CVE-2026-64684 modelcontextprotocol CVE debrief

Who should care

Defenders who use the RMCP crate in their applications should assess their exposure to potentially malicious or compromised MCP endpoints and review their usage of custom headers in the StreamableHttpClientTransportConfig.

Why it matters

CVE-2026-64684 is a vulnerability in the Rust SDK for the Model Context Protocol (RMCP) that can lead to the replay of custom API keys or authentication tokens to a new origin. Defenders should prioritize verifying the affected versions of the RMCP crate and upgrading to version 2.1.0 or later. They should also assess their exposure to potentially malicious or compromised MCP endpoints and review their usage of custom headers in the StreamableHttpClientTransportConfig.

• Potential replay of custom API keys or authentication tokens to a new origin.
• Possible unauthorized access to sensitive information.
• Need to verify the affected versions of the RMCP crate and upgrade to version 2.1.0 or later.
• Review and update usage of custom headers in the StreamableHttpClientTransportConfig.

Technical summary

The StreamableHttpClientTransport in the rmcp crate uses reqwest's automatic redirect policy and applies custom headers without marking them as sensitive. This can lead to the replay of custom API keys or authentication tokens to a new origin when a malicious or compromised MCP endpoint returns a cross-origin 307 or 308 redirect. Defenders should prioritize verifying the affected versions of the RMCP crate and upgrading to version 2.1.0 or later. They should also assess their exposure to potentially malicious or compromised MCP endpoints and review their usage of custom headers in the StreamableHttpClientTransportConfig. The CVE record and NVD vulnerability detail page provide information on the vulnerability,

Defensive priority

Defenders should prioritize verifying the affected versions of the RMCP crate and upgrading to version 2.1.0 or later. They should also assess their exposure to potentially malicious or compromised MCP endpoints and review their usage of custom headers in the StreamableHttpClientTransportConfig.

Recommended defensive actions

• Verify the version of the RMCP crate being used and upgrade to version 2.1.0 or later if necessary.
• Assess exposure to potentially malicious or compromised MCP endpoints.
• Review usage of custom headers in the StreamableHttpClientTransportConfig.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.

Evidence notes

The CVE record and NVD vulnerability detail page provide information on the vulnerability, including its description, CVSS score, and affected versions. The GitHub commits, pull requests, and release notes provide additional context on the fix.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.