CVE-2026-64684 RMCP: Custom HTTP headers leak to cross-origin redirect targets
Attackers can capture sensitive API keys and authentication tokens when the Rust MCP SDK follows cross-origin redirects, exposing credentials to malicious endpoints.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-64684?
CVE-2026-64684 is a medium-severity vulnerability in modelcontextprotocol rust-sdk, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.1.0, the rmcp crate's StreamableHttpClientTransport in crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs builds its default_http_client with reqwest's automatic redirect policy and applies caller-supplied values from StreamableHttpClientTransportConfig.custom_headers without marking them as sensitive. When a malicious or compromised MCP endpoint returns a cross-origin 307 or 308 redirect, reqwest follows the redirect and apply_custom_headers causes custom API keys or authentication tokens to be replayed to the new origin, where an attacker can capture and reuse them. The separate auth_header path is not affected because it uses the standard Authorization header, which reqwest strips on cross-origin redirects. This issue is fixed in version 2.1.0.
Weakness: CWE-200
Which versions are affected by CVE-2026-64684?
2 open source packages ship an affected version, including rmcp.
| Package | Affected | Fixed in |
|---|---|---|
rmcp crates.io | All versions | 2.1.0 |
rmcp crates.io | All versions | 2.1.0 |
How do I fix CVE-2026-64684?
Upgrade rmcp to 2.1.0 or later.
Is CVE-2026-64684 being exploited?
There is no known exploitation of CVE-2026-64684 in the wild and no public exploit code.
Sources
Also known as GHSA-9g45-5xwm-f3wc.