net/http: do not allow space or slash in Host headers

A malformed Host header can result in a malformed HTTP request.
Clean them to avoid this.

Updates #11206. We may come back and make this stricter for 1.6.

Change-Id: I23c7d821cd9dbf66c3c15d26750f305e3672d984
Reviewed-on: https://go-review.googlesource.com/11241
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/http_test.go

@@ -39,3 +39,20 @@ func TestForeachHeaderElement(t *testing.T) {
 		}
 	}
 }
+
+func TestCleanHost(t *testing.T) {
+	tests := []struct {
+		in, want string
+	}{
+		{"www.google.com", "www.google.com"},
+		{"www.google.com foo", "www.google.com"},
+		{"www.google.com/foo", "www.google.com"},
+		{" first character is a space", ""},
+	}
+	for _, tt := range tests {
+		got := cleanHost(tt.in)
+		if tt.want != got {
+			t.Errorf("cleanHost(%q) = %q, want %q", tt.in, got, tt.want)
+		}
+	}
+}

src/net/http/request.go

@@ -369,17 +369,23 @@ func (r *Request) WriteProxy(w io.Writer) error {
 
 // extraHeaders may be nil
 func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) error {
-	// According to RFC 6874, an HTTP client, proxy, or other
-	// intermediary must remove any IPv6 zone identifier attached
-	// to an outgoing URI.
-	host := removeZone(req.Host)
+	// Find the target host. Prefer the Host: header, but if that
+	// is not given, use the host from the request URL.
+	//
+	// Clean the host, in case it arrives with unexpected stuff in it.
+	host := cleanHost(req.Host)
 	if host == "" {
 		if req.URL == nil {
 			return errors.New("http: Request.Write on Request with no Host or URL set")
 		}
-		host = removeZone(req.URL.Host)
+		host = cleanHost(req.URL.Host)
 	}
 
+	// According to RFC 6874, an HTTP client, proxy, or other
+	// intermediary must remove any IPv6 zone identifier attached
+	// to an outgoing URI.
+	host = removeZone(host)
+
 	ruri := req.URL.RequestURI()
 	if usingProxy && req.URL.Scheme != "" && req.URL.Opaque == "" {
 		ruri = req.URL.Scheme + "://" + host + ruri
@@ -464,6 +470,22 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) err
 	return nil
 }
 
+// cleanHost strips anything after '/' or ' '.
+// Ideally we'd clean the Host header according to the spec:
+//   https://tools.ietf.org/html/rfc7230#section-5.4 (Host = uri-host [ ":" port ]")
+//   https://tools.ietf.org/html/rfc7230#section-2.7 (uri-host -> rfc3986's host)
+//   https://tools.ietf.org/html/rfc3986#section-3.2.2 (definition of host)
+// But practically, what we are trying to avoid is the situation in
+// issue 11206, where a malformed Host header used in the proxy context
+// would create a bad request. So it is enough to just truncate at the
+// first offending character.
+func cleanHost(in string) string {
+	if i := strings.IndexAny(in, " /"); i != -1 {
+		return in[:i]
+	}
+	return in
+}
+
 // removeZone removes IPv6 zone identifer from host.
 // E.g., "[fe80::1%en0]:8080" to "[fe80::1]:8080"
 func removeZone(host string) string {

src/net/http/request_test.go

@@ -513,6 +513,24 @@ func TestRequestWriteBufferedWriter(t *testing.T) {
 	}
 }
 
+func TestRequestBadHost(t *testing.T) {
+	got := []string{}
+	req, err := NewRequest("GET", "http://foo.com with spaces/after", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req.Write(logWrites{t, &got})
+	want := []string{
+		"GET /after HTTP/1.1\r\n",
+		"Host: foo.com\r\n",
+		"User-Agent: " + DefaultUserAgent + "\r\n",
+		"\r\n",
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("Writes = %q\n  Want = %q", got, want)
+	}
+}
+
 func TestStarRequest(t *testing.T) {
 	req, err := ReadRequest(bufio.NewReader(strings.NewReader("M-SEARCH * HTTP/1.1\r\n\r\n")))
 	if err != nil {