Hi, welcome. Thanks for attending our talk. This is Room for Escape,
Scribbling Outside the Lines of Template Security.
So, my name is Alvaro Munoz, also known as Pwntested in the social media.
I'm a security researcher with the GitHub security lab.
And with me, my colleague, Alexander Mirosh, also known as Oleg Mirosh in Twitter.
And he is a security researcher with Micro Focus Fortify team.
So, today, we will be reviewing the security of content management systems, or CMS.
And, basically, we will be focusing on the security of the template systems
that are used by CMSs. So, a content management system is basically an application that is used
to manage different types of web content, so that users can actually create, upload,
and publish different types of content. So, normally, this content is based on some
structures and some kind of documents called templates, which are used to generate and
create dynamic content. So, these templates normally allow a subset of different programming
language capabilities. So, they are powerful in the sense that they can be used to run arbitrary
code. Because of that, they are normally sandboxed in order to prevent arbitrary code execution and
remote code execution. So, our research will focus on both .NET and Java-based content
management systems, because these are the languages that are more prevalent across enterprises.
And our assumption is that the attacker can control the templates, right? So, we don't really
care about how the attacker gets control of these templates in the first place, if it's through
a server-side template injection, or if it's maybe through our cross-site scripting that allows him
to submit arbitrary templates in behalf of someone else. Or maybe they have permissions,
like if you are a user, for example, in SharePoint, if you have an account in SharePoint, you are
allowed to create your own sites and content. So, no matter how the attacker can edit the content
of a template, our research will focus on escaping and breaking all the mitigations put in place to
prevent arbitrary code execution. So, we will first start with .NET. Basically, we will focus
on SharePoint. We will present five different ways we were able to break the sandbox used by
SharePoint to prevent arbitrary code execution. And then we will move into the Java part, where
we will present four different template engines, including the most popular ones, such as Velocity
and FreeMarker. And then we will present 10 different content management systems, and we will
try our bypasses on them. So, we will wrap up with some takeaways and conclusions, and then open the
floor for Q&A. So, let's start with the .NET part. Thank you, Alvaro. In this part of our presentation,
we will present different types of security problems in content management systems, and give
examples of them in SharePoint. But before that, we need to explain some basics of SharePoint
security. SharePoint has two types of ASPx pages. The first type is application pages. Each of these
pages are the part of SharePoint server and implement some application logic. They are stored
in specific folders on the file system. Of course, users are not able to change them. They are regular
ASPx pages and are processed by server without any restrictions. In opposite to them, there are
side pages. They are stored in database and they can be customized even by users. SharePoint parses
them in special mode, also known as safe mode. Actually, they are more like some sort of templates
for presenting dynamic content. On this slide, we can see a diagram how server works with these
pages. Virtual provider can fetch content from file system or database. Based on the virtual path of
the current page, ASP page parse filter will decide either it is side page and safe mode should be
applied, or it is an application page and it will be processed without any restrictions. So, page
filter is a critical element from security design. Let's have a look on a simple ASPx page.
For our attacks, we are only interested in server-side components. For example, directives.
There are special instructions on how server should process the current page. Often, they have
some number of attributes. We can include server-side code either in special blocks or as
embedded server-side code. To be processed by server, control should have the runout server
attribute. The next element is server-side common block. It is ignored by server. And
server-side include directives allow us to include raw content of arbitrary file.
Now, we can better understand what safe mode means. Side page will not be compiled, so we cannot use
server-side code. We are not able to include files from file system. We can only use allowed
controls. This list is defined in safe control section of web config file. Page filter also
applies an allow list for directives and even for attributes of most of them. There are many
other restrictions. For example, event binding is blocked as well. As we can see, it is a set of
quite strong restrictions how it can be bypassed. As we know, safe mode is enforced by page filter.
So, our first question was, is there any place where SharePoint does not use it? And answer yes,
there are such places. Page filter will not be applied for the content argument of the parse
control method if it's called only with one argument or if the second one that actually has
name ignoreParseFilter is true. In addition, page filter is used at rendering time but is ignored
at design time. But even if page filter is not applied, SharePoint uses another way to verify
the input. It is a verify control on safe list method from editing page parse type that aims to
do the same task at the original page filter, block markup with unsafe elements. But let's
imagine for a second that we are able to bypass either this method or page filter. By the way,
each such bypass is separate vulnerability and we will present them in a few minutes.
So, how could we get remote code execution after escaping the safe mode? Remember,
since parse control method never calls compilation, we are not able to use
server-side code or perform any other attack that require compilation. But if we escape
safe mode, we can use unsafe controls or directives. Let's review several of them.
One of the most interesting unsafe controls is object data source. It allows us to invoke public
methods from arbitrary public type. We can see an example of payload that uses start method from
process type to launch calculator. So, actually, it is arbitrary code execution, jackpot. But let's
review a couple other unsafe controls. XML data source and XML controls allow us to get content
of arbitrary XML files. The similar results we can get by server-side include directives
that return us content of any text files. Let's see how we can escalate from arbitrary file read
to arbitrary code execution. If we can access web.config file, we will be able to get validation
key from machine key section. By the way, this value may be present in other places such as
internal SharePoint properties. We will see this example a bit later. With validation key,
we can generate a valid view state that will execute arbitrary code commands on the target
server. We can use tools such as userserial.net for this. Now, when we know what safe mode is and
how we can compromise SharePoint server if it is bypassed, we can move to our next part of this
talk. Here we will present five different types of security problem in content management system.
We will show examples of each type in SharePoint. We would like to stress here that our target is
SharePoint server with default configuration. All vulnerabilities allowed us to execute arbitrary
code on the target and all attacks were performed by regular user. No admin rights were needed.
Let's start. The first type of security problems is access to resources with sensitive information.
It can be configuration or business information in various files, logs, database, or even process
memory. Of course, proper sandbox should prevent this problem, but it is not always the case.
We already mentioned that page filter blocks server-side includes directives
and verify control on safe list method should do this as well, and actually it does.
But if it's called with false in block server-side includes argument, it will not block them.
And we found such usage of this method. It was used for design mode.
The next building block for our talk, how we can provide our markup for this mode. We can send it
in web part XML parameter of the render web part for edit method of the web page services.
So we can try to include web config file there. Here we can see the full payload for the mentioned
parameter and address to relevant web services. Server returned us the entire web config with
validation key. We use it for view state-based deserialization attack, which led us to
executing arbitrary OS command on the server.
Time for our next type of security problems in content management system.
Allow dangerous element that can be used for attacks.
We have found an example in SharePoint safe control list,
or should we call it not so safe control list in this case.
It is contact link suggestion micro view control. We can see snippet with interesting code from
implementation where request parameter is passed to data set read XML method.
If we can control input for this method, we can get our data deserialized by XML serializer
with control type. We already showed in one of our previous work that in this case we can get
arbitrary code execution. Unfortunately, this payload is too big for slides, but you can find
it in our white paper. The last piece of the puzzle is our site page with this dangerous control.
You can see it on this slide. For the attack, we just need to place the payload into the text box
and click on submit button and desired command will be executed on the SharePoint server.
For us, the next type of security problems looks quite interesting, so we will explain it with
more details and present two separate examples from SharePoint server. Dynamic content usually
means that we will have at least read access to some properties or attributes of some objects.
For some system like SharePoint, we can even write values there. Of course, some box should filter out
the access to dangerous property or attributes. Let's review basic type of such filtering.
The first case is when only one level of properties or attributes is allowed.
We can see examples on the slide. Name property of user object of select value from menu.
We can imagine the object and its properties like a branch with only leaves.
If an allow list is applied, it's relatively easy to review all elements. And after such review,
we can make sure that there is no dangerous elements. What if block list is used instead?
We still need to verify available objects and it can be a bit difficult.
So in general, we do not consider a block list a safe approach because of potential
gaps there. We will see in Java part a couple of examples of such bypasses.
The most interesting case for us is where access to nested properties or attributes
is allowed. For example, request.authUser.name or menu.selectedItem.text.
Because of branch-leaves analogy, many developers may imagine this case as branch with branches
and leaves. And they may apply a filter only for the first level objects. But it is not true.
It is a network. And we may jump from one branch to another branch or even to the trunk.
For example, on this slide, we can see how we can access application instance from a new web
control. Let's illustrate this problem by examples from SharePoint.
There is wikiContentWebParseControl. It is allowed and can pass our markup into the
parseControl method. Please note, it is called with false value in ignoreParseFilter argument.
So, PageFilter will not be ignored. The following snippet shows that in this case,
the virtual path will be created from AppRelativeVirtualPath property.
And if you remember, PageFilter will apply restriction based on this virtual path.
So, if we are able to change it, we can fool the PageFilter to not apply any restriction to our
markup. Let's try to graph such payload. First snippet fails, because when ASPxParser tries to
assign a new value to AppRelativeVirtualPath, the Page property of our control is not defined yet.
So, we need to delay this assignment a little bit. We can do this by data binding. So, by the time
the property is bound, the Page property will be already defined.
Here is our final payload. We can notice that our new path points to application page setting ASPx.
It is assigned to ToolTip property and later will be transferred to AppRelativeVirtualPath.
So, the PageFilter will think that this is system page and will allow any unsafe markup.
For example, we can use ObjectDataSource control to start calculator.
Let's see how the real attack may look.
We can see a site page with our payload. Path to system page. It will be assigned to
AppRelativeVirtualPath. Unsafe markup contains ObjectDataSource control.
We will invoke start method of the process type and we'll start calculator. Let's save it here.
For our attack, we need to upload it to the target server. In our case, it's SharePoint
with default configuration. That means regular users have access to their own personal sites
and we can use them to host our site page. We are notificated as regular user with name attacker.
Sneaky, isn't it? And here is his personal site. Let's upload our page here.
Now, to trigger our attack, we just need to click on it. But before that, let's take a look on the
right slide of our screen. We can see task manager on our SharePoint server and we will notice if
calculator start. Now, let's start our attack and here is our calculator.
Let's move to our next vulnerability in SharePoint. Now, we will try to exploit read access to nested
properties. We will use control parameter. It allows to bind value of property from another
control to select prop parameter. Also, it supports nested properties. We need to deliver value of
target property back to us. To do this, we will use XML URL data source control that sends value
of select parameters to external server. Now, we need to find accessible property with sensitive
information. Here, we should explain a little bit configuration process of SharePoint online
servers. Obviously, they are installed and configured automatically. But what about
configuration parameters that are unique for each installation? They will be provided in special
file that will be used for unintended configuration. What is important for our attack? All such
parameters, including validation key, will be stored in initialization setting property. So, we just need
to find a path of nested property to it from any allowed control. And here it is. Here is the whole
page that will send value of validation key to attackers.server.com. With this value, we
successfully got arbitrary code execution by unsafe deserialization in view state.
Now, let's move to the next type of security problems. It is connected with our previous work
on unsafe deserialization. Such kind of problem is relevant when text or binary data is converted
to an object and the type or class of this object is under attacker control. Plus, it is not properly
limited. Actually, it doesn't really matter what mechanism is used. Classical deserializers,
JSON unmarshalers, type converters, or even some custom approach. All of them are potentially
dangerous. For successful exploitation, attacker just may need to find proper gadget on the system.
We have found examples of such problems in SharePoint and were able to perform arbitrary
code execution attack using it. But patch is not available yet. So, we are going to publish all
details of this problem when fix is released. The last type of security problems in this part
is the classical time of check to time of use problems. They occur when the server invalidates
some input, but this input is later changed before the server uses it. We have found such problem in
web part editing surface page. User input is validated by the already mentioned verify control
on safe list method, but later the server may modify the input and remove substring that match
specific regex pattern. Let's look on the next input. For validation control on safe list, it is
just one server-side comment block, so it will pass validation. But SharePoint removes the highlighted
yellow substring and pass control will see two comments and some markup. Note, pass control has
only one argument, so patch filter will be ignored. Time for our payload. We can notice our unsafe
object data source control and calculator command. In this part of our talk, we saw multiple ways
to achieve arbitrary code execution on the SharePoint server by regular user.
These vulnerabilities revealed five different types of security problems in content management
system. Now let's switch back to Alvaro and he will present even more vulnerabilities with
templates for dynamic content in different Java frameworks and products. Thank you, Alexander.
So switching to the Java part, we will be reviewing four of the most popular Java template engines,
including FreeMarker, Velocity, Pebble, and GeneJava. And also we will be reviewing 10 different
content management systems that are using these engines and then we will be trying our bypasses on
these content management systems. So a brief introduction into what template engines are.
Basically these are systems that take two different inputs. One is the template where
we are mixing static and dynamic expressions and also the template context where we store objects
that are required by the engine in order to resolve the dynamic expressions in the template.
For example, here we have the user.name expression. In order to resolve it, we need to get access to
the user object in the context and then invoke the getName method. So because it's invoking Java
methods, it's basically dangerous in the sense that attackers can execute arbitrary Java code
and that's the reason why most of them are sandboxed in order to prevent arbitrary code execution.
So apart from the objects that are normally and directly used by the template,
there are a number of other objects that are exposed as part of a template API that is normally
provided by the content management systems, which basically provides objects such as, for example,
the request, the response, the separate context, or even the HTTP session. So those objects are
available in most of the template APIs and also many more that are part of this API.
So we will be discussing first some generic bypasses. That means that are independent
of which engine is used, no matter if it's FreeMarker, Velocity, Pebble, or GeneJava, right?
So those bypasses are based on finding different objects that we can use to get arbitrary code
execution. So we need to inspect the template context and look for this kind of objects.
In order to do that, if we have access to the JVM, we can basically debug it or we can
instrument it, dump the template context, and inspect and analyze all the objects in there.
However, if we don't have access to the JVM, we will have to read the template API documentation,
maybe do some brute forcing in order to guess names like, for example, request or rec or
response resp and things like that. And also in some of the engines, like, for example,
FreeMarker, it is possible to list all the variables or objects that are exposed in the
context. So apart from those objects that are directly exposed to the context, we have some
objects that are indirectly exposed. Like, for example, if we are exposing the HTTP session
because it contains some attributes, all of those attributes will be available for the users and
attackers to access and use. Same thing with the servlet request and same thing with the servlet
context. So those are stores of attributes, stores of objects that the attacker can get access to
and use to look for interesting objects. So let's see how many objects we can find in one
simple content management systems. In this case, it will be LiveArray. And in order to do that,
we will be generating this template that I'm showing you, which basically is listing all of
the context variables, then all of the request attributes, then we will show all the request,
sorry, the session attributes, then the servlet context attributes. And because the servlet
context will expose the Spring framework context, we will be also listing all of the Spring bins,
all the objects registered in the Spring context. So if we render that,
you will see that we have a bunch of objects directly exposed to the context. Those are part
of the template API. And some of them are very interesting. Others are exposed as part of the
request attributes, session attributes, and servlet context attributes. So as we can see here,
we have interesting objects like an instance manager or, for example, the web application
context for Spring that we use to list and access all the Spring bins. So as you can see, there are
hundreds and thousands of objects here. There is a total of almost 900 objects. And because
some of them will have public fields that we can get access to, then this number will increase to
thousands of objects. So back to the slides. This is why we call it object dumpster diving,
because this is how we felt when trying to find very few interesting objects that can lead to
remote code execution among, you know, thousands and thousands of objects. So now we will be
reviewing the top three objects that we found that lead to remote code execution. So for example,
the first one, the class loaders, was available in 10 out of the 10 content management systems
that we analyzed. So normally you can get access to this class loader instance through the get
class loader method in the class type. But that is normally blocked by any block list by any
sandbox. So you can still get access if you get access to a protection domain or a servlet context
as we saw before. So once that you get access to a class loader, you can obviously load arbitrary
classes and then also load arbitrary resources from the class path. But apart from that, you get
access to a URL object and then from there you can access any file from the file system that the
application server has permissions to access. So here you have an example. You get the URL using
the get resource method and then you point the URL to the password file and then you open a
connection and read all the bytes from that file. So apart from the standard class loaders,
because content management systems are normally deployed on top of servlet containers or
application servers, the instance of the class loader that you will be able to access is normally
an instance of a web application class loader that are slightly different because of the way that
class loading delegation work, but also because they expose a number of additional methods that
are not part of the abstract class loader class. So here are just some examples. Just mentioned
because we don't have time to cover them in this talk, although it's explained in the white paper
that we are publishing as part of this talk, there are different ways of using these
methods in this web application class loader to get remote code execution. Just to mention some of
the vectors, we will be able to upload web cells, instantiate arbitrary objects, perform JNDI
injections, or even initialize attacker-controlled classes. So we were able to find web application
class loaders in nine out of the ten content management systems that we analyzed. And then
the second object, the second most easy to find object in this template context was the instance
manager or the object factory. So this was found in nine out of the ten content management systems
and is normally found as part of the self-led context attributes, but also it can be accessed
through one of these web application class loaders. So if you get access to an instance manager,
what you can do is basically instantiate arbitrary types. So with that, it's very easy to
escalate to remote code execution. Here we have an example using the script engine manager, but
there are a bunch of classes that can lead to remote code execution, ones that you can instantiate
them and then invoke arbitrary methods on them. So the number three object was a Spring application
context. It was only available in four out of the ten CMSs that we analyzed. Actually, the only four
that were using the Spring framework underneath. And it's normally accessible through one of the
self-led context attributes, but also if the content management system is using a Spring MVC
framework for the model, controller, and view layers, then Spring will inject some additional
objects in the template context, like this Spring macro request context that will provide access to
the Spring application context. So once that we have access to the Spring context, we can get an
instance of a class loader and initiate a class loader-based attacks like we saw before. We can
access the Spring environment that will give us access to the system environment and also the
system properties and also Spring properties, including things like encryption keys and so on.
But probably more interesting, it will give us access to the Spring bins registered in this
Spring application context. So being able to access and manipulate those bins, we can basically
control the application logic and do things like register new users, delete users, perform
transactions, anything that the application is normally doing. So that's normally behind an
authorization layer. So no matter what roles we have, we will be able to perform
this kind of application logic control. So apart from that, we will also be able to disable the
template engine sandboxes if we are able to get access to the template engine configuration bins
and also instantiate arbitrary objects, like we will see later, by accessing JSON or XML and
marshalers. So those were like genetic bypasses, independent of what template engine was used
underneath. And now we will be reviewing some specific sandboxes bypasses for each of these
four different template engines. So we will start with FreeMarker because it's probably the most
popular one and also because, well, it was analyzed and researched by different researchers in the
Like, for example, James Kettle back in 2015, when he presented server-side template injection
vulnerability class. By the way, if you haven't watched this talk, it's highly recommended.
He presented a bypass that is based on a kind of add-on or module that is deployed on the
default configuration of FreeMarker. So Tony Torralba last year presented a different bypass,
but this one is not universal in the sense that it depends on a non-default configuration
and also being able to find gadgets in third-party libraries that may or may not be
available in a specific target. Then Ryan Hanson this year, probably at the same time that we were
doing this research, presented a vector that leads to remote code execution by uploading
a web shell, but this is not universal in the sense that it only works with Tomcat servers.
So FreeMarker sandbox is a method-based block list, which basically means that, for example,
methods like the getClassLoader method in the java.lang class is blocked and you cannot
get an instance of a class loader using this method. But as we mentioned before,
you can still use the separate context or the protection domain to get access to these
class loaders. Once you get access to a class loader, the class loader methods are not blocked
by the block list, so you can still interact with them, load classes, get resources,
or initiate remote code execution attacks that we saw before. And also reflection is
forbidden to set fields values, but is allowed to get and read field values.
So we use that to perform an attack, but this attack is not universal in the sense
that it depends on being able to find an instance of a URL class loader.
So this is normally the case in Tomcat, GlassFish, and Jetty, but it's not universal for other
application servers. So in this case, we are basically getting the URL class loader from
the protection domain or from the separate context. And then we are basically instantiating,
by invoking this new instance method, a new URL class loader that is pointing to the attacker
control jar file, in this case the pawn.jar file. Once we get access to this class loader,
we can load arbitrary classes from our own jar file, and we will be loading the pawn class.
And now the class is loaded, but it's not initialized. If we want to run arbitrary code,
we need to initialize the class. We cannot instantiate the class, we cannot invoke static
methods. So those vectors, those ways of initializing the class, are not a possibility
for us, but we can get access to public static fields. So just by adding a public static field
and getting access to that, the class will be initialized and the payload will be executed.
So that was not a universal bypass in the sense that we are required to find an instance of a URL
class loader, and we want to find something that works no matter if we are able to find that kind
of class loader or not. So we are interested in finding some classes that contains public static
fields of a given type, which contains a method that we can use to instantiate arbitrary types.
To find them, we use CodeQL, which is a language that you can use to query your source code in the
same way that you use, for example, SQL to query a database. And what we wrote was this query that
basically says, okay, find me all the public static fields which are of a given type that
can instantiate arbitrary types. We run this query on FreeMarker source code, and we found
four different results. Those are four different fields of different types, but all those types
extend from the bins wrapper type, which contains a new instance method, which we can use to
instantiate arbitrary types. Now, this is the universal remote code execution bypass for
FreeMarker. Basically, we need to get us access to any type of class loader from either the
protection domain, separate context, and then load the object wrapper interface and get the default
wrapper field from this interface. And then we can use that to instantiate arbitrary classes.
Since we want to have this payload basically contained in FreeMarker classes, we are using
the execute class in order to run arbitrary system commands. But you can use other payloads,
to get arbitrary code execution. This was fixed in version 2.30 of FreeMarker.
And also, if you get access to the Spring Bins, and you can get access to the FreeMarker
configuration, as we saw in some of the content management systems that we analyzed,
you can basically disable the sandbox. For example, you can get the class resolver for
the default configuration that is non-sandboxed, and then set that class resolver as the class
resolver for the current and sandboxed configuration. So effectively disabling
the sandbox and being able to execute the old payload from JamesKettle once again.
So now we will be reviewing eight different ways to escape the sandbox
and get remote code execution in LiveArray. So let's see
our LiveArray instance here. And what we will be doing is basically
using this template. And here, what we are going to do basically is, first of all, we are going to
use the class loader payload, which will allow us to read the password file using the URL trick
that we explained before. Then, because the instance of the class loader is a web application
class loader, we will be using that class loader to upload a web cell by using the write method.
This will be available in the shell JSP file. Then we will be also using the web application
class loader to get access to an instance manager and then instantiate arbitrary classes. In this
case, we are instantiating an expression language processor and then running the ID process.
Then we will be also showing a different payload that depends on using an instance manager. This
time is taken from the self-led context. And again, we are using the EL processor to run the ID
command. And then we will be using a JSON deserializer as an instance manager in the sense
that we will be using that to instantiate arbitrary types. We will be getting access to the
JSON deserializer from the Spring application context. And because this JSON deserializer uses
a while list and allow list here, we need to register or we can actually register any arbitrary
types for us to use in the JSON payload. So, now by deserializing this JSON payload,
we are effectively instantiating an EL processor and then again running the ID command.
Same thing, but instead of getting the JSON deserializer from the Spring context,
LiveRay directly exposed this JSON factory util, which we can use to create a JSON deserializer
and then once again use that to instantiate arbitrary types. And then we will see the two
free market specific bypasses that we explained a few minutes ago. The first one is using the URL
class loader that is the case here because LiveRay is deployed on Tomcat. And then we can
basically access our own jar file, load it, and then by getting a public static field,
the class will be initialized and the payload will be executed. And we will be able to check
the output of this payload by checking the standard output. And the last payload that worked
in LiveRay, actually there's a couple more, but eight was more than enough,
is this universal RC. So we are loading the object wrapper interface, loading the default
wrapper field, and then using that field to instantiate arbitrary types, in this case
the execute type that we will be using to run the ID. So now if we save this template and
we will be showing the results of these payloads. The first one was reading the password file,
the second one was deploying a web cell that we can now access and run arbitrary commands.
The third one was using different methods to run the ID command, as you can see here,
all of them worked. So also the URL class loader was basically running the static initializer
block of this class, which is basically printing pound in the standard output console. We can
check that. And here it is, so it worked. And back to the payloads, the universal RC
payload also worked, as we can see here. So switching to the Velocity Sandbox,
this one is based on a class and package-based block list. So things like, for example,
the whole Java lang reflect package is forbidden and you can get access to it.
All the methods in Java lang class and class loader are also blocked and you cannot use them.
But when we reviewed the implementation, we found that there is a flaw in the way that
this sandbox is checking the block list. So, for example, when you run the request
and then you get access to a class loader and run the load class method,
basically the execution of this method will be checked against the block list.
And the class name of the current class loader will be checked against the block list.
Instead of checking the whole hierarchy of classes, only the current object class will
be checked against the block list, which means that because Java lang class loader is an abstract
class, it won't never be the case that the current object class will be a Java lang class loader.
In this case, it's a parallel web app class loader, which is not present in the block list,
and therefore class loader methods are effectively allowed.
There is a feature in Velocity, which allows you to invoke a static method from a Java lang
class object. So once that we are able to load a class, we can just invoke any static method and
use that to get remote code execution easily. So now moving to GeneJava sandbox. This one is
based on a method block list. This is actually very short, only nine methods.
And what it contains is a piece of code that is pretty effective in protecting the sandbox.
So every time that method invocation happens as part of the evaluation of the template,
the result type will be checked. And if it's an instance of a Java lang class,
then it will throw an exception and it will refuse to return a Java lang class instance.
By not being able to get in access to arbitrary classes, we are normally not able to get
or to be able to instantiate arbitrary types. However, we found a flaw. As you can see here,
the invocation of a class cannot return a Java lang class instance, but it can return an array
of Java lang classes or a map including Java lang classes. So what we can do is find a method
returning an array of classes and then accessing any arbitrary item of this array.
So the second part of this bypass was basically being able to access a secret or hidden variable
in the context that is called __interpreter in lit speech. So by using this hidden variable,
we were able to access a bunch of different objects, including all the context objects.
And we were able to use those two parts to get an instance of a class loader.
So as you can see here, we use the interpreter to get the context. And from the context,
we got all the functions. And from the functions, we got the underlying methods,
which are instances of Java lang reflect method. And this class contains this get parameter types
method, which returns an array of Java lang classes. So as I explained before,
we are able to get an instance of any individual item of this array. And by doing that, we are
finally getting an instance of a Java lang class. From there, we can get the protection domain,
get the class loader, and then initiate any class loader-based attack.
So last, Pebbles and Box, we were able to find a couple of bypasses that we reported
to the maintainers. Unfortunately, they didn't have time to fix them on time. So we decided
not to leak these bypasses yet. And we are keeping them classified for a few months.
So with that, we will switch back to Alexander, who will wrap up the talk with some conclusions.
Thank you, Arvara. First of all, let's summarize our results after this research.
More than 30 new vulnerabilities were found and reported to the vendor.
More than 20 different products are affected. We can see their names on this slide. Based on
these results, we can make the next conclusions. These are not the problems of specific product
or framework. And content management system as a class should be on Red Team radar. Templates for
dynamic content that can be managed by user is the main target for such system. There are specific
areas with higher risk from security point of view. It's a good idea to perform security review and
testing for such places. Always try to reduce attack surface as much as you can. And stay safe!