CVE-2026-63127 RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery
A malicious MCP server can trick clients into sending valid access tokens to it, allowing impersonation of the victim against legitimate resources.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-63127?
CVE-2026-63127 is a high-severity authentication bypass vulnerability in modelcontextprotocol rust-sdk, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.0.0, the rmcp crate's OAuth implementation in crates/rmcp/src/transport/auth.rs omits the RFC 9728 resource field from ResourceServerMetadata and allows discover_oauth_server_via_resource_metadata to use protected-resource metadata without confirming that the returned resource identifier exactly matches the configured MCP server. A malicious MCP server can publish metadata for a different legitimate MCP resource and its authorization server, causing a victim who connects and completes the authorization flow to obtain a legitimate access token that the client subsequently sends to the malicious server. The attacker can capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes. This issue is fixed in version 2.0.0.
Weakness: CWE-345
Which versions are affected by CVE-2026-63127?
2 open source packages ship an affected version, including rmcp.
| Package | Affected | Fixed in |
|---|---|---|
rmcp crates.io | All versions | 2.0.0 |
rmcp crates.io | All versions | 2.0.0 |
How do I fix CVE-2026-63127?
Upgrade rmcp to 2.0.0 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-63127 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63127 exists.
Sources
Also known as GHSA-33f5-2c5q-wgwj.