look for requests from the agent host to IPv6 addresses that embed a metadata IP, such as ::169.254.169.254 or addresses in 64:ff9b:1::/48. Check cloud audit logs for use of the instance role's temporary credentials from unfamiliar addresses.

Summary

When an application using Pydantic AI opts a URL into force_download='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 force_download='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 force_download from external data:

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

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 force_download='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.