Among other things (which we will explain later, or maybe some eagle-eyed readers may spot it in the screenshot), the Handler module simply takes the path specified within the request, forms a path to the requested file, and renders it via the Template Toolkit template engine. The decoded webshell code is shown below. Specifically, within the csd function, which takes the value of the NSC_USER header supplied within the request and sets it as the $username variable.
Before we get into the details of bypassing the “adversary patch”, we will spend some time refreshing ourselves with what the vulnerability was, and how it is exploited. As well as backdoors, we were also able to identify specific exploitation artifacts.
Following the initial discovery of public exploitation of this vulnerability, the team at FireEye released their analysis of a new backdoor, named “NOTROBIN’, written in Golang. Of the 11 vulnerabilities patched by Citrix, attackers are attempting to exploit the following CVEs in the wild: CVE-2020-8193 is an authorization bypass vulnerability in the management interface on the device’s NSIP address. This appears to be a modified version of the PerlKit webshell. Therefore, we believe that our statistics represent just the tip of the iceberg. User supplied data added to $newBM variable: User controlled properties assigned to $doc before calling filewrite: Now we have a semi-controlled file-write where we can write an XML file anywhere on disk and control some of the content, however we need a way to leverage this to achieve code execution. At the time however, this particular attacker stood out as distinct from many other attackers, who appeared to be focused on deploying coin-miners. The majority of these compromised devices were situated in the US, with a total of 2057 backdoors and artifacts being identified. Post was not sent - check your email addresses! Possibly not. This is due to a race-condition in that our XML file is written and rendered within the same request, and thus executed before it can be deleted. An example request is shown below: Once the XML file has been written, we can then follow up with a request for the XML file, resulting in our code being executed: So now we can exploit the issue without any vulnerable Perl file existing on the target server! The contents of this file are controlled via the $doc variable, which depending on when it is called, contains various user-controlled data. Interestingly, these requests would write out a Perl backdoor line by line to a file named /netscaler/portal/scripts/loadcolourprefs.pl. The attacker must make two requests. Citrix SD-WAN 10.2.x before 10.2.3 and NetScaler SD-WAN 10.0.x before 10.0.8 have Improper Input Validation (issue 4 of 6). Note that @0x09AL also identified another method to execute code via the DATAFILE plugin. https://gist.github.com/rxwx/c51264441107c5159324080c920a96d8.jsView this gist on GitHub. However, we still have some other questions to answer. However, the csd function is also called outside of these Perl files. If we take a look again at the Handler.pm module we can see that the csd function is actually called automatically whenever the Handler is invoked, which includes any time a file is served via the /portal/templates/* path. Shodan reports ~6,000 across all ports. This resulted in some people (us included) believing that the following constraints could be relied upon for detection: Two days after the public exploits were released, @mpgn_x64 discovered that in fact any Perl file which called the csd function could be exploited, regardless of whether user-provided data was added to the written XML file. Cyber security is an arms race where both attackers and defenders continually update and improve their tools and ways of working. What if we craft a HTTP request that both writes and requests our XML file? Citrix disclosed on July 7th, 2020 a number of vulnerabilities in the Application Delivery Controller. This is also explained in the MDSec blog post. For example, when studying the “Iran Network Team” attacks, we noticed that the attacker would commonly stage secondary payloads within the public directory of the server, meaning that their presence could be easily detected.
The NSIP address is a specific device IP address dedicated to the management interface for Citrix devices.
This multidisciplinary team converts our leading cyber threat intelligence into powerful detection strategies. This multidisciplinary team converts our leading cyber threat intelligence into powerful detection strategies. Although the Template Toolkit can allow code execution via template directives such as PERL and RAWPERL, these are disabled in the configuration used on the Citrix server. Make another request to the `/vpns/portal/
Cyber security is an arms race where both attackers and defenders continually update and improve their tools and ways of working. This included patching CVE-2020-8195 and CVE-2020-8196 as well. In January 2020 a total of 1030 compromised servers were identified. NCC Group’s RIFT have been able to achieve compromise in certain, at the moment, esoteric configurations. : Effectively making the vulnerability only exploitable by an actor with prior knowledge of the infection key. Two HTTP requests are required in order to achieve code execution. This means that whenever a request is made for a file within the templates directory (via a request to /vpns/portal/<file> which maps via the httpd.conf to the templates directory), the vulnerable code-path will be hit automatically, even if the requested file is an HTML or XML file, for example. This included the following files: Shortly after this information was published, we started to see the first usage of this new exploitation technique deployed in the wild.
Note that aside from bypassing adversary patches that delete the “vulnerable” Perl files such as newbm.pl from the server, this method will also bypass the NOTROBIN method of checking for (and deleting) XML files within the template directory. This marked a shift, at least for one actor, to a new type of infection, which DCSO eloquently described as “palware” – a seemingly innocent piece of malware with the primary goal of preventing other actors from deploying their own malware. However, another vulnerability is needed to turn that unintended access into arbitrary file write, and eventual code execution. It can no longer be assumed that just because a device was patched, that it does not remain compromised. July 11th, 2020 @ 14:40 – v1.5 – added timeline graphicJuly 11th, 2020 @ 12:25 – v1.4 – added various exploitation attempts and volumesJuly 10th, 2020 @ 20:50 – v1.3 – Added exposure volumesJuly 10th, 2020 @ 20:35 – v1.2 – Discussed exploit development and impactJuly 10th, 2020 @ 20:01 – v1.1 – Sigma addedJuly 10th, 2020 @ 13:50 – v1.0 – Initial version, RIFT leverages our strategic analysis, data science, and threat hunting capabilities to create actionable threat intelligence, ranging from IoCs and detection capabilities to strategic reports on tomorrow’s threat landscape. Typically, these involved a webshell being deployed to the compromised device.
Essentially, exploitation of this issue can be broken down into two steps which we will discuss in detail later. Sorry, your blog cannot share posts by email. About the Research and Intelligence Fusion Team (RIFT):RIFT leverages our strategic analysis, data science, and threat hunting capabilities to create actionable threat intelligence, ranging from IoCs and detection capabilities to strategic reports on tomorrow’s threat landscape.
As an example, the following webshell was observed being dropped as part of a group of backdoors which we refer to as the “Iran Network Team” backdoors, first described in our Reddit live blog on January 13th 2020. Yes, it does. This means that not only that the backdoor would be removed, but that the entire script, and all its legitimate functionality would be wiped out with it. Now we know that exploitation of this issue was not simply confined to one specific “vulnerable” .pl file, and that attackers are constantly evolving their attack techniques in order to overcome our assumptions of constraints of specific vulnerability exploitation, i.e. It subsequently accepts some data provided by the user in the request, including a url, title and desc parameter.
And best of all, the “exploit” fits in fewer than 280 characters. Of course, in this scenario, the best course of action is to complete an examination of the server to identify any potential backdoors or attacker-deployed patches. SANS reported on July 9th that they saw initial scanning activity but it was unclear for which vulnerability. The following screenshot shows the Citrix/FireEye IOC scanner detecting exploitation via this technique: Readers may have read in FireEye’s “404 Exploit Not Found” blog post that the attacker behind the NOTROBIN attacks also used a single HTTP request method to exploit the issue. There appeared to be no specific sector that was targeted more than any other, however backdoors were observed on high-profile organisations from a number of industries including manufacturing, media, telecoms, healthcare, financial and technology. This allows the attacker to execute their code using a template directive such as the following: Further details regarding this “feature” can be found in the GitHub issue. Our advice is that patches should be deployed as soon as is possible.
On 10th January 2020, the first public exploits were released on GitHub. Once the signatures for each backdoor variant were developed, analysis of the available data was carried out. The Citrix Application Delivery Controller (formerly known as NetScaler ADC) and the Citrix Gateway (formerly known as NetScaler ADC since version 10.5).
Discounting the fact that the request method can be arbitrary, our method does not make use of the newbm.pl file. one request to trigger the XML file drop, another to the XML (similar to the original exploit), Result in a 200 response, but could also result in a 304, Contain a traversal `../` sequence in the request path – this depends on whether the request is made to the management or virtual IP interface, 8115 servers were identified that are still vulnerable to CVE-2019-19781, Of the 8115 vulnerable servers, 2508 (30.9%) have indicators of adversary patching, These 2508 servers remain vulnerable due to the new discovery of the exploit method described in this blog, A total of 3,332 unique servers were identified to contain, 23% of the compromised servers had been officially patched, but were, Many hosts contained multiple indicators and backdoors from distinct actors, in some cases up to 5 different indicators were observed, 49% of compromised devices were located in the US.
Requests to the /vpns/portal/ path are handled by the Handler.pm Perl module via the PerlResponseHandler mod_perl directive in the httpd.conf file.
Citrix has released security updates to address vulnerabilities in Citrix ADC (formerly known as NetScaler ADC), Citrix Gateway (formerly known as NetScaler Gateway), and Citrix SD-WAN WANOP appliance. The following screenshot shows how this works. In this blog post we will revisit CVE-2019-19781, a Remote Code Execution vulnerability affecting Citrix NetScaler / ADC.
However, there is one last hurdle. This provided some interesting results, some of which are detailed below. This means that we can now control the path of where the XML file is written, as well as some limited content, via the name, url or desc parameters.
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