diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index fd78d45..0d514f5 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -114,6 +114,16 @@
 	}
 }
 
+func cloneHeader(h http.Header) http.Header {
+	h2 := make(http.Header, len(h))
+	for k, vv := range h {
+		vv2 := make([]string, len(vv))
+		copy(vv2, vv)
+		h2[k] = vv2
+	}
+	return h2
+}
+
 // Hop-by-hop headers. These are removed when sent to the backend.
 // http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
 var hopHeaders = []string{
@@ -154,23 +164,16 @@
 		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
 	}
 
+	outreq.Header = cloneHeader(req.Header)
+
 	p.Director(outreq)
 	outreq.Close = false
 
-	// We are modifying the same underlying map from req (shallow
-	// copied above) so we only copy it if necessary.
-	copiedHeaders := false
-
 	// Remove hop-by-hop headers listed in the "Connection" header.
 	// See RFC 2616, section 14.10.
 	if c := outreq.Header.Get("Connection"); c != "" {
 		for _, f := range strings.Split(c, ",") {
 			if f = strings.TrimSpace(f); f != "" {
-				if !copiedHeaders {
-					outreq.Header = make(http.Header)
-					copyHeader(outreq.Header, req.Header)
-					copiedHeaders = true
-				}
 				outreq.Header.Del(f)
 			}
 		}
@@ -181,11 +184,6 @@
 	// connection, regardless of what the client sent to us.
 	for _, h := range hopHeaders {
 		if outreq.Header.Get(h) != "" {
-			if !copiedHeaders {
-				outreq.Header = make(http.Header)
-				copyHeader(outreq.Header, req.Header)
-				copiedHeaders = true
-			}
 			outreq.Header.Del(h)
 		}
 	}
diff --git a/src/net/http/httputil/reverseproxy_test.go b/src/net/http/httputil/reverseproxy_test.go
index 57503cc..37a9992 100644
--- a/src/net/http/httputil/reverseproxy_test.go
+++ b/src/net/http/httputil/reverseproxy_test.go
@@ -736,3 +736,36 @@
 		t.Errorf("got = %+v; want = %+v", got, want)
 	}
 }
+
+// Issue 18327: verify we always do a deep copy of the Request.Header map
+// before any mutations.
+func TestClonesRequestHeaders(t *testing.T) {
+	req, _ := http.NewRequest("GET", "http://foo.tld/", nil)
+	req.RemoteAddr = "1.2.3.4:56789"
+	rp := &ReverseProxy{
+		Director: func(req *http.Request) {
+			req.Header.Set("From-Director", "1")
+		},
+		Transport: roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			if v := req.Header.Get("From-Director"); v != "1" {
+				t.Errorf("From-Directory value = %q; want 1", v)
+			}
+			return nil, io.EOF
+		}),
+	}
+	rp.ServeHTTP(httptest.NewRecorder(), req)
+
+	if req.Header.Get("From-Director") == "1" {
+		t.Error("Director header mutation modified caller's request")
+	}
+	if req.Header.Get("X-Forwarded-For") != "" {
+		t.Error("X-Forward-For header mutation modified caller's request")
+	}
+
+}
+
+type roundTripperFunc func(req *http.Request) (*http.Response, error)
+
+func (fn roundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return fn(req)
+}