CVE-2026-64684mediumCWE-200

CVE-2026-64684: medium-severity vulnerability in modelcontextprotocol rust-sdk

RMCP: Custom HTTP headers leak to cross-origin redirect targets

Published · Updated

Follow this CVE+ Monitor rust-sdk+ Monitor rmcp

E-mail alert when it lands in KEV, gets a public PoC or its score goes up.

13Vexday Risk Score

No sign of exploitation. No public exploitation artifact known so far.

ssvc Trackcvss 6.8epss 0.5%
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.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
Affected products
modelcontextprotocol · rust-sdk
Related CVEs — modelcontextprotocol rust-sdk

In the same product, most dangerous first.

CVE-2026-63128HIGHRMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transport leads to remote denial-of-serviceEPSS 0.5%CVE-2026-42559HIGHRMCP: DNS rebinding vulnerability in rmcp Streamable HTTP server transportEPSS 0.3%CVE-2026-63127HIGHRMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata DiscoveryEPSS 0.2%