Repository navigation
[Vulnerability] modelcontextprotocol/rust-sdk: Header Validation Bypass #40
Description
Potential Security Vulnerability Detected
Repository: modelcontextprotocol/rust-sdk
Commit: 6677eee
Author: SIDDARTHA REDDY
Date: 2026-09-24T19:36:16Z
Commit Message
style: cargo fmt fixes on validate_standard_headers changes (#1275)
Signed-off-by: SIDDARTHA REDDY <75976672+SIDDARTHAREDDY8@users.noreply.github.com>
Pull Request
PR: #1275 - Fix: streamable-http server: a supplied Mcp-Method contradicting an initialize body is silently accepted
Labels: T-test, T-core, T-transport
Description:
Fixes #1271.
Implemented the fix for modelcontextprotocol/rust-sdk issue #1271 in crates/rmcp/src/transport/streamable_http_server/tower.rs. In validate_standard_headers, the InitializeRequest arm no longer returns Ok(()) blindly: a supplied Mcp-Method header is now checked against the body method (initialize), tolerating absence but rejecting contradiction with a header_mismatch JSON-RPC error (HTTP 400), mirroring the existing validate_header_matches_init_body pattern used for MCP-Protocol-Ve...
Analysis
Vulnerability Type: Header Validation Bypass
Severity: High
Description
The initialize handshake was accepting contradictory Mcp-Method headers without validation, allowing middleboxes that route on headers to be exploited. The patch adds validation to reject supplied Mcp-Method headers that contradict the body method, preventing routing confusion between the header and actual request body that could lead to request smuggling or authentication bypass.
Affected Code
if matches!(&req.request, ClientRequest::InitializeRequest(_)) {
return Ok(());
}
Proof of Concept
POST /mcp HTTP/1.1
Content-Type: application/json
Mcp-Method: tools/list
Mcp-Protocol-Version: 2026-07-28
{"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {"protocolVersion": "2026-07-28", "capabilities": {}, "clientInfo": {"name": "test", "version": "1.0"}}}
Before patch: Returns HTTP 200 and creates session despite contradictory header
After patch: Returns HTTP 400 with error code -32020 (header_mismatch)
This issue was automatically created by Vulnerability Spoiler Alert.
Detected at: 2026-10-06T07:48:26.865Z
Activity