CVE-2026-63127

RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

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

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.

CVE-2026-63127 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.0); upgrading removes the vulnerable code path.

Kodem intelligence

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1. Add the resource field to the struct:

   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:

   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
           )));
       }
   }
   

1. What is CVE-2026-63127? CVE-2026-63127 is a high-severity security vulnerability in rmcp (rust), affecting versions < 2.0.0. It is fixed in 2.0.0.
2. How severe is CVE-2026-63127? CVE-2026-63127 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of rmcp are affected by CVE-2026-63127? rmcp (rust) versions < 2.0.0 is affected.
4. Is there a fix for CVE-2026-63127? Yes. CVE-2026-63127 is fixed in 2.0.0. Upgrade to this version or later.
5. Is CVE-2026-63127 exploitable, and should I be worried? Whether CVE-2026-63127 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-63127 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-63127? Upgrade rmcp to 2.0.0 or later.