vulnerability / NVI-00013257 / Revision 1
rmcp OAuth client fetches server-controlled resource_metadata URLs
GitHub lists this reviewed advisory for rmcp. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
rmcp
source listed< 2.0.0
## Summary The `rmcp` OAuth client accepts a server-controlled `resource_metadata=` 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 resource_metadata="..."` 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 `extract_www_authenticate_params()` accepts an absolute URL from the server-controlled header: ```rust let resource_key = "resource_metadata="; while let Some(pos) = header_lowercase[search_offset
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-c9xm-49cp-xcr9 ↗
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Public revision history
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- Revision 1 ·
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