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mcp: verify 'Origin' and 'Content-Type' headers (#842)

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This PR introduces protection against cross-origin requests by
installing `http.CrossOriginProtection`.
By default, the zero value is used which does not contain any trusted
origins or bypass patterns.
The protection can be customized by providing a custom
`http.CrossOriginProtection` in `mcp.StreamableHTTPOptions`. Currently,
the deny handler set on the protection is ignored.
This default has an MCPGODEBUG option (`disableoriginverification`) to
disable the default protection.
It will be available until `v1.6.0` of the SDK.
We also increase the Go version required by the SDK to 1.25, since this
is the version that introduced `http.CrossOriginProtection`. This is in
line with the SDK's policy of supporting two newest Go versions (1.26
was released in February 2026).

Additionally, we start validating if the `Content-Type` header for
`POST` requests is set to `application/json` to avoid accepting
CORS-safelisted requests.
This commit is contained in:
Maciej Kisiel 2026-03-13 11:36:41 +01:00 • committed by GitHub
commit a433a831d6
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1 changed files with 252 additions and 11 deletions
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37
mcp/streamable.go
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@ -174,6 +174,14 @@ type StreamableHTTPOptions struct {
// Only disable this if you understand the security implications.
// See: https://modelcontextprotocol.io/specification/2025-11-25/basic/security_best_practices#local-mcp-server-compromise
DisableLocalhostProtection bool
// CrossOriginProtection allows to customize cross-origin protection.
// The deny handler set in the CrossOriginProtection through SetDenyHandler
// is ignored.
// If nil, default (zero-value) cross-origin protection will be used.
// Use `disablecrossoriginprotection` MCPGODEBUG compatibility parameter
// to disable the default protection until v1.6.0.
CrossOriginProtection *http.CrossOriginProtection
}
// NewStreamableHTTPHandler returns a new [StreamableHTTPHandler].
@ -190,8 +198,10 @@ func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *Strea
h.opts = *opts
}
if h.opts.Logger == nil { // ensure we have a logger
h.opts.Logger = ensureLogger(nil)
h.opts.Logger = ensureLogger(h.opts.Logger)
if h.opts.CrossOriginProtection == nil {
h.opts.CrossOriginProtection = &http.CrossOriginProtection{}
}
return h
@ -226,6 +236,13 @@ func (h *StreamableHTTPHandler) closeAll() {
// The option will be removed in the 1.6.0 version of the SDK.
var disablelocalhostprotection = mcpgodebug.Value("disablelocalhostprotection")
// disablecrossoriginprotection is a compatibility parameter that allows to disable
// the verification of the 'Origin' and 'Content-Type' headers, which was added in
// the 1.4.1 version of the SDK. See the documentation for the mcpgodebug package
// for instructions how to enable it.
// The option will be removed in the 1.6.0 version of the SDK.
var disablecrossoriginprotection = mcpgodebug.Value("disablecrossoriginprotection")
func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
// DNS rebinding protection: auto-enabled for localhost servers.
// See: https://modelcontextprotocol.io/specification/2025-11-25/basic/security_best_practices#local-mcp-server-compromise
@ -238,6 +255,22 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
}
}
if disablecrossoriginprotection != "1" {
// Verify the 'Origin' header to protect against CSRF attacks.
if err := h.opts.CrossOriginProtection.Check(req); err != nil {
http.Error(w, err.Error(), http.StatusForbidden)
return
}
// Validate 'Content-Type' header.
if req.Method == http.MethodPost {
contentType := req.Header.Get("Content-Type")
if contentType != "application/json" {
http.Error(w, "Content-Type must be 'application/json'", http.StatusUnsupportedMediaType)
return
}
}
}
// Allow multiple 'Accept' headers.
// https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept#syntax
accept := strings.Split(strings.Join(req.Header.Values("Accept"), ","), ",")