[release-branch.go1.19] net/http: go 1.20.6 host validation breaks setting Host to a unix socket address

src/net/http/request.go

@@ -582,8 +582,29 @@ func (r *Request) write(w io.Writer, usingProxy bool, extraHeaders Header, waitF
 	if err != nil {
 		return err
 	}
+	// Validate that the Host header is a valid header in general,
+	// but don't validate the host itself. This is sufficient to avoid
+	// header or request smuggling via the Host field.
+	// The server can (and will, if it's a net/http server) reject
+	// the request if it doesn't consider the host valid.
 	if !httpguts.ValidHostHeader(host) {
-		return errors.New("http: invalid Host header")
+		// Historically, we would truncate the Host header after '/' or ' '.
+		// Some users have relied on this truncation to convert a network
+		// address such as Unix domain socket path into a valid, ignored
+		// Host header (see https://go.dev/issue/61431).
+		//
+		// We don't preserve the truncation, because sending an altered
+		// header field opens a smuggling vector. Instead, zero out the
+		// Host header entirely if it isn't valid. (An empty Host is valid;
+		// see RFC 9112 Section 3.2.)
+		//
+		// Return an error if we're sending to a proxy, since the proxy
+		// probably can't do anything useful with an empty Host header.
+		if !usingProxy {
+			host = ""
+		} else {
+			return errors.New("http: invalid Host header")
+		}
 	}
 
 	// According to RFC 6874, an HTTP client, proxy, or other

src/net/http/request_test.go

@@ -770,16 +770,23 @@ func TestRequestWriteBufferedWriter(t *testing.T) {
 	}
 }
 
-func TestRequestBadHost(t *testing.T) {
+func TestRequestBadHostHeader(t *testing.T) {
 	got := []string{}
 	req, err := NewRequest("GET", "http://foo/after", nil)
 	if err != nil {
 		t.Fatal(err)
 	}
-	req.Host = "foo.com with spaces"
-	req.URL.Host = "foo.com with spaces"
-	if err := req.Write(logWrites{t, &got}); err == nil {
-		t.Errorf("Writing request with invalid Host: succeded, want error")
+	req.Host = "foo.com\nnewline"
+	req.URL.Host = "foo.com\nnewline"
+	req.Write(logWrites{t, &got})
+	want := []string{
+		"GET /after HTTP/1.1\r\n",
+		"Host: \r\n",
+		"User-Agent: " + DefaultUserAgent + "\r\n",
+		"\r\n",
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("Writes = %q\n  Want = %q", got, want)
 	}
 }