Error when redirect target is too long. (letsencrypt#4775)

jsha · web-flow · commit bc528cf8cdf4 · 2020-04-15T13:44:26.000-07:00
This can happen when a misconfiguration redirects a certain path to
itself, doubled. After 10 redirects the error message can get quite
long. Instead we halt things at 2000 bytes, which should be more than
enough.
diff --git a/va/http.go b/va/http.go
@@ -28,8 +28,11 @@ const (
 	// HTTP-01 challenge response. The expected payload should be ~87 bytes. Since
 	// it may be padded by whitespace which we previously allowed accept up to 128
 	// bytes before rejecting a response (32 byte b64 encoded token + . + 32 byte
-	// b64 encoded key fingerprint)
+	// b64 encoded key fingerprint).
 	maxResponseSize = 128
+	// maxPathSize is the maximum number of bytes we will accept in the path of a
+	// redirect URL.
+	maxPathSize = 2000
 	// whitespaceCutset is the set of characters trimmed from the right of an
 	// HTTP-01 key authorization response.
 	whitespaceCutset = "\n\r\t "
@@ -521,6 +524,10 @@ func (va *ValidationAuthorityImpl) processHTTPValidation(
 		}
 
 		redirPath := req.URL.Path
+		if len(redirPath) > maxPathSize {
+			return berrors.ConnectionFailureError("Redirect target too long")
+		}
+
 		// If the redirect URL has query parameters we need to preserve
 		// those in the redirect path
 		redirQuery := ""
diff --git a/va/http_test.go b/va/http_test.go
@@ -558,6 +558,14 @@ func httpTestSrv(t *testing.T) *httptest.Server {
 		fmt.Fprint(resp, tooLargeBuf)
 	})
 
+	mux.HandleFunc("/redir-path-too-long", func(resp http.ResponseWriter, req *http.Request) {
+		http.Redirect(
+			resp,
+			req,
+			"https://example.com/this-is-too-long-01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789",
+			http.StatusMovedPermanently)
+	})
+
 	// A path that redirects to an uppercase public suffix (#4215)
 	mux.HandleFunc("/redir-uppercase-publicsuffix", func(resp http.ResponseWriter, req *http.Request) {
 		http.Redirect(
@@ -811,6 +819,22 @@ func TestFetchHTTP(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name: "Redirect to long path",
+			Host: "example.com",
+			Path: "/redir-path-too-long",
+			ExpectedProblem: probs.ConnectionFailure(
+				"Fetching https://example.com/this-is-too-long-01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789: Redirect target too long"),
+			ExpectedRecords: []core.ValidationRecord{
+				{
+					Hostname:          "example.com",
+					Port:              strconv.Itoa(httpPort),
+					URL:               "http://example.com/redir-path-too-long",
+					AddressesResolved: []net.IP{net.ParseIP("127.0.0.1")},
+					AddressUsed:       net.ParseIP("127.0.0.1"),
+				},
+			},
+		},
 		{
 			Name: "Wrong HTTP status code",
 			Host: "example.com",
