A pentester’s take on (Not)Petya

By now, everybody has probably read many articles on the latest viral outbreak, ranging from detailed analysis of the virus itself to rants about why companies don’t apply updates in a timely fashion and why employees continue to open malicious attachments in emails.

In this post, I thought I’d take a step back and look at some of the virus’ features that make it interesting from my perspective as a penetration tester, as it mimics some of the attacks we use on a weekly basis during our penetration tests. It also highlights the importance of some of the recommendations we provide that are often overlooked because the threat is rarely fully understood.

Some of the details are still a little blurry, but one of the initial infection vectors seems to have been the update mechanism of a software used by many companies in Ukraine. This is far from the classic phishing attacks where everybody can blame the end user for the infection. And despite the fact that this new attack also uses the EternalBlue exploit to propagate internally, it is most likely not the entry point into any company, in part due to the recent WannaCry ransomware which urged companies to finally apply the appropriate updates. For once we can’t just blame missing updates and irresponsible employees!

The second interesting part of this particular malware is the fact that it leverages mimikatz-like functionality to steal passwords and re-use them to infect other machines using PsExec or WMIC. This is very similar to what is performed during targeted attacks or penetration tests. After compromising an initial host, the attacker moves laterally through the network with any compromised credentials in order to take control of as many other hosts until the desired sensitive data is compromised. Obviously, the more privileged the credentials that were initially compromised are, the easier it will be for the attacker to gain access to other hosts. In the case of the current outbreak, given the fact that there seems to be no recovery possibility, and on top of this your backups happen to be accessible on the network, you will lose everything.

If at any point during an attack a domain administrator account (or similarly privileged account) is compromised, it is pretty much game over for the company, as the attacker has essentially compromised the whole company, and recovering from this is an extremely painful process. Some people will say that their most sensitive applications and data are not located on Windows servers and therefore not impacted by this, but in all likelihood, the legitimate employees who have access to the data do it from a Windows workstation. An attacker could therefore profit from this legitimate access to compromise the “secure” data as well.

Given the Ukrainian connection to the infection vector, the impact in Switzerland will probably be relatively low, but it is not unreasonable to believe that a similar attack could be performed against software that is regularly used within this country too. I can certainly think of a number of banking software that are used by a majority of Swiss companies that might be an interesting target. It gets worse if you think about Adobe, Java or even Windows (anybody remember Flame?).

So what can we do to reduce the risk of this happening in the future?

The answer in my eyes lies within two of the fundamentals of information security that are unfortunately very rarely implemented correctly:

  • Network segmentation & filtering
  • Least privileges

Network segmentation is generally pretty well understood and most companies have dedicated VLANs for various types of devices, but too often there is no firewalling performed between the different networks, making it all too easy to move around laterally or exploit vulnerabilities in obscure protocols that run on exotic ports that could very easily be blocked.

On top of a network firewall, we often recommend the use of Windows’ integrated firewall to block off SMB and RDP ports to avoid lateral movement within the same subnet. Typically these ports should be accessed only by a restricted team to manage the workstations or for support purposes. These rules can for example be deployed by GPOs.

If these rules had been deployed in the case of the current outbreak, if an initial host was compromised, the attack would be contained to that machine, as it couldn’t use PsExec, WMIC or EternalBlue to replicate, therefore drastically reducing its impact.

Nowadays, every company has one or several firewalls, but are unfortunately rarely used to their full potential. They can be used for a lot more than just routing traffic from one subnet to another. It takes a little time to set them up correctly to allow only authorised, necessary traffic, but this remains one of the best preventive measures against virus outbreaks and attacks.

For the second aspect of “least privileges”, I have very rarely encountered a company that truly performs this well at all levels, and I will group several different weaknesses that we see in pretty much every penetration test we perform:

  1. Use of excessive privileges by services
  2. Use of the same local administrator password on multiple hosts
  3. Excessive use of domain administrator (or other privileged) accounts

Before delving into why these issues are so important, I have to mention that access to a Windows workstation or server can be granted to either local users that are defined on the host itself, or domain users (as long as the machine is part of a domain) that are defined in the Active Directory.

It is also important to understand that apart from in certain circumstances, a user with local administrator privileges can use tools such as mimikatz to recover not only the NTLM password of all locally defined accounts, but also the plain-text password (or some times only NTLM hash) of any domain account authenticated to the host. Given that the NTLM hash can essentially be used as a password thanks to pass-the-hash techniques, all local accounts and any connected domain accounts can be completely compromised by a local administrator.

This should illustrate quite well why the three weaknesses above are so important.

  1. Any compromised service that is running with local administrator (SYSTEM) privileges will give the attacker the possibility of compromising all the local and connected domain accounts. Some of the services that we exploit the most that are nearly always affected by this are Tomcat, JBoss and SQL Server.
  2. If a local administrator account is compromised and that the same password is used on other hosts, an attacker can very easily use pass-the-hash attacks to compromise all of these other servers. Essentially, by using the same password on multiple machines, you’re transforming a local administrator account into a “global” administrator account. Microsoft has published a tool called LAPS which makes it pretty easy to manage a separate password for all domain-connected Windows machines, though it is also possible to simply disable the account all together.
  3. “Domain admin” accounts should only ever be used to connect to a domain controller and as sparingly as possible. They should never be used in day-to-day administration tasks. As was explained above, any time you connect to a server or workstation that has previously been compromised by an attacker, you are gifting him your account and privileges. It is therefore pretty obvious that privileged accounts should be used as sparingly as possible to avoid this scenario. We’ll often recommend segmenting the Active Directory into various groups granting access to only a limited subset of the company’s servers or workstation to reduce the impact the compromise of such an account can have.

It is well understood nowadays that it is not a question of “if” an attacker or virus gets in, but “when”. This does not mean that securing your perimeter is pointless, but it means that you have to prepare for a breach and do your best to reduce its impact.

So I’ll wrap this up with the following recommendations that cover the points mentioned above and that we give in most of our penetration tests:

  • Use your firewalls for what they were designed for and filter access between your various subnets
  • Use Window’s built-in firewall to restrict access to management ports
  • Use LAPS or a similar password management system to avoid having the same password on multiple systems
  • Disable local administrator accounts if you don’t actually use them
  • Avoid running services with local admin privileges when it is not required
  • Make sure your day-to-day accounts do not have admin privileges, and have separate accounts for administrative tasks. (It seems obvious, but given recent pentest results, it’s worth mentioning that the passwords for these accounts should be different…)
  • Put your privileged accounts in the “Protected Users” group to avoid credentials theft
  • Only use your domain admin accounts on domain controllers, and use them as sparingly as possible
  • Make sure your backups are stored securely and that they are recoverable even in the case of a complete domain compromise

Often in the past it has been difficult to promote security within a company because it has always been hard to establish the business impact of a breach, but by reading the current news, you’ll see many companies crippled by the fact they have lost control of all their Windows machines, and maybe in some cases their backups. I’ll let you imagine the consequences that could have in your company.

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.

======================
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.

======================
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

 

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

Metasploit psexec resurrect

What a joy !

I just received tonight this nice email from github :

Meatballs1 merged commit 1a3b319 into  from 

My 2 years old pull request to metasploit was just accepted !

Long story short

Annoyed to have to chain msfencode and msfencode and msfencode to bypass anti-virus during penetration testing, we wanted to create some packers that do the job. Better than that, we wanted to integrate it in metasploit to use it with all the framework features and improve our performances :D.

I firstly figured it out that most of AVs detect ‘exe’ loader creation technique (from msfpayload) even if you put a “foobar” payload : echo -n “foobar” | msfencode -t exe -e generic/none => HIGH SCORE on virustotal.

I proposed “exe-only” technique. Shortly, it write the payload at the original entry-point of your exe template and put the section RWX so it reduces the loader signature to one RWX section only.

So next we could focus on the payload encoding.

For information, I scanned every native windows exe and find that ntkrnlpa.exe and ntoskrnl.exe contains RWX section (if AVs shoots files for having RWX sections, it would shoot Windows native exe too).

After some debate this exe-only technique was added to metasploit.

Next part was to use it with the famous psexec module that nobody use anymore because every AVs trigger it.

It’s simply because service executable created by psexec module use subsitution method, replacing “PAYLOAD:” with the payload in a template. Again, AVs trigger template regardless of the payload and to create a working template it was such a pain that we prefered use a “normal” executable and send it using psexec custom_exe feature…

So I wanted to use the previously merged “exe-only” technique to create a register service payload prepended to the user encoded payload.

That’s that stuff that took two years to land in Metasploit, mostly because I’m a noob in ruby and git (booo) and a little bit of scepticism from some metasploit guys.

Anyway, I’m proud it’s finally merged, you could just track it for fun :

07/09/2012 – https://dev.metasploit.com/redmine/issues/7231

14/10/2012 – https://github.com/rapid7/metasploit-framework/pull/903

19/05/2013 – https://github.com/rapid7/metasploit-framework/pull/1850

20/11/2013 – https://github.com/rapid7/metasploit-framework/pull/2657

07/06/2014 – Merged !

I hope you will re-use psexec now and I’m sure it bypass a lot of BIG AV at this moment because their sandbox executes the service PE that actually register itself to the SVC manager and exit. SVC manager then re run the PE beginning at the registered service entry-point.

It was very cool to speak with Metasploit guys and I know I would have to persevere for my next pull request !

NeDi Remote Code Execution

During a recent intrusion test, we discovered that NeDi was used in our target infrastructure. Since this application’s source code is freely available on the developer’s website (www.nedi.ch) I thought I’d have a look and see whether it would be possible to take control of a server through it.

It didn’t take too long to discover a call to PHP’s eval function in file inc/graph.php:


function drawFunction($function, $dx = 0.1) {
$xold = $x = $this->x0;
eval("$yold=".$function.";");
for ($x += $dx; $x <= $this->x1; $x += $dx) {
eval("$y = ".$function.";");
imageLine($this->img, $this->posX0+$xold*$this->scale,
$this->posY0-$yold*$this->scale,
$this->posX0+$x*$this->scale,
$this->posY0-$y*$this->scale, $this->grn);
$xold = $x;
$yold = $y;
}
}

We see that parameter $function is used directly in the eval(). Where does this particular parameter come from? Well drawFunction is called from page OtherPlot.php:


$cmd = isset($_GET['cmd']) ? $_GET['cmd'] : '';
$res = isset($_GET['res']) ? $_GET['res'] : 'vga';
$xf = isset($_GET['xf']) ? $_GET['xf'] : 4;
$yf = isset($_GET['yf']) ? $_GET['yf'] : 4;
$xt = isset($_GET['xt']) ? $_GET['xt'] : 4;
$yt = isset($_GET['yt']) ? $_GET['yt'] : 4;
$f = isset($_GET['function']) ? $_GET['function'] : 'sin(30 * $x) * 1 / cos($x) / $x';
# $f='tan($x - $x * cos(pi() * $x))';

if ($cmd=="img"){
include_once ("inc/graph.php");
$graph = new FunctionGraph($xf,$yf);
$graph->drawAxes();
$graph->drawFunction($f, 0.01);
$graph->writePNG();
$graph->destroy();
die;
}

As you can see, the parameter which is sent to the vulnerable function is taken directly from the function GET parameter.

Moreover, this particular piece of code can be called by an unauthenticated user, meaning that it is possible to execute arbitrary code (unless there is some sort of PHP hardening going on) on the web server without needing any credentials.

After discussing the issue with Remo Rickli, who wrote the application, it turns out this is a piece of legacy code which is not used any more and will be removed in future versions. For the time being, the recommendation is to remove the Other-Plot.php and inc/graph.php files from your servers (http://www.nedi.ch/nedi-news/).