Analyzing Dark Crystal RAT, a C# backdoor

The FireEye
Mandiant Threat Intelligence Team helps protect our customers by
tracking cyber attackers and the malware they use. The FLARE Team
helps augment our threat intelligence by reverse engineering malware
samples. Recently, FLARE worked on a new C# variant of Dark Crystal
RAT (DCRat) that the threat intel team passed to us. We reviewed open
source intelligence and prior work, performed sandbox testing, and
reverse engineered the Dark Crystal RAT to review its capabilities and
communication protocol. Through publishing this blog post we aim to
help defenders look for indicators of compromise and other telltale
signs of Dark Crystal RAT, and to assist fellow malware researchers
new to .NET malware, or who encounter future variants of this sample.

Discovering Dark Crystal RAT

The threat intel team provided FLARE with an EXE sample, believed to
contain Dark Crystal RAT, and having the MD5 hash
b478d340a787b85e086cc951d0696cb1. Using sandbox testing, we found that
this sample produced two executables, and in turn, one of those two
executables produced three more. Figure 1 shows the relationships
between the malicious executables discovered via sandbox testing.

Figure 1: The first sample we began analyzing ultimately
produced five executables.

Armed with the sandbox results, our next step was to perform a
triage analysis on each executable. We found that the original sample
and mnb.exe were droppers, that dal.exe was a clean-up utility to
delete the dropped files, and that daaca.exe and fsdffc.exe were
variants of Plurox, a family with existing reporting. Then we moved to
analyzing the final dropped sample, which was dfsds.exe. We found
brief public reporting by @James_inthe_box on the same sample,
identifying
it as DCRat and as a RAT and credential stealer. We also found a
public
sandbox run that included the same sample. Other public
reporting described
DCRat, but actually analyzed the daaca.exe Plurox component
bundled along with DCRat in the initial sample.

Satisfied that dfsds.exe was a RAT lacking detailed public
reporting, we decided to perform a deeper analysis.

Analyzing Dark Crystal RAT

Initial Analysis

Shifting aside from our sandbox for a moment, we performed static
analysis on dfsds.exe. We chose to begin static analysis using CFF
Explorer, a good tool for opening a PE file and breaking down its
sections into a form that is easy to view. Having viewed dfsds.exe in
CFF Explorer, as shown in Figure 2, the utility showed us that it is a
.NET executable. This meant we could take a much different path to
analyzing it than we would on a native C or C++ sample. Techniques we
might have otherwise used to start narrowing down a native sample’s
functionality, such as looking at what DLLs it imports and what
functions from those DLLs that it uses, yielded no useful results for
this .NET sample. As shown in Figure 3, dfsds.exe imports only the
function _CorExeMain from mscoree.dll. We
could have opened dfsds.exe in IDA Pro, but IDA Pro is usually not the
most effective way of analyzing .NET samples; in fact, the free
version of IDA Pro cannot handle .NET Common Language Infrastructure
(CLI) intermediate code.

Figure 2: CFF Explorer shows that dfsds.exe is a .NET executable.

Figure 3: The import table for dfsds.exe is not useful as it
contains only one function.

Instead of using a disassembler like IDA Pro on dfsds.exe, we used a
.NET decompiler. Luckily for the reverse engineer, decompilers operate
at a higher level and often produce a close approximation of the
original C# code. dnSpy is a great .NET decompiler. dnSpy’s interface
displays a hierarchy of the sample’s namespaces and classes in the
Assembly Explorer and shows code for the selected class on the right.
Upon opening dfsds.exe, dnSpy told us that the sample’s original name
at link time was DCRatBuild.exe, and that its entry point is at <PrivateImplementationDetails>{63E52738-38EE-4EC2-999E-1DC99F74E08C}.Main,
shown in Figure 4. When we browsed to the Main method using the
Assembly Explorer, we found C#-like code representing that method in
Figure 5. Wherever dnSpy displays a call to another method in the
code, it is possible to click on the target method name to go to it
and view its code. By right-clicking on an identifier in the code, and
clicking Analyze in the context menu, we caused dnSpy to look for all
occurrences where the identifier is used, similar to using
cross-references in IDA Pro.

Figure 4: dnSpy can help us locate the
sample’s entry point

Figure 5: dnSpy decompiles the Main
method into C#-like code

We went to the SchemaServerManager.Main
method that is called from the entry point method, and observed that
it makes many calls to ExporterServerManager.InstantiateIndexer with
different integer arguments, as shown in Figure 6. We browsed to the
ExporterServerManager.InstantiateIndexer
method, and found that it is structured as a giant switch statement
with many goto statements and labels; Figure 7 shows an excerpt. This
does not look like typical dnSpy output, as dnSpy often reconstructs a
close approximation of the original C# code, albeit with the loss of
comments and local variable names. This code structure, combined with
the fact that the code refers to the CipherMode.CBC constant, led us to believe that
ExporterServerManager.InstantiateIndexer
may be a decryption or deobfuscation routine. Therefore, dfsds.exe is
likely obfuscated. Luckily, .NET developers often use obfuscation
tools that are somewhat reversible through automated means.

Figure 6: SchemaServerManager.Main makes
many calls to ExporterServerManager.InstantiateIndexer

Figure 7:
ExporterServerManager.InstantiateIndexer looks like it may be a
deobfuscation routine

Deobfuscation

De4dot is a .NET deobfuscator that knows how to undo many types of
obfuscations. Running de4dot -d (for detect) on dfsds.exe (Figure 8)
informed us that .NET Reactor was used to obfuscate it.

Figure 8: dfsds.exe is obfuscated with .NET Reactor

After confirming that de4dot can deobfuscate dfsds.exe, we ran it
again to deobfuscate the sample into the file dfsds_deob.exe (Figure 9).

Figure 9: de4dot successfully deobfuscates dfsds.exe

After deobfuscating dfsds.exe, we ran dnSpy again on the resulting
dfsds_deob.exe. When we decompiled SchemaServerManager.Main again, the results were
much different, as shown in Figure 10. Contrasting the new output with
the obfuscated version shown previously in Figure 6, we found the
deobfuscated code much more readable. In the deobfuscated version, all
the calls to ExporterServerManager.InstantiateIndexer were
removed; as suspected, it was apparently a string decoding routine. In
contrast, the class names shown in the Assembly Explorer did not
change; the obfuscator must have irrecoverably replaced the original
class names with meaningless ones obtained from a standard list. Next,
we noted that ten lines in Figure 10 hold base64-encoded data. Once
the sample was successfully deobfuscated, it was time to move on to
extracting its configuration and to follow the sample’s code path to
its persistence capabilities and initial beacon.

Figure 10: Deobfuscating dfsds.exe shows
that the method begins with some path manipulation and then accesses
Base64-encoded data

Configuration, Persistence and Initial Beacon

Recall that in Figure 10 we found that the method SchemaServerManager.Main has a local variable
containing Base64-encoded data; decoding that data revealed what it
contains. Figure 11 shows the decoded configuration (with C2 endpoint
URLs de-fanged):

Figure 11: Decoding the base64 data in
SchemaServerManager.Main reveals a configuration string

Figure 11 shows that the data decoded to a configuration string
containing four values: MHost, BHost, MX, and TAG. We analyzed the
code that parses this string and found that MHost and BHost were used
as its main and backup command and control (C2) endpoints. Observe
that the MHost and BHost values in Figure 11 are identical, so this
sample did not have a backup C2 endpoint.

In dnSpy it is possible to give classes and methods meaningful names
just as it is possible to name identifiers in IDA Pro. For example,
the method SchemaServerManager.StopCustomer
picks the name of a random running process. By right-clicking the
StopCustomer identifier and choosing Edit
Method, it is possible to change the method name to PickRandomProcessName, as shown in Figure 12.

Figure 12: Assigning meaningful names to
methods makes it easier to keep analyzing the program

Continuing to analyze the SchemaServerManager.Main method revealed that the
sample persists across reboots. The persistence algorithm can be
summarized as follows:

1. The malware picks the name of a random running process, and
   then copies itself to %APPDATA% and C:. For example, if svchost.exe is selected,
   then the malware copies itself to %APPDATA%svchost.exe and C:svchost.exe.
2. The malware creates a
   shortcut %APPDATA%dotNET.lnk pointing to
   the copy of the malware under %APPDATA%.
3. The malware creates a shortcut
   named dotNET.lnk in the logged-on user’s Startup folder pointing to
   %APPDATA%dotNET.lnk.
4. The
   malware creates a shortcut C:Sysdll32.lnk
   pointing to the copy of the malware under C:.
5. The malware creates a shortcut named
   Sysdll32.lnk in the logged-on user’s Startup folder pointing to
   C:Sysdll32.lnk.
6. The malware
   creates the registry value HKCUSoftwareMicrosoftWindowsCurrentVersionRunscrss
   pointing to %APPDATA%dotNET.lnk.
7. The malware creates the registry value HKCUSoftwareMicrosoftWindowsCurrentVersionRunWininit
   pointing to C:Sysdll32.lnk.

After its persistence steps, the malware checks for multiple
instances of the malware:

1. The malware sleeps for a random interval between 5 and 7
   seconds.
2. The malware takes the MD5 hash of the
   still-base64-encoded configuration string, and creates the mutex
   whose name is the hexadecimal representation of that hash. For this
   sample, the malware creates the mutex bc2dc004028c4f0303f5e49984983352. If this fails
   because another instance is running, the malware exits.

The malware then beacons, which also allows it to determine whether
to use the main host (MHost) or backup host (BHost). To do so, the
malware constructs a beacon URL based on the MHost URL, makes a
request to the beacon URL, and then checks to see if the server
responds with the HTTP response body “ok.” If the server does not send
this response, then the malware unconditionally uses the BHost; this
code is shown in Figure 13. Note that since this sample has the same
MHost and BHost value (from Figure 11), the malware uses the same C2
endpoint regardless of whether the check succeeds or fails.

Figure 13: The malware makes an HTTP
request based on the MHost URL to determine whether to use the MHost
or BHost

The full algorithm to obtain the beacon URL is as follows:

1. Obtain the MHost URL, i.e., hxxp://domalo[.]online/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54
   /46kqbjvyklunp1z56txzkhen7gjci3cyx8ggkptx25i74mo6myqpx9klvv3/akcii239my
   zon0xwjlxqnn3b34w.
2. Calculate the SHA1 hash of the full
   MHost URL, i.e., 56743785cf97084d3a49a8bf0956f2c744a4a3e0.
3. Remove the last path component from the MHost URL, and then
   append the SHA1 hash from above, and ?data=active. The full beacon
   URL is therefore hxxp://domalo[.]online/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54
   /46kqbjvyklunp1z56txzkhen7gjci3cyx8ggkptx25i74mo6myqpx9klvv3/56743785cf
   97084d3a49a8bf0956f2c744a4a3e0.php?data=active.

After beaconing the malware proceeds to send and receive messages
with the configured C2.

Messages and Capabilities

After performing static analysis of dfsds.exe to determine how it
selects the C2 endpoint and confirming the C2 endpoint URL, we shifted
to dynamic analysis in order to collect sample C2 traffic and make it
easier to understand the code that generates and accepts C2 messages.
Luckily for our analysis, the malware continues to generate requests
to the C2 endpoint even if the server does not send a valid response.
To listen for and intercept requests to the C2 endpoint
(domalo[.]online) without allowing the malware Internet access, we
used FLARE’s
FakeNet-NG tool. Figure 14 shows some of the C2 requests that
the malware made being captured by FakeNet-NG.

Figure 14: FakeNet-NG can capture the
malware’s HTTP requests to the C2 endpoint

By comparing the messages generated by the malware and captured in
FakeNet-NG with the malware’s decompiled code, we determined its
message format and types. Observe that the last HTTP request visible
in Figure 14 contains a list of running processes. By tracing through
the decompiled code, we found that the method SchemaServerManager.ObserverWatcher.NewMerchant
generated this message. We renamed this method to taskThread and
assigned meaningful names to the other methods it calls; the resulting
code for this method appears in Figure 15.

Figure 15: The method that generates the
list of running processes and sends it to the C2 endpoint

By analyzing the code further, we identified the components of the
URLs that the malware used to send data to the C2 endpoint, and how
they are constructed.

Beacons

The first type of URL is a beacon, sent only once when the malware
starts up. For this sample, the beacon URL was always
hxxp://domalo[.]online/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54/46kqbjvyklunp1z56txzk
hen7gjci3cyx8ggkptx25i74mo6myqpx9klvv3/<hash>.php?data=active,
where <hash> is the SHA1 hash of the MHost URL, as described earlier.

GET requests, format 1

When the malware needs to send data to or receive data from the C2,
it sends a message. The first type of message, which we denote as
“format 1,” is a GET request to URLs of the form hxxp://domalo[.]online/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54/46kqb
jvyklunp1z56txzkhen7gjci3cyx8ggkptx25i74mo6myqpx9klvv3/akcii239myzon0xwjlxqnn
3b34w/<hash>.php?
type=__ds_setdata&__ds_setdata_user=<user_hash>&__ds_setdata_ext=<message_hash>&__ds_setdata_data=<message>, where:

• 
  <hash> is MD5(SHA1(MHost)), which for this sample, is
  212bad81b4208a2b412dfca05f1d9fa7.
• 
  <user_hash> is a unique
  identifier for the machine on which the malware is running. It is
  always calculated as SHA1(OS_version +
  machine_name + user_name) as provided by the .NET
  System.Environment class.
• 
  <message_hash> identifies what
  kind of message the malware is sending to the C2 endpoint. The <message_hash> is calculated as MD5(<message_type> + <user_hash>),
  where <message_type> is a short
  keyword identifying the type of message, and <user_hash> is as calculated above.
  • Values for <message_type> exist for each command that
    the malware supports; for possible values, see the “msgs”
    variable in the code sample shown in Figure 19.
  • Observe
    that this makes it difficult to observe the message type
    visually from log traffic, or to write a static network
    signature for the message type, since it varies for every
    machine due to the inclusion of the <user_hash>.
  • One type of
    message uses the value u instead of a hash for <message_hash>.
• 
  <message> is the message data,
  which is not obscured in any way.

The other type of ordinary message is a getdata message. These are GET requests to URLs of
the form hxxp://domalo[.]online/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54/46kqb
jvyklunp1z56txzkhen7gjci3cyx8ggkptx25i74mo6myqpx9klvv3/akcii239myzon0xwjlxqnn
3b34w/<hash>.php?
type=__ds_getdata&__ds_getdata_user=<user_hash>&__ds_getdata_ext=<message_hash>&__ds_getdata_key=<key>, where:

• 
  <hash> and
  <user_hash> are calculated as
  described above for getdata messages.
• 
  <message_hash> is also
  calculated as described above for getdata messages, but describes
  the type of message the malware is expecting to receive in the
  server’s response.
• 
  <key> is MD5(<user_hash>).

The server is expected to respond to a getdata message with an appropriate response for
the type of message specified by <message_hash>.

GET requests, format 2

A few types of messages from the malware to the C2 use a different
format, which we denote as “format 2.” These messages are GET requests
of the form hxxp://domalo[.]online
/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54/46kqbjvyklunp1z56txzkhen7gj
ci3cyx8ggkptx25i74mo6myqpx9klvv3/akcii239myzon0xwjlxqnn3b34w/<user_hash>.<mes
sage_hash>, where:

• 
  <user_hash> is calculated as
  described above for getdata messages.
• 
  <message_hash> is also
  calculated as described above for getdata messages, but describes
  the type of message the malware is expecting to receive in the
  server’s response. <message_hash>
  may also be the string comm.

Table 1 shows possible <message_types> that may be incorporated
into <message_hash> as part of format
2 messages to instruct the server which type of response is desired.
In contrast to format 1 messages, format 2 messages are only used for
a handful of <message_type> values.

Table 1: Message types when the malware uses a
special message to request tasking from the server

POST requests

When the malware needs to upload large files, it makes a POST
request. These POST requests are sent to hxxp://domalo[.]online/ksezblxlvou3kcmbq8l7hf3f4cy5xgeo4udla91dueu3qa54/46kqb
jvyklunp1z56txzkhen7gjci3cyx8ggkptx25i74mo6myqpx9klvv3/akcii239myzon0xwjlxqnn
3b34w/<hash>.php, with the following parameters in the
POST data:

• name is <user_hash> + “.”
  + <message_type>, where <user_hash> is calculated as described
  above and <message_type> is the type
  of data being uploaded.
• upload is a file with the data being sent to the server.

Table 2 shows possible <message_type> values along with the type of
file being uploaded.

Table 2: Message types when files are uploaded
to the server

Capabilities

By analyzing the code that handles the responses to the comm message (format 2), it was possible for us to
inventory the malware’s capabilities. Table 3 shows the keywords used
in responses along with the description of each capability.

Table 3: Capabilities of DCRat

Proof-of-Concept Dark Crystal RAT Server

After gathering information from Dark Crystal RAT about its
capabilities and C2 message format, another way to illustrate the
capabilities and test our understanding of the messages was to write a
proof-of-concept server. Here is a code snippet that we wrote
containing a barebones
DCRat server written in Python. Unlike a real RAT server, this
one does not have a user interface to allow the attacker to pick and
launch commands. Instead, it has a pre-scripted command list that it
sends to the RAT.

When the server starts up, it uses the Python BaseHTTPServer to
begin listening for incoming web requests (lines 166-174). Incoming
POST requests are assumed to hold a file that the RAT is uploading to
the server; this server assumes all file uploads are screenshots and
saves them to “screen.png” (lines 140-155). For GET requests, the
server must distinguish between beacons, ordinary messages, and
special messages (lines 123-138). For ordinary messages, __ds_setdata messages are simply printed to
standard output, while the only __ds_getdata
message type supported is s_comm (screenshot
communications), to which the server responds with the desired
screenshot dimensions (lines 63-84). For messages of type comm, the
server sends four types of commands in sequence: first, it hides the
desktop icons; then, it causes the string “Hello this is tech support”
to be spoken; next, it displays a message box asking for a password;
finally, it launches the Windows Calculator (lines 86-121).

Figure 16 shows the results when Dark Crystal RAT is run on a system
that has been configured to redirect all traffic to domalo[.]online to
the proof-of-concept server we wrote.

Figure 16: The results when a Dark
Crystal RAT instance communicates with the proof-of-concept server

Other Work and Reconnaissance

After reverse engineering Dark Crystal RAT, we continued
reconnaissance to see what additional information we could find. One
limitation to our analysis was that we did not wish to allow the
sample to communicate with the real C2, so we kept it isolated from
the Internet. To learn more about Dark Crystal RAT we tried two
approaches: the first was to browse the Dark Crystal RAT website
(files.dcrat[.]ru) using Tor, and the other was to take a look at
YouTube videos of others’ experiments with the “real” Dark Crystal RAT server.

Dark Crystal RAT Website

We found that Dark Crystal RAT has a website at files.dcrat[.]ru,
shown in Figure 17. Observe that there are options to download the RAT
itself, as well as a few plugins; the DCLIB extension is consistent
with the plugin loading code we found in the RAT.

Figure 17: The website files.dcrat[.]ru
allows users to download Dark Crystal RAT and some of its plugins

Figure 18 shows some additional plugins, including plugins with the
ability to resist running in a virtual machine, disable Windows
Defender, and disable webcam lights on certain models. No plugins were
bundled with the sample we studied.

Figure 18: Additional plugins listed on
the Dark Crystal RAT website

Figure 19 lists software downloads on the RAT page. We took some
time to look at these files; here are some interesting things we discovered:

• The DCRat listed on the website is actually a “builder” that
  packages a build of the RAT and a configuration for the attacker to
  deploy. This is consistent with the name DCRatBuild.exe shown back
  in Figure 4. In our brief testing of the builder, we found that it
  had a licensing check. We did not pursue bypassing it once we found
  public YouTube videos of the DCRat builder in operation, as we show
  later.
• The DarkCrystalServer is not self-contained, rather,
  it is just a PHP file that allows the user to supply a username and
  password, which causes it to download and install the server
  software. Due to the need to supply credentials and communicate back
  with dcrat[.]ru (Figure 20), we did not pursue further analysis of
  DarkCrystalServer.

Figure 19: The RAT page lists software
for the RAT, the server, an API, and plugin development

Figure 20: The DarkCrystalServer asks for
a username and password and calls back to dcrat[.]ru to download
software, so we did not pursue it further

YouTube Videos

As part of confirming our findings about Dark Crystal RAT
capabilities that we obtained through reverse engineering, we found
some YouTube demonstrations of the DCRat builder and server.

The YouTube user LIKAR has a YouTube
demonstration of Dark Crystal RAT. The author demonstrates use
of the Dark Crystal RAT software on a server with two active RAT
instances. During the video, the author browses through the various
screens in the software. This made it easy to envision how a cyber
threat would use the RAT, and to confirm our suspicions of how it works.

Figure 21 shows a capture from the video at 3:27. Note that the Dark
Crystal RAT builder software refers to the DCRatBuild package as a
“server” rather than a client. Nonetheless, observe that one of the
options was a type of Java, or C# (Beta). By watching this YouTube
video and doing some additional background research, we discovered
that Dark Crystal RAT has existed for some time in a Java version. The
C# version is relatively new. This explained why we could not find
much detailed prior reporting about it.

Figure 21: A YouTube demonstration
revealed that Dark Crystal RAT previously existed in a Java version,
and the C# version we analyzed is in beta

Figure 22 shows another capture from the video at 6:28. The
functionality displayed on the screen lines up nicely with the
“msgbox”, “browseurl”, “clipboard”, “speak”, “opencd”, “closecd”, and
other capabilities we discovered and enumerated in Table 6.

Figure 22: A YouTube demonstration
confirmed many of the Dark Crystal RAT capabilities we found in
reverse engineering

Conclusion

In this post we walked through our analysis of the sample that the
threat intel team provided to us and all its components. Through our
initial triage, we found that its “dfsds.exe” component is Dark
Crystal RAT. We found that Dark Crystal RAT was a .NET executable, and
reverse engineered it. We extracted the malware’s configuration, and
through dynamic analysis discovered the syntax of its C2
communications. We implemented a small proof-of-concept server to test
the correct format of commands that can be sent to the malware, and
how to interpret its uploaded screenshots. Finally, we took a second
look at how actual threat actors would download and use Dark Crystal RAT.

To conclude, indicators of compromise for this version of Dark
Crystal RAT (MD5: 047af34af65efd5c6ee38eb7ad100a01) are given in Table 4.

Indicators of Compromise

Dark Crystal RAT (dfsds.exe)

Table 4: IoCs for this instance of DCRat

FireEye Product Support for Dark Crystal RAT

Table 5 describes how FireEye products react to the initial sample
(MD5: b478d340a787b85e086cc951d0696cb1) and its Dark Crystal RAT
payload, or in the case of Mandiant Security Validation, allow a
stakeholder to validate their own capability to detect Dark Crystal RAT.

Table 5: Support in FireEye products to detect
Dark Crystal RAT or validate detection capability