CVE-2026-63127: RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

RFC 9728 says a client must confirm that the resource value in the metadata equals the server URL it used, and must discard the metadata if not. In crates/rmcp/src/transport/auth.rs, the ResourceServerMetadata struct has no resource field, and discover_oauth_server_via_resource_metadata() uses whatever it fetches.

The attack in the advisory: the attacker runs fake-mcp.com/mcp and serves metadata at /.well-known/oauth-protected-resource that names real-mcp.com/mcp as the resource and lists the real server's authorization servers. A victim points an rmcp-based client at the fake server. The client starts OAuth with the real authorization server, and the victim sees a genuine consent prompt and approves. The token, valid for the real server, is then sent to the fake server on later requests. The attacker can replay it to impersonate the victim.

The attacker needs the victim to connect to a server they control, which is user interaction in the CVSS vector. Any agent or tool built on rmcp that uses OAuth-protected MCP servers is affected. The consent screen is real, so users have no visible warning.

Affected: the Rust crate rmcp before 2.0.0, for clients that use its OAuth transport (transport/auth.rs) with OAuth-protected MCP servers. Check which version your project resolves:

cargo tree -i rmcp

Check Cargo.lock for the locked rmcp version too.

Upgrade to rmcp 2.0.0.

cargo update -p rmcp

If the lockfile stays below 2.0.0, raise the version requirement in Cargo.toml to 2.0.0. The advisory's recommended fix adds the resource field to the metadata struct and returns an error when it does not match the configured base URL. The sources name no workaround. Until you upgrade, only connect to MCP servers you trust, and revoke tokens issued during sessions with unknown servers.

Review which MCP server URLs your rmcp clients were configured with, looking for hosts you do not own whose OAuth metadata names a different resource. Fetch /.well-known/oauth-protected-resource from each and compare its resource value with the server URL. At the authorization server, look for token grants followed by use of the token from unexpected IP addresses. After upgrading, a mismatch produces a MetadataError reading "Resource metadata mismatch" if the fix follows the advisory's suggestion.

Summary

The rmcp library does not validate the resource parameter in OAuth Protected Resource metadata (RFC 9728), allowing a malicious MCP server to redirect OAuth flows to a legitimate authorization server and steal the resulting access tokens.

Details

RFC 9728 specifies two MUST requirements for resource parameter validation:

• Section 7.3: the client MUST ensure that the resource identifier URL it is using as the prefix for the metadata request exactly matches the resource value in the returned metadata document.
• Section 3.3: if the resource value returned is not identical to the URL the client used, the data MUST NOT be used.

In the current implementation (crates/rmcp/src/transport/auth.rs), the ResourceServerMetadata struct (lines 390–394) does not include a resource field:

struct ResourceServerMetadata {
    authorization_server: Option<String>,
    authorization_servers: Option<Vec<String>>,
    scopes_supported: Option<Vec<String>>,
}

And discover_oauth_server_via_resource_metadata() (lines 1446–1465) proceeds without any resource URL validation.

Recommended fix

1. Add the resource field to the struct: ``rust struct ResourceServerMetadata { resource: Option<String>, // RFC 9728 REQUIRED field authorization_server: Option<String>, authorization_servers: Option<Vec<String>>, scopes_supported: Option<Vec<String>>, } ``
2. Add validation logic after fetching metadata: ```rust let Some(resource_metadata) = self .fetch_resource_metadata_from_url(&resource_metadata_url) .await? else { return Ok(None); };

// RFC 9728: validate that the resource identifier matches our target server if let Some(resource) = &resource_metadata.resource { if resource.trim_end_matches('/') != self.base_url.as_str().trim_end_matches('/') { return Err(AuthError::MetadataError(format!( "Resource metadata mismatch: expected '{}', got '{}'", self.base_url, resource ))); } }

### PoC

1. Attacker sets up a malicious MCP server at `fake-mcp.com/mcp`. 
2. At `fake-mcp.com/mcp/.well-known/oauth-protected-resource`, the attacker serves metadata declaring:
    - resource: `real-mcp.com/mcp` (the legitimate server)
    - authorization_servers: the legitimate authorization server(s) of `real-mcp.com/mcp`


3. Victim configures any MCP client using `rmcp` to connect to `fake-mcp.com/mcp`.

4. `rmcp` fetches the protected resource metadata and, without validating that the `resource` field (`real-mcp.com/mcp`) differs from the configured server (`fake-mcp.com/mcp`), initiates an OAuth flow with the legitimate authorization server.
5. The victim sees a legitimate authorization prompt and completes the flow.
6. The resulting access token — valid for `real-mcp.com/mcp` — is sent to `fake-mcp.com/mcp` in subsequent requests.
7. The attacker captures the token and can impersonate the victim on `real-mcp.com/mcp`.

### Impact
This is an access token theft vulnerability via OAuth resource metadata spoofing. All MCP clients built on `rmcp` that rely on OAuth-protected MCP servers are affected. An attacker who tricks a user into connecting to a malicious MCP server can steal valid access tokens for any legitimate MCP server, enabling full impersonation of the victim.

#### Credit
Jian Cui, Minsun Shim, Zhou Li, Xiaojing Liao
University of Illinois Urbana-Champaign (UIUC)
University of California, Irvine (UCI)