CVE-2026-64684: Information Disclosure (CVSS 6.8)

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.

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CVSS v3.1 Score

6.8

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

Weakness Type (CWE)

CWE-200 CWE-200

References

Other References

https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b https://github.com/modelcontextprotocol/rust-sdk/pull/936 https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0 https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9g45-5xwm-f3wc

Frequently Asked Questions

What is 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. It has a CVSS v3.1 base score of 6.8 (MEDIUM).

How severe is CVE-2026-64684? +

CVE-2026-64684 has a CVSS v3.1 score of 6.8 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

How do I check if I'm vulnerable to CVE-2026-64684? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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