rmcp OAuth client fetches server-controlled resource_metadata URLs

Overview

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:

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:

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:

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 fix

Before accepting or fetching a resource_metadata 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:

fn is_same_origin(base: &Url, candidate: &Url) -> bool {
    base.scheme() == candidate.scheme()
        && base.host() == candidate.host()
        && base.port_or_known_default() == candidate.port_or_known_default()
}

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

Affected packages

• rmcp < 2.0.0

Remediation

Upgrade to a patched release:

• rmcp 2.0.0