Duplicated standard MCP headers: only the first value is checked #1321

HttpServletMcpTransport.validateHeaders reads Mcp-Method, Mcp-Name and MCP-Protocol-Version with HttpServletRequest#getHeader, which returns only the first value. Any further values of the same header are ignored.

Impact

SEP-2243 headers exist so intermediaries (gateways, load balancers, WAFs) can route and apply policy without parsing the body. Intermediaries disagree on which copy of a duplicated header counts: some take the first, some the last, some join them with , . A request such as:

Mcp-Method: tools/list
Mcp-Method: tools/call

with a tools/call body can be routed or authorized by a last-value proxy as tools/list, then accepted by the server, which only looks at the first value. The same applies to Mcp-Name (e.g. a policy of the form "allow tool X").

Expected behaviour

A standard header with more than one value is malformed. The server should reject the request with -32020 (HeaderMismatch) and HTTP 400. This applies to repeated header lines and to a single comma-joined value alike.

• RFC 9110 §5.3: repeated header lines are equivalent to one comma-joined value. These headers hold a single value, not a list, so a joined value is invalid.
• The spec lists "a header value contains invalid characters" as a validation failure.
• This is already how Mcp-Param-{Name} headers are handled (McpHeaderDecoder.paramHeaders).

Proposed fix

Read the headers with getHeaders(name) and require exactly one value for Mcp-Method, Mcp-Name and MCP-Protocol-Version. Add tests for duplicated values of each.