src/net/http/transport.go      |  7 ++++---
 src/net/http/transport_test.go | 41 +++++++++++++++++++++++++++++++++++++++++

diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index e946760963c5b9877c4193de9ad6e7c035b6e1ca..df41cbd7743d8d884befd3eef71d6545c782846e 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -1309,14 +1309,15 @@ 		hdr := t.ProxyConnectHeader
 		if hdr == nil {
 			hdr = make(Header)
 		}
+		if pa := cm.proxyAuth(); pa != "" {
+			hdr = hdr.clone()
+			hdr.Set("Proxy-Authorization", pa)
+		}
 		connectReq := &Request{
 			Method: "CONNECT",
 			URL:    &url.URL{Opaque: cm.targetAddr},
 			Host:   cm.targetAddr,
 			Header: hdr,
-		}
-		if pa := cm.proxyAuth(); pa != "" {
-			connectReq.Header.Set("Proxy-Authorization", pa)
 		}
 		connectReq.Write(conn)
 
diff --git a/src/net/http/transport_test.go b/src/net/http/transport_test.go
index 5e5438a708b48aef3e3a720d4c93a02b64ff079f..a2f95911d938585acb3b3a034b8f51e5605d8774 100644
--- a/src/net/http/transport_test.go
+++ b/src/net/http/transport_test.go
@@ -1371,6 +1371,47 @@ 		t.Errorf("Got error %#v; want %#v", oe, want)
 	}
 }
 
+// Issue 36431: calls to RoundTrip should not mutate t.ProxyConnectHeader.
+//
+// (A bug caused dialConn to instead write the per-request Proxy-Authorization
+// header through to the shared Header instance, introducing a data race.)
+func TestTransportProxyDialDoesNotMutateProxyConnectHeader(t *testing.T) {
+	setParallel(t)
+	defer afterTest(t)
+
+	proxy := httptest.NewTLSServer(NotFoundHandler())
+	defer proxy.Close()
+	c := proxy.Client()
+
+	tr := c.Transport.(*Transport)
+	tr.Proxy = func(*Request) (*url.URL, error) {
+		u, _ := url.Parse(proxy.URL)
+		u.User = url.UserPassword("aladdin", "opensesame")
+		return u, nil
+	}
+	h := tr.ProxyConnectHeader
+	if h == nil {
+		h = make(Header)
+	}
+	tr.ProxyConnectHeader = make(Header, len(h))
+	for k, vals := range h {
+		tr.ProxyConnectHeader[k] = append([]string(nil), vals...)
+	}
+
+	req, err := NewRequest("GET", "https://golang.fake.tld/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = c.Do(req)
+	if err == nil {
+		t.Errorf("unexpected Get success")
+	}
+
+	if !reflect.DeepEqual(tr.ProxyConnectHeader, h) {
+		t.Errorf("tr.ProxyConnectHeader = %v; want %v", tr.ProxyConnectHeader, h)
+	}
+}
+
 // TestTransportGzipRecursive sends a gzip quine and checks that the
 // client gets the same value back. This is more cute than anything,
 // but checks that we don't recurse forever, and checks that