CVE-2026-42559: CWE-346: Origin Validation Error in modelcontextprotocol rust-sdk
RMCP is an official Rust SDK for the Model Context Protocol. Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface. This vulnerability is fixed in 1.4.0.
AI Analysis
Technical Summary
The vulnerability in the rmcp crate's Streamable HTTP server transport (prior to version 1.4.0) is due to missing validation of the Host header in incoming HTTP requests. This lack of origin validation allows an attacker to perform a DNS rebinding attack, tricking the MCP server into accepting authenticated requests from a malicious website. This can lead to unauthorized access and potentially full compromise of confidentiality, integrity, and availability of the MCP server. The issue is tracked as CWE-346 (Origin Validation Error) and CWE-350 (Missing Verification of Data Authenticity). The vulnerability has a CVSS 3.1 score of 8.8, indicating high severity. The fix is included starting with version 1.4.0 of the rust-sdk.
Potential Impact
Successful exploitation allows a remote attacker controlling a malicious website to bypass origin checks and send authenticated requests to an MCP server on the victim's local or private network interface. This can lead to full compromise of the server's confidentiality, integrity, and availability. There are no known exploits in the wild as of the published date.
Mitigation Recommendations
Upgrade the modelcontextprotocol rust-sdk to version 1.4.0 or later, where the vulnerability is fixed by proper validation of the Host header. Since this is a library component, users should ensure their deployments incorporate the patched version. Patch status is confirmed by the vendor advisory stating the issue is fixed in version 1.4.0.
CVE-2026-42559: CWE-346: Origin Validation Error in modelcontextprotocol rust-sdk
Description
RMCP is an official Rust SDK for the Model Context Protocol. Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface. This vulnerability is fixed in 1.4.0.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in the rmcp crate's Streamable HTTP server transport (prior to version 1.4.0) is due to missing validation of the Host header in incoming HTTP requests. This lack of origin validation allows an attacker to perform a DNS rebinding attack, tricking the MCP server into accepting authenticated requests from a malicious website. This can lead to unauthorized access and potentially full compromise of confidentiality, integrity, and availability of the MCP server. The issue is tracked as CWE-346 (Origin Validation Error) and CWE-350 (Missing Verification of Data Authenticity). The vulnerability has a CVSS 3.1 score of 8.8, indicating high severity. The fix is included starting with version 1.4.0 of the rust-sdk.
Potential Impact
Successful exploitation allows a remote attacker controlling a malicious website to bypass origin checks and send authenticated requests to an MCP server on the victim's local or private network interface. This can lead to full compromise of the server's confidentiality, integrity, and availability. There are no known exploits in the wild as of the published date.
Mitigation Recommendations
Upgrade the modelcontextprotocol rust-sdk to version 1.4.0 or later, where the vulnerability is fixed by proper validation of the Host header. Since this is a library component, users should ensure their deployments incorporate the patched version. Patch status is confirmed by the vendor advisory stating the issue is fixed in version 1.4.0.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-04-28T16:56:50.192Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a05e885ec166c07b0eee255
Added to database: 5/14/2026, 3:21:41 PM
Last enriched: 5/14/2026, 3:37:05 PM
Last updated: 5/15/2026, 5:36:49 AM
Views: 4
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