OAuth client SSRF filter checks host strings, not resolved addresses

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

not scored

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

From 2.0.0 until 3.7.0, the rmcp OAuth client decided whether it may fetch an authorization server's metadata URL by inspecting only the host string. It rejected literal loopback, private and link-local IP addresses and known cloud metadata hostnames, but it did not resolve other hostnames. A hostname that resolves to an internal address passed the check. A malicious MCP server can return protected-resource metadata that names an authorization server whose hostname resolves to an internal address. The client then sends GET requests to that address during discovery. The same release fixes two related bypasses: - Bracketed IPv6 literals such as `http://[::1]:6379/` were not recognized as internal, because the check kept the brackets from the URL host. - `token_endpoint` and `registration_endpoint` from authorization server metadata were not checked at all, and token requests follow redirects. A malicious authorization server could make the client send POST requests to an internal…

Published by the source

22 Oct 2026, 12:00 UTC

Last modified by the source

9 Nov 2026, 10:00 UTC

Source last checked

10 Nov 2026, 06:45 UTC

1. Revision 1 · created · 10 Nov 2026

   Published from osv record RUSTSEC-2026-0457

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# Patch Next advisory brief: GHSA-5p52-q7pq-9fq2

Advisory: OAuth client SSRF filter checks host strings, not resolved addresses
IDs: GHSA-5p52-q7pq-9fq2
Record: https://patchnext.com/advisories/rustsec-2026-0457 (revision 1, source checked 10 Nov 2026, 06:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rmcp 3.7.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rmcp: affected >= 2.0.0, < 3.7.0; fixed in 3.7.0

## Suggested inspection (read-only)
- crates.io rmcp: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on rmcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/RUSTSEC-2026-0457 (OSV record RUSTSEC-2026-0457 lists affected ranges: crates.io rmcp >=2.0.0 <3.7.0 (fixed))

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