From 6e1d364d604ffc7f504ce381fdb0418472818275 Mon Sep 17 00:00:00 2001
From: Jakob Borg <jakob@nym.se>
Date: Sun, 3 Jan 2016 21:56:19 +0100
Subject: [PATCH] Don't allow in use CSRF tokens to expire (fixes #1008)

---
 cmd/syncthing/gui_csrf.go | 31 ++++++++++++++++++++++++-----
 cmd/syncthing/gui_test.go | 42 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 5 deletions(-)
 create mode 100644 cmd/syncthing/gui_test.go

diff --git a/cmd/syncthing/gui_csrf.go b/cmd/syncthing/gui_csrf.go
index ccb90b61f..74e425717 100644
--- a/cmd/syncthing/gui_csrf.go
+++ b/cmd/syncthing/gui_csrf.go
@@ -17,9 +17,16 @@ import (
 	"github.com/syncthing/syncthing/lib/sync"
 )
 
+// csrfTokens is a list of valid tokens. It is sorted so that the most
+// recently used token is first in the list. New tokens are added to the front
+// of the list (as it is the most recently used at that time). The list is
+// pruned to a maximum of maxCsrfTokens, throwing away the least recently used
+// tokens.
 var csrfTokens []string
 var csrfMut = sync.NewMutex()
 
+const maxCsrfTokens = 25
+
 // Check for CSRF token on /rest/ URLs. If a correct one is not given, reject
 // the request with 403. For / and /index.html, set a new CSRF cookie if none
 // is currently set.
@@ -36,6 +43,7 @@ func csrfMiddleware(unique, prefix, apiKey string, next http.Handler) http.Handl
 		if !strings.HasPrefix(r.URL.Path, prefix) {
 			cookie, err := r.Cookie("CSRF-Token-" + unique)
 			if err != nil || !validCsrfToken(cookie.Value) {
+				httpl.Debugln("new CSRF cookie in response to request for", r.URL)
 				cookie = &http.Cookie{
 					Name:  "CSRF-Token-" + unique,
 					Value: newCsrfToken(),
@@ -47,7 +55,13 @@ func csrfMiddleware(unique, prefix, apiKey string, next http.Handler) http.Handl
 		}
 
 		if r.Method == "GET" {
-			// Allow GET requests unconditionally
+			// Allow GET requests unconditionally, but if we got the CSRF
+			// token cookie do the verification anyway so we keep the
+			// csrfTokens list sorted by recent usage. We don't care about the
+			// outcome of the validity check.
+			if cookie, err := r.Cookie("CSRF-Token-" + unique); err == nil {
+				validCsrfToken(cookie.Value)
+			}
 			next.ServeHTTP(w, r)
 			return
 		}
@@ -66,8 +80,15 @@ func csrfMiddleware(unique, prefix, apiKey string, next http.Handler) http.Handl
 func validCsrfToken(token string) bool {
 	csrfMut.Lock()
 	defer csrfMut.Unlock()
-	for _, t := range csrfTokens {
+	for i, t := range csrfTokens {
 		if t == token {
+			if i > 0 {
+				// Move this token to the head of the list. Copy the tokens at
+				// the front one step to the right and then replace the token
+				// at the head.
+				copy(csrfTokens[1:], csrfTokens[:i+1])
+				csrfTokens[0] = token
+			}
 			return true
 		}
 	}
@@ -78,9 +99,9 @@ func newCsrfToken() string {
 	token := randomString(32)
 
 	csrfMut.Lock()
-	csrfTokens = append(csrfTokens, token)
-	if len(csrfTokens) > 10 {
-		csrfTokens = csrfTokens[len(csrfTokens)-10:]
+	csrfTokens = append([]string{token}, csrfTokens...)
+	if len(csrfTokens) > maxCsrfTokens {
+		csrfTokens = csrfTokens[:maxCsrfTokens]
 	}
 	defer csrfMut.Unlock()
 
diff --git a/cmd/syncthing/gui_test.go b/cmd/syncthing/gui_test.go
new file mode 100644
index 000000000..f286fc2e4
--- /dev/null
+++ b/cmd/syncthing/gui_test.go
@@ -0,0 +1,42 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package main
+
+import "testing"
+
+func TestCSRFToken(t *testing.T) {
+	t1 := newCsrfToken()
+	t2 := newCsrfToken()
+
+	t3 := newCsrfToken()
+	if !validCsrfToken(t3) {
+		t.Fatal("t3 should be valid")
+	}
+
+	for i := 0; i < 250; i++ {
+		if i%5 == 0 {
+			// t1 and t2 should remain valid by virtue of us checking them now
+			// and then.
+			if !validCsrfToken(t1) {
+				t.Fatal("t1 should be valid at iteration", i)
+			}
+			if !validCsrfToken(t2) {
+				t.Fatal("t2 should be valid at iteration", i)
+			}
+		}
+
+		// The newly generated token is always valid
+		t4 := newCsrfToken()
+		if !validCsrfToken(t4) {
+			t.Fatal("t4 should be valid at iteration", i)
+		}
+	}
+
+	if validCsrfToken(t3) {
+		t.Fatal("t3 should have expired by now")
+	}
+}