Maddin, Montag, 14. August 2006, 17:42 (somewhat) breaking the same-origin policy by undermining dns-pinning A small contribution to the current “hacking the intranet with JavaScript” meme:
Introduction
J. Grossman, RSnake, SPI Dynamics, pdp and others have demonstrated lately that it is possible for a malicious JavaScript
a) to obtain the (internal) IP address of the hosting web browser,
b) to portscan the lan to locate intranet http servers,
c) to fingerprint these http servers using well known URLs
d) and (sometimes) to exploiting them via CSRF.
During my research on that topic I discovered, that with some tweaking, it is also possible for the script to obtain read access, allowing the leakage of internal information and more precise fingerprinting.
Technical background
The basis of the attack is rather old. It was described by the Princeton University in 1996 [1] and was recently brought to my attention by Amit Klein [3]. For the attack to succeed the attacker needs to control the DNS entry for his web server (www.attacker.org in the following example).
Attacking an intranet host located at 10.10.10.10 would roughly work like this:
- The victim downloads a malicious script from www.attacker.org
- After the script has been downloaded, the attacker modifies the DNS answer for www.attacker.org to 10.10.10.10
- The malicious script requests a web page from www.attacker.org (e.g via loading it into an iframe)
- The web browser again does a DNS lookup request for www.attacker.org, now resolving to the intranet host at 10.10.10.10
- The web browser assumes that the domain values of the malicious script and the intranet server match, at therefore grants the script unlimited access to the intranet server.
To prevent this type of attack, modern web browsers implement “DNS Pinning” - DNS lookup results are kept unchanged for the entire browser session, even though the DNS entry’s lifetime may be shorter. Mohammad A. Haque describes in [2] how the attack method still can work, providing that the malicious script survives in the browser cache. The described scenario requires the victim to quit his web browser and to access the malicious script a second time, which renders the attack to be somewhat unlikely.
The refined attack: Undermining DNS pinning by rejecting connections
As it turns out, it is also possible to force the browser to renew the DNS entry for a given domain “on the fly”. The following sequence of events worked for me (tested on IE6 xpsp2 and Firefox 1.5.0.6):
1) The victim loads the script from www.attacker.org.
2) The attacker changes the DNS entry of www.attacker.org to 10.10.10.10
3) Further more the attacker quits the web server that was running on www.attacker.org’s original IP
4) The script uses a timed event (setIntervall or setTimeout) to load a web page from www.attacker.org
5) The web browser tries to connect to the IP which is bound to www.attacker.org from the previous request. As the web server there is shut down now, this connection attempt is rejected.
6) Because of this (and probably because of the DNS entry’s short lifetime), the browser drops the DNS pinning and does a new DNS lookup request, resulting in 10.10.10.10 (sometimes it takes more than one loading attempt to trigger the lookup request).
7) The script is now able to access the intranet server’s content and to leak it to the outside.
I successfully tested the described approach on two different computers in two different networks. Still the result is purely experimental. As I have not read the web browser’s source code, I can only guess why the attack works. For this reason it may be possible, that the attack fails on different setups.
Outlook
This technique obviously can be automated. Instead of quitting the web server on attacker.org completely, dynamic firewall rules could be used to reject further connections from the victim’s IP after the initial script was delivered.
The attack only woks, if the attacked server does not check the http host property, as this property would still be “www.attacker.org”. For the same reasons all virtual hosts are out of the attacker’s reach.
Update (12/2006): Kanatoko Anvil from jumperz.net found out that it is not necessary to shut down the web server. It is sufficient for the malicious script to access a closed port on the intranet server (e.g. attacker.org:81) to cause the web browser to initiate a new DNS query. See here for a demo. Wow.
References
[1] DNS Attack Scenario, www.cs.princeton.edu
[2] Josh Soref: DNS: Spoofing and Pinning, viper.haque.net
[3] Amit Klein: Re: Detecting, Analyzing, and Exploiting Intranet Applications using JavaScript (Posting to the WebAppSec-Mailinglist), www.webappsec.org
Maddin, Freitag, 19. Mai 2006, 16:55 Entering Bliki Igor just finished revamping our NerdAlert webpage - nerdalert.de is know proudly run using a Bliki, a Frankenstein’s monster between a blog and a Wiki. I was sceptical at first because I could not see the advantages over the regular wiki we have used before. Man, was I wrong. I sold now. The Wikipedia as usual has the best explanation:
The main advantage of combining the two concepts, however, is in leveraging the utility of wikis at making connections between ideas; this effectively turns blog posts into proper wiki articles, but maintains the former's immediate nature. Thus, a bliki can evolve as a whole over time, and past information is not merely jettisoned into the aether and lost in the shuffle.
I think it is a perfect fit for our needs (documenting a monthly radio show and some additional activities). And the webpage looks sooooooo gorgeous now.
Maddin, Donnerstag, 4. Mai 2006, 12:31 Travelling: PH-Neutral / OWASP AppSec 2006 At the end of May I have the honour to present some of my research work. First I will attend PH-Neutral in Berlin and talk a little bit about my results in developing transparent websecurity techniques. Later the same week, I will present our anti CSRF proxy at the OWASP AppSec06 conference in Leuven. Dear reader, if you are attending one these events please let me know, so that we can share experiences and beer at the fireplace.
Maddin, Mittwoch, 22. März 2006, 16:08 Web Montag Hamburg – Memories and Slides What a fun night. I met nice people, listened to some enlightening talks, had geeky conversations and drank cold beer. Hamburg’s first Web Montag was quite nice.
Lately, I did some research on “Session Riding” (aka CSRF). As Session Riding is a vulnerability class that is rather unknown, I used the opportunity of this event to raise some attention. I gave a short talk on the subject and showed an unfixed CSRF exploit. Hopefully I didn’t bore my audience to death. Check out my slides here.
Maddin, Donnerstag, 16. Februar 2006, 13:44 Cross Domain XMLHttpRequests are *really* not a good idea There was more than one moment in the last month, in which I wondered about the reasons behind the same-origin restriction on the XMLHttpRequest JavaScript object’s destination URL. This restriction effectively prevents JavaScript initiated cross domain http requests. None of the other network aware elements of HTML/JavaScript are subject to such a policy (e.g. images, iframes or scripts).
Finally somebody convinced me: Lucas Carlson describes in a recent Blog entry how cross domain XMLHttpRequests could be employed to subvert firewall protection. A malicious JavaScript executed by a web browser behind a firewall would be able to communicate the content of any http intranet server, which would otherwise be protected by the firewall, to any host on the internet. Such a script is furthermore able to do a complete port scan on the intranet, thus discovering services and further attack targets.
What a pity, cross domain Ajax would be so much fun otherwise.
Maddin, Montag, 9. Januar 2006, 13:08 Firefox search plug-in for the Google Duel The Google Duel is a nice web application that takes two terms and uses Google to find out, which one can be found more often in the almighty internet. Whenever I am unsure about some construct of the English language or the spelling of some name, I use the Google Duel for a quick and dirty first impression what might be right (hmmm, is this guy spelled ‘Kwitschko' or 'Klitschko'?). The outcome is by no means authorative but sometimes a majority vote like this is enough.
I wanted to use the duel from the search field of my Firefox, therefore I wrote a little connector and a search plug-in (click to install). The two duelling terms have to be separated by a semicolon (;).
Maddin, Mittwoch, 16. November 2005, 12:05 Using DNS queries to estimate backdoor propagation A backdoor that tries to phone home usually uses DNS-queries to locate the host they should report to. These DNS queries are cached by the DNS server for some time. Dan Kaminski uses this behaviour to estimate the number of PCs that are infected by Sony’s DRM rootkit(he found more than 500.000 DNS servers that received a query related to the rootkit, leading to a conservative estimate that the number of infected PCs is in the millions).
The image shows the distribution of the located DNS servers in Europe (click here for larger maps: USA, Asia, Europe). The more I learn about DNS, the more I am intrigued by this often overlooked protocol.
Maddin, Dienstag, 6. September 2005, 11:34 Windows of exposure revisited David Wheeler took the time and had a closer look on the time spans in which no unpatched exploit for a couple of popular web browsers existed. His findings where somewhat devastating, especially for IE:
It turns out that there were only 7 days in 2004 that you could have somewhat safely used Internet Explorer (it was October 12-17), even assuming that attackers only used publicly-known attacks, and that you were only worried about the worst kind of attacks.
For the rest of 2004 (249 days) there was at least one public known, unpatched exploit for IE. In comparison: Opera had 65 days with unpached security problems, the Mozilla family 56 (still a lot, if you ask me). For more details read David’s well written article.
Maddin, Dienstag, 16. August 2005, 16:20 Remembering the rain: A show about What The Hack In tomorrow's NerdAlert we will talk about the wet four days of What The Hack. In particular we will rebroadcast a sort of "best of" of the shows we did for Subether Radio. Tune in for interviews with:
Andreas Bogk about the end of Unix and the operating system of the future
