rmcp OAuth client fetches server-controlled resource_metadata URLs

Exploitation

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

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary The rmcp OAuth client accepts a server-controlled resourcemetadata= URL from the WWW-Authenticate header and fetches it without same-origin or private-network validation. An attacker-controlled MCP server can return a 401 WWW-Authenticate: Bearer resourcemetadata="..." header pointing at an internal URL, including localhost, RFC 1918 addresses, or cloud metadata endpoints. The client then performs an outbound GET to that URL from the victim application's network context. Affected version - Repository: modelcontextprotocol/rust-sdk - Crate: rmcp - Current main reviewed: c330fede90e4729c234f8e87fdbc5ea27a1dd10c - Commit date: 2026-05-19 - Affected file: crates/rmcp/src/transport/auth.rs - File blob: 3aa3e91310662c409af9dc38c9d584d6df217e9c - Severity framing: High/Medium SSRF, depending on the embedding application's network position and whether response parsing gives the attacker useful success/failure or chained fetch behavior. Source evidence extractwwwauthenticateparams()…

Published by the source

2 Oct 2026, 16:12 UTC

Last modified by the source

2 Oct 2026, 16:30 UTC

Source last checked

26 Oct 2026, 15:43 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-c9xm-49cp-xcr9

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-c9xm-49cp-xcr9

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# Patch Next advisory brief: GHSA-c9xm-49cp-xcr9

Advisory: rmcp OAuth client fetches server-controlled resource_metadata URLs
IDs: GHSA-c9xm-49cp-xcr9
Record: https://patchnext.com/advisories/ghsa-c9xm-49cp-xcr9 (revision 3, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rmcp 2.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rmcp: affected < 2.0.0; fixed in 2.0.0

## 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/GHSA-c9xm-49cp-xcr9 (OSV record GHSA-c9xm-49cp-xcr9 lists affected ranges: crates.io rmcp >=0 <2.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-c9xm-49cp-xcr9 (GitHub-reviewed advisory GHSA-c9xm-49cp-xcr9 lists affected ranges: crates.io rmcp >=0 <2.0.0)

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