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mcp: remove default cross origin protection (#906)

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Fixes #902
This commit is contained in:
Maciej Kisiel 2026-04-24 11:35:22 +02:00 • committed by GitHub
commit 41e1f941bb
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GPG key ID: B5690EEEBB952194
6 changed files with 41 additions and 118 deletions
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4
docs/rough_edges.md
View file

@ -59,3 +59,7 @@ v2.
wrapper) we need to first unmarshal into a `map[string]any` in order to do
server-side validation of required fields. CallToolParams could have just had
a map[string]any.
- `StreamableHTTPOptions.CrossOriginProtection` should not have been part of
the SDK API. Cross-origin protection is a general HTTP concern, not specific
to MCP, and can be applied as standard HTTP middleware.

4
internal/docs/rough_edges.src.md
View file

@ -58,3 +58,7 @@ v2.
wrapper) we need to first unmarshal into a `map[string]any` in order to do
server-side validation of required fields. CallToolParams could have just had
a map[string]any.
- `StreamableHTTPOptions.CrossOriginProtection` should not have been part of
the SDK API. Cross-origin protection is a general HTTP concern, not specific
to MCP, and can be applied as standard HTTP middleware.

30
mcp/sse.go
View file

@ -10,6 +10,7 @@ import (
"crypto/rand"
"fmt"
"io"
"mime"
"net"
"net/http"
"net/url"
@ -64,14 +65,6 @@ type SSEOptions 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 v1.7.0.
CrossOriginProtection *http.CrossOriginProtection
}
// NewSSEHandler returns a new [SSEHandler] that creates and manages MCP
@ -97,10 +90,6 @@ func NewSSEHandler(getServer func(request *http.Request) *Server, opts *SSEOptio
s.opts = *opts
}
if s.opts.CrossOriginProtection == nil {
s.opts.CrossOriginProtection = &http.CrossOriginProtection{}
}
return s
}
@ -212,20 +201,13 @@ func (h *SSEHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
}
}
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)
// Validate 'Content-Type' header.
if req.Method == http.MethodPost {
mediaType, _, err := mime.ParseMediaType(req.Header.Get("Content-Type"))
if err != nil || mediaType != "application/json" {
http.Error(w, "Content-Type must be 'application/json'", http.StatusUnsupportedMediaType)
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
}
}
}
sessionID := req.URL.Query().Get("sessionid")

75
mcp/sse_test.go
View file

@ -12,7 +12,6 @@ import (
"net"
"net/http"
"net/http/httptest"
"strings"
"sync/atomic"
"testing"
@ -320,77 +319,3 @@ func TestSSELocalhostProtection(t *testing.T) {
})
}
}
func TestSSEOriginProtection(t *testing.T) {
server := NewServer(testImpl, nil)
tests := []struct {
name string
protection *http.CrossOriginProtection
requestOrigin string
wantStatusCode int
}{
{
name: "default protection with Origin header",
protection: nil,
requestOrigin: "https://example.com",
wantStatusCode: http.StatusForbidden,
},
{
name: "custom protection with trusted origin and same Origin",
protection: func() *http.CrossOriginProtection {
p := http.NewCrossOriginProtection()
if err := p.AddTrustedOrigin("https://example.com"); err != nil {
t.Fatal(err)
}
return p
}(),
requestOrigin: "https://example.com",
wantStatusCode: http.StatusNotFound, // origin accepted; session not found
},
{
name: "custom protection with trusted origin and different Origin",
protection: func() *http.CrossOriginProtection {
p := http.NewCrossOriginProtection()
if err := p.AddTrustedOrigin("https://example.com"); err != nil {
t.Fatal(err)
}
return p
}(),
requestOrigin: "https://malicious.com",
wantStatusCode: http.StatusForbidden,
},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
opts := &SSEOptions{
CrossOriginProtection: tt.protection,
}
handler := NewSSEHandler(func(req *http.Request) *Server { return server }, opts)
httpServer := httptest.NewServer(handler)
defer httpServer.Close()
// Use POST with a valid session-like URL to test origin protection
// without creating a hanging GET connection.
reqReader := strings.NewReader(`{"jsonrpc":"2.0","id":1,"method":"ping"}`)
req, err := http.NewRequest(http.MethodPost, httpServer.URL+"?sessionid=nonexistent", reqReader)
if err != nil {
t.Fatal(err)
}
req.Header.Set("Content-Type", "application/json")
req.Header.Set("Origin", tt.requestOrigin)
resp, err := http.DefaultClient.Do(req)
if err != nil {
t.Fatal(err)
}
defer resp.Body.Close()
if got := resp.StatusCode; got != tt.wantStatusCode {
body, _ := io.ReadAll(resp.Body)
t.Errorf("Status code: got %d, want %d (body: %s)", got, tt.wantStatusCode, body)
}
})
}
}

42
mcp/streamable.go
View file

@ -180,9 +180,15 @@ type StreamableHTTPOptions struct {
// 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 v1.7.0.
// If nil, no cross-origin protection is applied. Use the `enableoriginverification`
// MCPGODEBUG compatibility parameter to enable the default protection until v1.8.0.
//
// Deprecated: wrap the handler with cross-origin protection middleware
// instead. For example:
//
// handler := mcp.NewStreamableHTTPHandler(...)
// protection := http.NewCrossOriginProtection()
// protectedHandler := protection.Handler(handler)
CrossOriginProtection *http.CrossOriginProtection
}
@ -202,7 +208,7 @@ func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *Strea
h.opts.Logger = ensureLogger(h.opts.Logger)
if h.opts.CrossOriginProtection == nil {
if h.opts.CrossOriginProtection == nil && enableoriginverification == "1" {
h.opts.CrossOriginProtection = &http.CrossOriginProtection{}
}
@ -235,15 +241,16 @@ func (h *StreamableHTTPHandler) closeAll() {
// disablelocalhostprotection is a compatibility parameter that allows to disable
// DNS rebinding protection, which was added in the 1.4.0 version of the SDK.
// See the documentation for the mcpgodebug package for instructions how to enable it.
// The option will be removed in the 1.7.0 version of the SDK.
// 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 1.7.0 version of the SDK.
var disablecrossoriginprotection = mcpgodebug.Value("disablecrossoriginprotection")
// enableoriginverification is a compatibility parameter that restores the
// default cross-origin protection behavior from v1.4.1-v1.5.0. When set to
// "1", a zero-value CrossOriginProtection will be applied if none is
// explicitly provided in StreamableHTTPOptions.
// See the documentation for the mcpgodebug package for instructions how to enable it.
// The option will be removed in the 1.8.0 version of the SDK.
var enableoriginverification = mcpgodebug.Value("enableoriginverification")
func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
// DNS rebinding protection: auto-enabled for localhost servers.
@ -257,17 +264,18 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
}
}
if disablecrossoriginprotection != "1" {
if h.opts.CrossOriginProtection != nil {
// 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 && baseMediaType(req.Header.Get("Content-Type")) != "application/json" {
http.Error(w, "Content-Type must be 'application/json'", http.StatusUnsupportedMediaType)
return
}
}
// Validate 'Content-Type' header.
if req.Method == http.MethodPost && baseMediaType(req.Header.Get("Content-Type")) != "application/json" {
http.Error(w, "Content-Type must be 'application/json'", http.StatusUnsupportedMediaType)
return
}
// Allow multiple 'Accept' headers.

4
mcp/streamable_test.go
View file

@ -2587,10 +2587,10 @@ func TestStreamableOriginProtection(t *testing.T) {
wantStatusCode int
}{
{
name: "default protection with Origin header",
name: "no protection with Origin header",
protection: nil,
requestOrigin: "https://example.com",
wantStatusCode: http.StatusForbidden,
wantStatusCode: http.StatusOK,
},
{
name: "custom protection with trusted origin and same Origin",