GHSA-c9xm-49cp-xcr9: SSRF

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() accepts an absolute URL from the server-controlled header:

rust let resourcekey = "resourcemetadata="; while let Some(pos) = headerlowercase[searchoffset..].find(resourcekey) { let globalpos = searchoffset + pos + resourcekey.len(); let valueslice = &header[globalpos..]; if let Some((value, consumed)) = Self::parsenextheadervalue(valueslice) { if let Ok(url) = Url::parse(&value) { params.resourcemetadataurl = Some(url); break; } if let Ok(url) = baseurl.join(&value) { params.resourcemetadataurl = 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.

fetchresourcemetadatafromurl() then performs the GET:

rust let response = match self .httpclient .get(resourcemetadataurl.clone()) .header(HEADERMCPPROTOCOLVERSION, "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 resourcemetadata="http://169.254.169.254/latest/meta-data/"

3. The client parses the resourcemetadata 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 resourcemetadata SSRF returned no open or closed matches. - Public issue search for resourcemetadata 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 fix

Before accepting or fetching a resourcemetadata URL:

- require the URL to be same-origin with the original MCP server URL, unless the spec intentionally permits a narrow trusted exception; - reject loopback, link-local, RFC 1918, unique-local IPv6, and known cloud metadata hostnames; - reject non-HTTP(S) schemes; - apply the same validation to redirect targets and any follow-on metadata URLs.

A minimal same-origin check would compare scheme, host, and effective port:

rust fn issameorigin(base: &Url, candidate: &Url) -> bool { base.scheme() == candidate.scheme() && base.host() == candidate.host() && base.portorknowndefault() == candidate.portorknowndefault() }

If cross-origin resourcemetadata is required for compatibility, it should be explicitly opted into and protected by private-network blocking.