Information disclosure in Modelcontextprotocol Rust-Sdk
CVE-2026-64684
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.
Vulnerability class: Information Disclosure
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N.
EPSS exploit prediction
EPSS: 0.005 (42.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 42.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: 0.000
Affected products
- Modelcontextprotocol Rust-Sdk — versions < 2.1.0
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-64684?
- CVE-2026-64684 is a medium-severity vulnerability in Modelcontextprotocol Rust-Sdk, classified under Information Disclosure. CVSS score: 6.8/10. Published 2026-09-16.
- How severe is CVE-2026-64684?
- Medium severity. CVSS v3 base score is 6.8 out of 10.