Insomni’Hack 2017 – FPS Write-Ups

For the 10th Insomni’Hack anniversary, new hacking challenge categories were available during the CTF. They consisted of social engineering, hacking room, and a multiplayer FPS game.

This article will cover several write-ups for this last category. It is a great occasion to understand quickly some basis of modern game hacking.

The game was compiled with Unity Engine, methods demonstrated here could work on other games compiled by the same engine. The binaries and installation instructions will be available at the end of the article.

Challenges

The following mention was written into every challenge description: “You will find almost all of the logic code into Data\Managed\Assembly-CSharp.dll.“

A .NET assembly editor and decompiler are needed as CSharp uses .NET platform. The decompiler has to feature a decent search method (a game .dll to analyze is likely to have a large number of classes). For this write-up, the dnSpy tool was used. It is available on Github.

Escape Room

The game developers are implementing a tutorial level called “Basement”. They tried to block players from accessing the level by setting obstacles. Show them you are still able to reach the main room.

This is the first and only challenge to do on the “Basement” map.

Once connected, we spawn into a small room. There seems to be only one way out :

Our jump power is way too low to go there. So we could try to implement a fly hack but it may be too overkill for this first challenge. A smarter way would be to find a variable that increases our jump enough to go through this way.

We launch dnSpy and decompile the earlier mentioned .dll.

We should start by searching for the “Jump” string into DnSpy. At this point, we already note that a lot of our search results are located in the vp_FPController class and see some interesting names containing the JumpForce string which sounds like an interesting name. We refine our search:

(On dnSpy, a string being referenced several times in the same file will only be shown once into the search assemblies (bottom window) results. You must then use the other search bar in the editor window at the upper right corner to navigate through the several references from the same file.)

The MotorJumpForce is the only candidate for a jump variable that is located in this vp_FPController class that we see so often in the results. Let’s start from there and if it doesn’t work we will check the ones in the other classes.

This variable is only referenced into the OnStart_Jump callback whose code is below:

(Note the usage of the class member MotorFreeFly on the second line whose name is interesting. In this method, this class member would cancel the jump if set to true and if the player isn’t grounded, this will be useful for the next challenge).

Now that we found this function, let’s multiply by 10 our jump force. Edit Method (Ctrl+Shift+E) and replace the next to last line for this one:

Now press the Compile button at the bottom-right of the window to close the method edition and Save All (Ctrl+Shift+S) to overwrite the existing .dll.

We’ve succeeded to through this way, but at some point, there are mines blocking the way. If we listen carefully we hear a trigger sound before the explosion occurs. If the explosion occurs after a delay, and that we succeed to trigger it running fast enough, we can avoid damage.

Now that we patched the jump force, we should try to patch the player’s acceleration or movement speed by searching some related words like “speed” or “acceleration” in the same vp_FPController class.

We fall in two methods coming from the same caller UpdateMotor() :

If we suppose the MotorFreeFly variable is not set, we have to edit the last line of the UpdateThrottleWalk() method, where m_MotorThrottle.z get reassigned. Z-axis corresponds to the W and S controller keys.

Once we increased this variable, we are able to pass this obstruction, we get the flag and can finally move on the next map.

Reach the Top

Positioning is the key to any tactical operation, especially if you are alone. Prove you are the best player by reaching the top of the building:

(Using the previous jump hack doesn’t directly help to solve this challenge because you need to be alive for several seconds to get the flag).

The first idea is to remove gravity from our player. m_GravityMultiplier and PhysicsGravityModifier look good candidates especially the second one since he’s located in vp_Controller.

Once again we find some references in the vp_Controller class, in a method called UpdateForces that is not taking any parameter.

This method is called every frame and calculates the falling speed the player.

We can replace the 0.002f coefficient to 0f, and recompile. The plan is to jump and see if we could go above the building.

In this demo, the player is getting stuck after his first jump. The player cannot jump a second time because the controller checks whether you are grounded or not before allowing your next jump.

Remember the MotorFreeFly variable we found several times earlier? Let’s inspect it a little more, and find references to it. Right-click on it and choose “Analyze” (Ctrl + Shift + R).

Again, we see that the functions come from the same vp_FPController class we edited earlier. The MotorFreeFly variable is used 4 times in it. In the CanStart_Jump and OnStart_Jump functions, this variable is used to check if the player should perform a jump, but the UpdateJump and UpdateMotor functions, that are called every frame, reveal something more interesting:

As we saw earlier, we understand that the controller will handle walk and jump functions differently depending on whether the MotorFreeFly variable is set to “True” or not.

A lot of FPS games have an advanced “free fly” mode implemented (called “no clipping” and prevents the first person camera from being obstructed by other objects and also permits the camera to move in any direction). The code implemented for this feature could be triggered by a player to cheat without having to understand the game physics.

To solve this part, we should find all references to MotorFreeFly in any conditional expression, remove the ‘!’ negation operator when prefixed, and conversely append it when not present.

Re-toast it!

A bug was reported to the developers about an unstable toasting feature. They patched the toaster to restrict players from triggering it by adding a keypad. Once the toaster is activated, you don’t have much time to enter the password. Could you show them that you can still make a toast?

Let’s search for the “Toast” keyword:

We find an interesting function name, it takes a string in an argument called “code”.

This function seems to be a wrapper for a RPC call destined to the Master (second argument) that checks if the submitted code matches. The RPC function code is the following one, it’s executed server-side:

No anti-brute force mechanism is implemented, then, surrounding the RPC call with a basic loop will brute-force the PIN:

Finally, trying any combination in-game results in defusing the toaster and receiving the flag.

Install instructions

The game was not designed to be publicly released. It is still far away from a real game and this is why won’t find it on any game platform. Nevertheless, it has a great educational value and we want to share it for these purposes.

To play the game, first, register (free) on PhotonEngine and download the Server SDK.

Run the PhotonControl application, and set the Server IP to 127.0.0.1.

Then, start the LoadBalancing application.

Finally, launch the game depending on your platform:

Heap Overflow Vulnerability in Citrix NetScaler Gateway (CVE-2017-7219)

After presenting my findings on the Swisscom router at the CybSecConference last year, I started looking for a new product to analyze. I quickly found that it’s possible to download virtual “demo” appliances of Citrix products, so I went on to download a Netscaler VPX, which at the time was at version 11.10 (b50.10). The goal as always was to discover a way to compromise the device remotely, which is what led me to discover a heap overflow vulnerability which allows an authenticated user to compromise the device with root privileges. During the research, I (un)fortunately wasn’t able to find a way to exploit the flaw without credentials.

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TL;DR;
======================

A heap overflow in the “ping” functionality allows an authenticated VPN user to trigger a use-after-free condition in order to execute arbitrary commands on the appliance. (CVE-2017-7219)

The following Metasploit module can be used to exploit the vulnerability (use at your own risk…), though it will probably only function against the version that was analyzed.

======================
DETAILS
======================

As mentioned above, I began by downloading the virtual appliance and started it up on my machine. Since I’ve never used or configured a Netscaler appliance in the past, it took a while to get things going and configuring it in some kind of standard mode.

Once the appliance is started, it is possible to log into the console with the standard nsroot account. This gives access to a “limited” shell, but Citrix were nice enough to add a shell command which gives root access to the box, so I used this to extract the filesystem and analyze what was going on.

Going through the various files on the system, I found one that seemed promising which was named /netscaler/nsvpnd. As it’s name hints at, it is used to handle requests sent to the VPN web interface. Though only authenticated requests seem to get here, as authentication itself is performed by another binary on the system.

One of the requests that is performed by the nsvpnd binary is the ping request.

This results in the following HTTP request:

POST /cvpn/aHR0cDovLzEyNy4wLjAuMQ/fs/ping HTTP/1.1
Host: 
[...]
Cookie: NSC_AAAC=b2f85f0b72ef21c82eac5ac4d314a4170af182cd945525d5f4f58455e445a4a42; NSC_FSSO=0
DNT: 1
Connection: close
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded
Content-Length: 41


host=127.0.0.1&nsc_nonce=kPIueluG6ubF37J0

Apparently the /cvpn/aHR0cDovLzEyNy4wLjAuMQ part of the URL is not actually required, so it can safely be removed. In any case, this request is eventually handled by one of two vulnerable functions that contain an unbounded strcpy with our host parameter, as shown below.

This is where the overflow happens, though we are not overwriting a stack variable, but one of the members of a struct, which is expected to be at most 256 bytes long. Our parameter on the other hand can go up to 512 bytes, which is what allows us to overflow this buffer.

So it is possible to write up to 256 bytes after the host member of the structure, therefore overwriting any other members of the structure after the host, which is where things get interesting. One of the following members is actually a pointer to another structure (a parameter list) on the heap which was previously allocated and eventually gets free‘d by the application when the request has completely been processed.  This means we can essentially free an arbitrary memory location.

Before going any further, a quick analysis of the system and binary show that FreeBSD uses jemalloc instead of dlmalloc, the heap is not executable, but the stack is, and ASLR is not enabled (this was 2016 after all). Another thing that was helpful in exploiting this particular issue is that all requests to the web interface are handled by one single process, which means we can actually interact many times with the process by sending multiple HTTP requests if required.

At this point, my idea to gain code execution was the following:

  1. Find a function pointer somewhere in the application, as well as the size that was used to allocate that memory
  2. Free the memory address of this function pointer
  3. On the next malloc of the appropriate size, the same address should be returned
  4. Overwrite the function pointer with user-controlled data (a pointer to my shellcode) when it is copied to this memory address
  5. Trigger the function pointer to call my shellcode

The only remaining problem is getting a shellcode to some predictable location. Thankfully, as mentioned earlier, ASLR is disabled, the stack is executable and the value we send in our host parameter is actually stored on the stack! All we need is to get this address and plug it into the function pointer to get the shellcode to execute. Obviously, despite ASLR being disabled, the stack will not always be exactly at the same place, so I used a super-l33t technique consisting of pre-pending my shellcode with lots of NOPs (because 2016).

So we can now break down each step and look at how we can achieve them:

  1. With some reverse engineering and debugging, I found one function pointer that was always allocated at address 0x2840a0c0. This function seems to be used to decode parameters sent in the HTTP requests. The memory address is initially allocated at 0x08097fb9 with a call to malloc(32).
  2. Use the overflow to overwrite the pointer to the parameter list with 0x2840a0c0. The address is then free‘d when the request has finished being processed. Here, we also need to take a note of where our host parameter is located on the stack, as this is where we will store our shellcode.
  3. While searching through the binary’s code, I found one place where a malloc is called with a length which can be specified by the VPN user directly. This is when providing the username and password to log into a SMB server. There may be other parts of the code that could be exploited in a more reliable manner, but this is the first I found and decided to go with it. The only problem is that it means we need to initiate a SMB login to a server that is accessible to the Netscaler appliance.
  4. As long as our password is between 16 and 32 characters, the previously free‘d address is returned and we can therefore overwrite the function pointer with the value of our password. It must therefore be the address of our shellcode, which we discovered was placed on the stack when performing the ping.
  5. The function pointer is actually called at regular intervals by the application while processing data, so we can just wait until it is called to get our code executed.

As you’ve probably deduced by now, in order to exploit the vulnerability, we are going to use two separate HTTP requests. The first one is used to put the shellcode on the stack and trigger the overflow, while the second is used to overwrite the function pointer with the address of our shellcode and actually execute our payload. This is summarised here:

Request 1 (ping host)

→ Start of host value contains shellcode which is conveniently placed on the stack

→ Use overflow to overwrite pointer to parameter list with 0x2840a0c0

→ When the request has been entirely processed, the program frees the address 0x2840a0c0

Request 2 (smb login)

→ Specify a password parameter of length between 16 and 32, this forces malloc to return the address that was previously freed.

→ Password value (shellcode location) overwrites function pointer that was previously located there

→ While processing the request, the overwritten function pointer is called, executing the shellcode

So that’s pretty much it. The following MSF module, should be able to exploit the flaw, but use it at your own risk. I’ve only tested it on a controlled lab environment.

For those of you who participated in our Insomni’hack teaser this year, you’ll notice many similarities with “The Great Escape – part 2” challenge, as it was very much inspired by this flaw.

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TIMELINE
======================

  • 08.12.2016: Initial report sent to Citrix
  • 09.12.2016: Case opened by Citrix to investigate the issue
  • 14.12.2016: Vulnerability acknowledged and reproduced by Citrix team
  • February-March 2017: Rollout of fixed Netscaler versions
  • 12.04.2017: Release of security bulletin: https://support.citrix.com/article/CTX222657

 

SCRT is proud to announce the opening of its new office in Bern

To sustain its growth and continue to maintain relations of proximity with its customers and partners, SCRT is glad to announce the opening of its new office in Bern.

SCRT is one of the leaders in Information Security in Switzerland, with headquarters in Préverenges (Vaud) and a branch office in Bordeaux (France). This new SCRT location will facilitate the provision of professional IT Security Services to new and existing clients in the German speaking part of Switzerland.

Ideally located in the Monbijou district, close to the train station, this office offers a dynamic, welcoming and optimal setting for our customers and staff. SCRT will continue to offer the same services at our new address in Bern and ensure our well known proximity and reactivity all over Switzerland.

We look forward to welcoming you!

SecureIT Valais – Workshop Buffer Overflow

La première édition de SecureIT s’est déroulée vendredi le 17 février à Sierre. L’événement organisé par l’AVPC (Association Valaisanne pour la Promotion de la Cybersécurité) en collaboration avec la HES-SO Valais-Wallis, Parti Pirate et le groupe de hackers étiques Fourchette Bombe, a rassemblé près de 300 participants.

J’y ai présenté un workshop sur l’exploitation d’un Buffer Overflow, vulnérabilité très ancienne et pourtant largement exploitée même de nos jours. Le but du workshop était de faire une introduction à l’exploitation de cette vulnérabilité, sans toutefois avoir la prétention d’un cours complet sur le sujet.

Voici les slides ainsi que l’exercice complet de mon workshop.

Mot de passe pour l’archive: scrt

SecureIT – Valais

Voici les slides de ma présentation de Vendredi dernier pour Secure-IT. J’y ai présenté quelques-unes des techniques les plus communément exploitées en test d’intrusion pour compromettre un domaine Windows ainsi que les différentes remédiations possibles.

Pour ceux qui n’auraient pas le temps de parcourir la totalité des slides, voici un bref résumé des recommendations:

  • Désactiver WPAD et les protocoles de résolutions de noms LLMNR et NetBios
  • Utiliser un système come LAPS pour gérer les mots de passe des administrateurs locaux
  • Limiter l’utilisation des comptes privilégiés (surtout les “admins du domaine”)
  • Utiliser AppLocker pour empêcher l’exécution de programmes non autorisés
  • Améliorer le filtrage réseau entre les VLANs internes et même au sein du même VLAN (Firewall local)
  • Limiter les privilèges utilisés par les applications (notamment Tomcat et serveurs SQL)
  • Utiliser un système de corrélation de logs pour pouvoir détecter les anomalies sur le réseau

L’utilisation de tests d’intrusion de type “Red Team” ou “Purple Team” permet ensuite de valider la pertinence des logs et des alertes remontées, ceci dans le but d’accélérer la réponse à incident pour pouvoir palier à une réelle attaque.