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

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.

.github/workflows/codeql.yml

@@ -8,6 +8,9 @@ on:
   schedule:
     - cron: '31 9 * * 4'
 
+# Declare default permissions as read only.
+permissions: read-all
+
 jobs:
   analyze:
     name: Analyze (${{ matrix.language }})

.github/workflows/conformance.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.26"
       - name: Start everything-server
         run: |
           go run ./conformance/everything-server/main.go -http=":3001" &
@@ -45,7 +45,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.26"
       - name: "Run conformance tests" 
         uses: modelcontextprotocol/conformance@a2855b03582a6c0b31065ad4d9af248316ce61a3 # v0.1.15
         with:

.github/workflows/docs-check.yml

@@ -17,6 +17,8 @@ jobs:
     steps:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
+        with:
+          go-version: "^1.26"
       - name: Check out code
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - name: Check docs are up-to-date

.github/workflows/nightly.yml

@@ -27,7 +27,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
       with:
-        go-version: "^1.25"
+        go-version: "^1.26"
     - name: Set up Node.js
       uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
       with:

.github/workflows/test.yml

@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.24", "1.25"]
+        go: ["1.25", "1.26"]
     steps:
       - name: Check out code
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
@@ -52,6 +52,8 @@ jobs:
         run: go test -v ./...
 
   race-test:
+    # Temporarily disable until fixes are prepared.
+    if: false
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
@@ -59,6 +61,6 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
-          go-version: "1.24"
+          go-version: "1.25"
       - name: Test with -race
         run: go test -v -race ./...

docs/README.md

@@ -39,6 +39,11 @@ protocol.
 
 See [troubleshooting.md](troubleshooting.md) for a troubleshooting guide.
 
+# Backwards compatibility
+
+See [mcpgodebug.md](mcpgodebug.md) for a list of backwards incompatible behavior changes
+and description how they can be temporarily undone.
+
 # Rough edges
 
 See [rough_edges.md](rough_edges.md) for a list of rough edges or API

docs/mcpgodebug.md

@@ -0,0 +1,41 @@
+<!-- Autogenerated by weave; DO NOT EDIT -->
+ # Backwards compatibility and MCPGODEBUG
+
+ According to our compatibility promise, we can't break backward compatibility
+ of the SDK API. However, sometimes we need to change the behavior of the SDK
+ in a backward-incompatible way in order to fix bugs or security issues.
+ In those cases we introduce temporary compatibility parameters, that can be
+ used to opt-out of the new behavior. They are usually maintained for two
+ minor release cycles and then removed.
+
+ The compatibility parameters are provided via the `MCPGODEBUG` environment
+ variable. The value of the variable is a comma-separated list of parameter
+ value assignments, e.g.:
+
+ ```
+ MCPGODEBUG=parameter1=value1,parameter2=value2
+ ```
+
+## `MCPGODEBUG` history
+
+### 1.4.1
+
+Options listed below will be removed in the 1.6.0 version of the SDK.
+
+- `disablecrossoriginprotection` added. If set to `1`, newly added cross-origin
+  protection will be disabled. The default behavior was changed to enable
+  cross-origin protection.
+
+### 1.4.0
+
+Options listed below will be removed in the 1.6.0 version of the SDK.
+
+- `jsonescaping` added. If set to `1`, JSON marshaling will preserve the previous
+  behavior of escaping HTML characters in JSON strings. The default behavior
+  was changed to not escape HTML characters, to be consistent with other SDKs.
+
+- `disablelocalhostprotection` added. If set to `1`, newly added DNS rebinding
+  protection will be disabled. The default behavior was changed to enable DNS rebinding
+  protection. The protection can also be disabled by setting the
+  `DisableLocalhostProtection` field in the `StreamableHTTPOptions` struct to
+  `true`, which is the recommended way to disable the protection long term.

docs/protocol.md

@@ -360,7 +360,7 @@ and step-up authentication (when the server returns `insufficient_scope` error).
 ## Security
 
 Here we discuss the mitigations described under
-the MCP spec's [Security Best Practices](https://modelcontextprotocol.io/specification/2025-06-18/basic/security_best_practices) section, and how we handle them.
+the MCP's [Security Best Practices](https://modelcontextprotocol.io/docs/tutorials/security/security_best_practices) section, and how we handle them.
 
 ### Confused Deputy
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/modelcontextprotocol/go-sdk
 
-go 1.24.0
+go 1.25.0
 
 require (
 	github.com/golang-jwt/jwt/v5 v5.3.0

internal/docs/README.src.md

@@ -38,6 +38,11 @@ protocol.
 
 See [troubleshooting.md](troubleshooting.md) for a troubleshooting guide.
 
+# Backwards compatibility
+
+See [mcpgodebug.md](mcpgodebug.md) for a list of backwards incompatible behavior changes
+and description how they can be temporarily undone.
+
 # Rough edges
 
 See [rough_edges.md](rough_edges.md) for a list of rough edges or API

internal/docs/doc.go

@@ -9,6 +9,7 @@
 //go:generate weave -o ../../docs/server.md ./server.src.md
 //go:generate weave -o ../../docs/troubleshooting.md ./troubleshooting.src.md
 //go:generate weave -o ../../docs/rough_edges.md ./rough_edges.src.md
+//go:generate weave -o ../../docs/mcpgodebug.md ./mcpgodebug.src.md
 
 // The doc package generates the documentation at /doc, via go:generate.
 //

internal/docs/mcpgodebug.src.md

@@ -0,0 +1,40 @@
+ # Backwards compatibility and MCPGODEBUG
+
+ According to our compatibility promise, we can't break backward compatibility
+ of the SDK API. However, sometimes we need to change the behavior of the SDK
+ in a backward-incompatible way in order to fix bugs or security issues.
+ In those cases we introduce temporary compatibility parameters, that can be
+ used to opt-out of the new behavior. They are usually maintained for two
+ minor release cycles and then removed.
+
+ The compatibility parameters are provided via the `MCPGODEBUG` environment
+ variable. The value of the variable is a comma-separated list of parameter
+ value assignments, e.g.:
+
+ ```
+ MCPGODEBUG=parameter1=value1,parameter2=value2
+ ```
+
+## `MCPGODEBUG` history
+
+### 1.4.1
+
+Options listed below will be removed in the 1.6.0 version of the SDK.
+
+- `disablecrossoriginprotection` added. If set to `1`, newly added cross-origin
+  protection will be disabled. The default behavior was changed to enable
+  cross-origin protection.
+
+### 1.4.0
+
+Options listed below will be removed in the 1.6.0 version of the SDK.
+
+- `jsonescaping` added. If set to `1`, JSON marshaling will preserve the previous
+  behavior of escaping HTML characters in JSON strings. The default behavior
+  was changed to not escape HTML characters, to be consistent with other SDKs.
+
+- `disablelocalhostprotection` added. If set to `1`, newly added DNS rebinding
+  protection will be disabled. The default behavior was changed to enable DNS rebinding
+  protection. The protection can also be disabled by setting the
+  `DisableLocalhostProtection` field in the `StreamableHTTPOptions` struct to
+  `true`, which is the recommended way to disable the protection long term.

internal/docs/protocol.src.md

@@ -285,7 +285,7 @@ and step-up authentication (when the server returns `insufficient_scope` error).
 ## Security
 
 Here we discuss the mitigations described under
-the MCP spec's [Security Best Practices](https://modelcontextprotocol.io/specification/2025-06-18/basic/security_best_practices) section, and how we handle them.
+the MCP's [Security Best Practices](https://modelcontextprotocol.io/docs/tutorials/security/security_best_practices) section, and how we handle them.
 
 ### Confused Deputy
 

mcp/streamable.go

@@ -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(h.opts.Logger)
-		h.opts.Logger = ensureLogger(nil)
+
+	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"), ","), ",")

mcp/streamable_test.go

@@ -832,6 +832,36 @@ func TestStreamableServerTransport(t *testing.T) {
 			},
 			wantSessions: 0,
 		},
+		{
+			name: "content type headers",
+			requests: []streamableRequest{
+				initialize,
+				initialized,
+				{
+					// Request with incorrect Content-Type should be rejected.
+					method:         "POST",
+					headers:        http.Header{"Content-Type": {"text/plain"}},
+					messages:       []jsonrpc.Message{req(3, "tools/call", &CallToolParams{Name: "tool"})},
+					wantStatusCode: http.StatusUnsupportedMediaType,
+				},
+				{
+					// Request with empty Content-Type should be rejected.
+					method:         "POST",
+					headers:        http.Header{"Content-Type": {""}},
+					messages:       []jsonrpc.Message{req(4, "tools/call", &CallToolParams{Name: "tool"})},
+					wantStatusCode: http.StatusUnsupportedMediaType,
+				},
+				{
+					// Correct Content-Type should pass.
+					method:         "POST",
+					headers:        http.Header{"Content-Type": {"application/json"}},
+					messages:       []jsonrpc.Message{req(5, "tools/call", &CallToolParams{Name: "tool"})},
+					wantStatusCode: http.StatusOK,
+					wantMessages:   []jsonrpc.Message{resp(5, &CallToolResult{Content: []Content{}}, nil)},
+				},
+			},
+			wantSessions: 1,
+		},
 		{
 			name: "accept headers",
 			requests: []streamableRequest{
@@ -1409,10 +1439,16 @@ func (s streamableRequest) do(ctx context.Context, serverURL, sessionID string,
 	if sessionID != "" {
 		req.Header.Set(sessionIDHeader, sessionID)
 	}
-	req.Header.Set("Content-Type", "application/json")
+	if s.method == http.MethodPost {
+		req.Header.Set("Content-Type", "application/json")
+	}
 	req.Header.Set("Accept", "application/json, text/event-stream")
 	maps.Copy(req.Header, s.headers)
 
+	if req.Header.Get("Content-Type") == "" {
+		req.Header.Del("Content-Type")
+	}
+
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return "", 0, nil, fmt.Errorf("request failed: %v", err)
@@ -2436,3 +2472,76 @@ func TestStreamableLocalhostProtection(t *testing.T) {
 		})
 	}
 }
+
+func TestStreamableOriginProtection(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.StatusOK,
+		},
+		{
+			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 := &StreamableHTTPOptions{
+				Stateless:             true, // avoid session ID requirement
+				CrossOriginProtection: tt.protection,
+			}
+			handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, opts)
+			httpServer := httptest.NewServer(handler)
+			defer httpServer.Close()
+
+			reqReader := strings.NewReader(`{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}`)
+			req, err := http.NewRequest(http.MethodPost, httpServer.URL, reqReader)
+			if err != nil {
+				t.Fatal(err)
+			}
+			req.Header.Set("Content-Type", "application/json")
+			req.Header.Set("Origin", tt.requestOrigin)
+			req.Header.Set("Accept", "application/json, text/event-stream")
+
+			resp, err := http.DefaultClient.Do(req)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer resp.Body.Close()
+
+			if got := resp.StatusCode; got != tt.wantStatusCode {
+				t.Errorf("Status code: got %d, want %d", got, tt.wantStatusCode)
+			}
+		})
+	}
+}