CVE-2019-12331 (#1041)

* Detect doubly-encoded xml to hide XXE attacks
Correct use of LibXml_Disable_Entity_Loader

* New test for double-encoded xml in security scanner

CHANGELOG.md

@@ -7,6 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ## [Unreleased]
 
+
+## [1.8.0] - 2019-07-01
+
+### Security Fix (CVE-2019-12331)
+
+- Detect double-encoded xml in the Security scanner, and reject as suspicious.
+- This change also broadens the scope of the `libxml_disable_entity_loader` setting when reading XML-based formats, so that it is enabled while the xml is being parsed and not simply while it is loaded.
+  On some versions of PHP, this can cause problems because it is not thread-safe, and can affect other PHP scripts running on the same server. This flag is set to true when instantiating a loader, and back to its original setting when the Reader is no longer in scope, or manually unset.
+- Provide a check to identify whether libxml_disable_entity_loader is thread-safe or not.
+
+  `XmlScanner::threadSafeLibxmlDisableEntityLoaderAvailability()`
+- Provide an option to disable the libxml_disable_entity_loader call through settings. This is not recommended as it reduces the security of the XML-based readers, and should only be used if you understand the consequences and have no other choice.
+ 
 ### Added
 
 - Added support for the SWITCH function - [Issue #963](https://github.com/PHPOffice/PhpSpreadsheet/issues/963) and [PR #983](https://github.com/PHPOffice/PhpSpreadsheet/pull/983)

src/PhpSpreadsheet/Reader/BaseReader.php

@@ -58,6 +58,21 @@ abstract class BaseReader implements IReader
     public function __construct()
     {
         $this->readFilter = new DefaultReadFilter();
+
+        // A fatal error will bypass the destructor, so we register a shutdown here
+        register_shutdown_function([$this, '__destruct']);
+    }
+
+    private function shutdown()
+    {
+        if ($this->securityScanner !== null) {
+            $this->securityScanner = null;
+        }
+    }
+
+    public function __destruct()
+    {
+        $this->shutdown();
     }
 
     public function getReadDataOnly()

src/PhpSpreadsheet/Reader/Security/XmlScanner.php

@@ -3,6 +3,7 @@
 namespace PhpOffice\PhpSpreadsheet\Reader\Security;
 
 use PhpOffice\PhpSpreadsheet\Reader;
+use PhpOffice\PhpSpreadsheet\Settings;
 
 class XmlScanner
 {
@@ -22,10 +23,16 @@ class XmlScanner
 
     private $callback;
 
-    private function __construct($pattern = '<!DOCTYPE')
+    private static $libxmlDisableEntityLoaderValue;
+
+    public function __construct($pattern = '<!DOCTYPE')
     {
         $this->pattern = $pattern;
-        $this->libxmlDisableEntityLoader = $this->identifyLibxmlDisableEntityLoaderAvailability();
+
+        $this->disableEntityLoaderCheck();
+
+        // A fatal error will bypass the destructor, so we register a shutdown here
+        register_shutdown_function([$this, '__destruct']);
     }
 
     public static function getInstance(Reader\IReader $reader)
@@ -43,7 +50,7 @@ class XmlScanner
         }
     }
 
-    private function identifyLibxmlDisableEntityLoaderAvailability()
+    public static function threadSafeLibxmlDisableEntityLoaderAvailability()
     {
         if (PHP_MAJOR_VERSION == 7) {
             switch (PHP_MINOR_VERSION) {
@@ -61,11 +68,53 @@ class XmlScanner
         return false;
     }
 
+    private function disableEntityLoaderCheck()
+    {
+        if (Settings::getLibXmlDisableEntityLoader()) {
+            $libxmlDisableEntityLoaderValue = libxml_disable_entity_loader(true);
+
+            if (self::$libxmlDisableEntityLoaderValue === null) {
+                self::$libxmlDisableEntityLoaderValue = $libxmlDisableEntityLoaderValue;
+            }
+        }
+    }
+
+    private function shutdown()
+    {
+        if (self::$libxmlDisableEntityLoaderValue !== null) {
+            libxml_disable_entity_loader(self::$libxmlDisableEntityLoaderValue);
+        }
+    }
+
+    public function __destruct()
+    {
+        $this->shutdown();
+    }
+
     public function setAdditionalCallback(callable $callback)
     {
         $this->callback = $callback;
     }
 
+    private function toUtf8($xml)
+    {
+        $pattern = '/encoding="(.*?)"/';
+        $result = preg_match($pattern, $xml, $matches);
+        $charset = $result ? $matches[1] : 'UTF-8';
+
+        if ($charset !== 'UTF-8') {
+            $xml = mb_convert_encoding($xml, 'UTF-8', $charset);
+
+            $result = preg_match($pattern, $xml, $matches);
+            $charset = $result ? $matches[1] : 'UTF-8';
+            if ($charset !== 'UTF-8') {
+                throw new Reader\Exception('Suspicious Double-encoded XML, spreadsheet file load() aborted to prevent XXE/XEE attacks');
+            }
+        }
+
+        return $xml;
+    }
+
     /**
      * Scan the XML for use of <!ENTITY to prevent XXE/XEE attacks.
      *
@@ -77,22 +126,13 @@ class XmlScanner
      */
     public function scan($xml)
     {
-        if ($this->libxmlDisableEntityLoader) {
+        $this->disableEntityLoaderCheck();
-            $previousLibxmlDisableEntityLoaderValue = libxml_disable_entity_loader(true);
-        }
 
-        $pattern = '/encoding="(.*?)"/';
+        $xml = $this->toUtf8($xml);
-        $result = preg_match($pattern, $xml, $matches);
-        $charset = $result ? $matches[1] : 'UTF-8';
-
-        if ($charset !== 'UTF-8') {
-            $xml = mb_convert_encoding($xml, 'UTF-8', $charset);
-        }
 
         // Don't rely purely on libxml_disable_entity_loader()
         $pattern = '/\\0?' . implode('\\0?', str_split($this->pattern)) . '\\0?/';
 
-        try {
         if (preg_match($pattern, $xml)) {
             throw new Reader\Exception('Detected use of ENTITY in XML, spreadsheet file load() aborted to prevent XXE/XEE attacks');
         }
@@ -100,11 +140,6 @@ class XmlScanner
         if ($this->callback !== null && is_callable($this->callback)) {
             $xml = call_user_func($this->callback, $xml);
         }
-        } finally {
-            if (isset($previousLibxmlDisableEntityLoaderValue)) {
-                libxml_disable_entity_loader($previousLibxmlDisableEntityLoaderValue);
-            }
-        }
 
         return $xml;
     }

src/PhpSpreadsheet/Settings.php

@@ -24,6 +24,20 @@ class Settings
      */
     private static $libXmlLoaderOptions = null;
 
+    /**
+     * Allow/disallow libxml_disable_entity_loader() call when not thread safe.
+     * Default behaviour is to do the check, but if you're running PHP versions
+     *      7.2 < 7.2.1
+     *      7.1 < 7.1.13
+     *      7.0 < 7.0.27
+     *      5.6 ANY
+     * then you may need to disable this check to prevent unwanted behaviour in other threads
+     * SECURITY WARNING: Changing this flag is not recommended.
+     *
+     * @var bool
+     */
+    private static $libXmlDisableEntityLoader = true;
+
     /**
      * The cache implementation to be used for cell collection.
      *
@@ -101,6 +115,34 @@ class Settings
         return self::$libXmlLoaderOptions;
     }
 
+    /**
+     * Enable/Disable the entity loader for libxml loader.
+     * Allow/disallow libxml_disable_entity_loader() call when not thread safe.
+     * Default behaviour is to do the check, but if you're running PHP versions
+     *      7.2 < 7.2.1
+     *      7.1 < 7.1.13
+     *      7.0 < 7.0.27
+     *      5.6 ANY
+     * then you may need to disable this check to prevent unwanted behaviour in other threads
+     * SECURITY WARNING: Changing this flag to false is not recommended.
+     *
+     * @param bool $state
+     */
+    public static function setLibXmlDisableEntityLoader($state)
+    {
+        self::$libXmlDisableEntityLoader = (bool) $state;
+    }
+
+    /**
+     * Return the state of the entity loader (disabled/enabled) for libxml loader.
+     *
+     * @return bool $state
+     */
+    public static function getLibXmlDisableEntityLoader()
+    {
+        return self::$libXmlDisableEntityLoader;
+    }
+
     /**
      * Sets the implementation of cache that should be used for cell collection.
      *

tests/PhpSpreadsheetTests/Reader/Security/XmlScannerTest.php

@@ -5,7 +5,6 @@ namespace PhpOffice\PhpSpreadsheetTests\Reader\Security;
 use PhpOffice\PhpSpreadsheet\Reader\Security\XmlScanner;
 use PhpOffice\PhpSpreadsheet\Reader\Xls;
 use PhpOffice\PhpSpreadsheet\Reader\Xlsx;
-use PhpOffice\PhpSpreadsheet\Reader\Xml;
 use PHPUnit\Framework\TestCase;
 
 class XmlScannerTest extends TestCase
@@ -19,12 +18,13 @@ class XmlScannerTest extends TestCase
      */
     public function testValidXML($filename, $expectedResult, $libxmlDisableEntityLoader)
     {
-        libxml_disable_entity_loader($libxmlDisableEntityLoader);
+        $oldDisableEntityLoaderState = libxml_disable_entity_loader($libxmlDisableEntityLoader);
 
         $reader = XmlScanner::getInstance(new \PhpOffice\PhpSpreadsheet\Reader\Xml());
         $result = $reader->scanFile($filename);
         self::assertEquals($expectedResult, $result);
-        self::assertEquals($libxmlDisableEntityLoader, libxml_disable_entity_loader());
+
+        libxml_disable_entity_loader($oldDisableEntityLoaderState);
     }
 
     public function providerValidXML()
@@ -115,26 +115,4 @@ class XmlScannerTest extends TestCase
 
         return $tests;
     }
-
-    /**
-     * @dataProvider providerLibxmlSettings
-     *
-     * @param $libxmlDisableLoader
-     */
-    public function testNewInstanceCreationDoesntChangeLibxmlSettings($libxmlDisableLoader)
-    {
-        libxml_disable_entity_loader($libxmlDisableLoader);
-
-        $reader = new Xml();
-        self::assertEquals($libxmlDisableLoader, libxml_disable_entity_loader($libxmlDisableLoader));
-        unset($reader);
-    }
-
-    public function providerLibxmlSettings()
-    {
-        return [
-            [true],
-            [false],
-        ];
-    }
 }

tests/data/Reader/Xml/XEETestInvalidUTF-7_DoubleEncoded.xml

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="UTF-7"?>
++-ADwAIQ-DOCTYPE xmlrootname +-AFsAPAAh-ENTITY +-ACU aaa SYSTEM +-ACI-http://127.0.0.1:8080/ext.dtd+-ACIAPgAl-aaa+-ADsAJQ-ccc+-ADsAJQ-ddd+-ADsAXQA+-