diff --git a/extensions/debug/CHANGELOG.md b/extensions/debug/CHANGELOG.md
index 9207c4544e..e4e81598c0 100644
--- a/extensions/debug/CHANGELOG.md
+++ b/extensions/debug/CHANGELOG.md
@@ -4,7 +4,7 @@ Yii Framework 2 debug extension Change Log
 2.0.2 under development
 -----------------------
 
-- no changes in this release.
+- Bug #4820: Fixed reading incomplete debug index data in case of high request concurrency (martingeorg, samdark)
 
 
 2.0.1 December 07, 2014
diff --git a/extensions/debug/controllers/DefaultController.php b/extensions/debug/controllers/DefaultController.php
index 62381aeabd..57849c907d 100644
--- a/extensions/debug/controllers/DefaultController.php
+++ b/extensions/debug/controllers/DefaultController.php
@@ -118,7 +118,17 @@ class DefaultController extends Controller
                 clearstatcache();
             }
             $indexFile = $this->module->dataPath . '/index.data';
-            if (is_file($indexFile) && is_readable($indexFile) && ($content = file_get_contents($indexFile)) !== false) {
+
+            $content = '';
+            $fp = @fopen($indexFile, 'r');
+            if ($fp !== false) {
+                @flock($fp, LOCK_SH);
+                $content = fread($fp, filesize($indexFile));
+                @flock($fp, LOCK_UN);
+                fclose($fp);
+            }
+
+            if ($content !== '') {
                 $this->_manifest = array_reverse(unserialize($content), true);
             } else {
                 $this->_manifest = [];