GHSA-c9xm-49cp-xcr9

rmcp OAuth client fetches server-controlled resource_metadata URLs

Published

2026-10-02

Modified

2026-10-02

Sources

github-advisory

Summary

## 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..].find(resource_key) { let global_pos = search_offset + pos + resource_key.len(); let value_slice = &header[global_pos..]; if let Some((value, consumed)) = Self::parse_next_header_value(value_slice) { if let Ok(url) = Url::parse(&value) { params.resource_metadata_url = Some(url); break; } if let Ok(url) = base_url.join(&value) { params.resource_metadata_url = Some(url); break; } ``` There is no check that the parsed URL shares origin with the original MCP server, and no block for loopback, link-local, RFC 1918, or metadata hostnames. `fetch_resource_metadata_from_url()` then performs the GET: ```rust let response = match self .http_client .get(resource_metadata_url.clone()) .header(HEADER_MCP_PROTOCOL_VERSION, "2024-11-05") .send() .await ``` Again, there is no same-origin, scheme, host, DNS, or IP-range validation before the request. ## Attack scenario 1. A Rust MCP client using `rmcp` connects to an attacker-controlled MCP server, or to a compromised MCP server. 2. The server responds to OAuth discovery with a `401` and a `WWW-Authenticate` header such as: ```text WWW-Authenticate: Bearer resource_metadata="http://169.254.169.254/latest/meta-data/" ``` 3. The client parses the `resource_metadata` value as a URL. 4. The client sends a GET request to that URL from the victim application's network context. This can be used to probe internal services. In environments where the target endpoint returns JSON matching the expected metadata shape, the flow can also chain into additional authorization-server metadata fetches. ## Impact The direct impact is SSRF from any application embedding the rmcp OAuth client. Depending on where the client runs, this can reach: - cloud metadata endpoints; - localhost-only services; - private VPC or container-network services; - internal HTTP APIs not reachable from the attacker. The attacker controls the URL through the MCP server response. The victim only needs to connect to the attacker's MCP server or a compromised server that can emit the crafted `WWW-Authenticate` header. ## Public duplicate check On 2026-05-27: - Visible repository advisories were: - `GHSA-9g45-5xwm-f3wc`: custom HTTP headers leak to cross-origin redirect targets. - `GHSA-89vp-x53w-74fx`: DNS rebinding in Streamable HTTP server transport. - Public issue search for `WWW-Authenticate resource_metadata SSRF` returned no open or closed matches. - Public issue search for `resource_metadata` returned one closed implementation issue, `#517`, about authorization discovery fallback behavior, not SSRF/resource validation. This report is distinct from the redirect-header advisory and the DNS-rebinding advisory. ## Suggested f

Affected packages

Remediation: Upgrade to 2.0.0 or later.

References

• https://github.com/advisories/GHSA-c9xm-49cp-xcr9