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GAME OVER: Detecting and Stopping an APT41 Operation

In August 2019, FireEye released
the “Double Dragon” report on our newest graduated threat group,
APT41. A China-nexus dual espionage and financially-focused group,
APT41 targets industries such as gaming, healthcare, high-tech, higher
education, telecommunications, and travel services. APT41 is known to
adapt quickly to changes and detections within victim environments,
often recompiling malware within hours of incident responder activity.
In multiple situations, we also identified APT41 utilizing
recently-disclosed vulnerabilities, often weaponzing and exploiting
within a matter of days.
Our knowledge of this group’s targets and activities are rooted in
our Incident Response and Managed Defense services, where we encounter
actors like APT41 on a regular basis. At each encounter, FireEye works
to reverse malware, collect intelligence and hone our detection
capabilities. This ultimately feeds back into our Managed Defense and
Incident Response teams detecting and stopping threat actors earlier
in their campaigns.
In this blog post, we’re going to examine a recent instance where FireEye
Managed Defense came toe-to-toe with APT41. Our goal is to
display not only how dynamic this group can be, but also how the
various teams within FireEye worked to thwart attacks within hours of
detection – protecting our clients’ networks and limiting the threat
actor’s ability to gain a foothold and/or prevent data exposure.
GET TO DA CHOPPA!
In April 2019, FireEye’s Managed Defense team identified suspicious
activity on a publicly-accessible web server at a U.S.-based research
university. This activity, a snippet of which is provided in Figure 1,
indicated that the attackers were exploiting CVE-2019-3396,
a vulnerability in Atlassian Confluence Server that allowed for path
traversal and remote code execution.

Figure 1: Snippet of PCAP showing
attacker attempting CVE-2019-3396 vulnerability
This vulnerability relies on the following actions by the attacker:
Customizing the _template field to utilize a template that
allowed for command execution. Inserting a cmd field that provided the command to be
executed.
Through custom JSON POST requests, the attackers were able to run
commands and force the vulnerable system to download an additional
file. Figure 2 provides a list of the JSON data sent by the attacker.

Figure 2: Snippet of HTTP POST requests
exploiting CVE-2019-3396
As shown in Figure 2, the attacker utilized a template located at
hxxps[:]//github[.]com/Yt1g3r/CVE-2019-3396_EXP/blob/master/cmd.vm.
This publicly-available template provided a vehicle for the attacker
to issue arbitrary commands against the vulnerable system. Figure 3
provides the code of the file cmd.vm.

Figure 3: Code of cmd.vm, used by the
attackers to execute code on a vulnerable Confluence system
The HTTP POST requests in Figure 2, which originated from the IP
address 67.229.97[.]229, performed system
reconnaissance and utilized Windows certutil.exe to download a file
located at hxxp[:]//67.229.97[.]229/pass_sqzr.jsp and save it
as test.jsp (MD5:
84d6e4ba1f4268e50810dacc7bbc3935). The file test.jsp was ultimately identified to be a variant
of a China
Chopper webshell.
A Passive Aggressive Operation
Shortly after placing test.jsp on the
vulnerable system, the attackers downloaded two additional files onto
the system:

64.dat (MD5:
51e06382a88eb09639e1bc3565b444a6)
Ins64.exe (MD5:
e42555b218248d1a2ba92c1532ef6786)
Both files were hosted at the same IP address utilized by the
attacker, 67[.]229[.]97[.]229. The file
Ins64.exe was used to deploy the HIGHNOON
backdoor on the system. HIGHNOON is a backdoor that consists of
multiple components, including a loader, dynamic-link library (DLL),
and a rootkit. When loaded, the DLL may deploy one of two embedded
drivers to conceal network traffic and communicate with its command
and control server to download and launch memory-resident DLL plugins.
This particular variant of HIGHNOON is tracked as HIGHNOON.PASSIVE by
FireEye. (An exploration of passive backdoors and more analysis of the
HIGHNOON malware family can be found in our full APT41 report).
Within the next 35 minutes, the attackers utilized both the test.jsp web shell and the HIGHNOON backdoor to
issue commands to the system. As China Chopper relies on HTTP
requests, attacker traffic to and from this web shell was easily
observed via network monitoring. The attacker utilized China Chopper
to perform the following:
Movement of 64.dat and Ins64.exe
to C:\Program
Files\Atlassian\Confluence Performing a directory
listing of C:\Program
Files\Atlassian\Confluence Performing a directory
listing of C:\Users
Additionally, FireEye’s FLARE team reverse engineered the custom
protocol utilized by the HIGHNOON backdoor, allowing us to decode the
attacker’s traffic. Figure 4 provides a list of the various commands
issued by the attacker utilizing HIGHNOON.

Figure 4: Decoded HIGHNOON commands
issued by the attacker
Playing Their ACEHASH Card
As shown in Figure 4, the attacker utilized the HIGHNOON backdoor to
execute a PowerShell command that downloaded a script from PowerSploit,
a well-known PowerShell Post-Exploitation Framework. At the time of
this blog post, the script was no longer available for downloading.
The commands provided to the script – “privilege::debug sekurlsa::logonpasswords exit
exit” – indicate that the unrecovered script was likely a copy
of Invoke-Mimikatz,
reflectively loading Mimikatz 2.0 in-memory. Per the observed HIGHNOON
output, this command failed.
After performing some additional reconnaissance, the attacker
utilized HIGHNOON to download two additional files into the C:\Program Files\Atlassian\Confluence directory:

c64.exe (MD5:
846cdb921841ac671c86350d494abf9c)
F64.data (MD5:
a919b4454679ef60b39c82bd686ed141)
These two files are the dropper and encrypted/compressed payload
components, respectively, of a malware family known as ACEHASH.
ACEHASH is a credential theft and password dumping utility that
combines the functionality of multiple tools such as Mimikatz,
hashdump, and Windows Credential Editor (WCE).
Upon placing c64.exe and F64.data on the system, the attacker ran the command

c64.exe f64.data “9839D7F1A0 -m”
This specific command provided a password of “9839D7F1A0” to decrypt the contents of F64.data, and a switch of “-m”, indicating the attacker wanted to replicate
the functionality of Mimikatz. With the correct password provided,
c64.exe loaded the decrypted and
decompressed shellcode into memory and harvested credentials.
Ultimately, the attacker was able to exploit a vulnerability,
execute code, and download custom malware on the vulnerable Confluence
system. While Mimikatz failed, via ACEHASH they were able to harvest a
single credential from the system. However, as Managed Defense
detected this activity rapidly via network signatures, this operation
was neutralized before the attackers progressed any further.
Key Takeaways From This Incident
APT41 utilized multiple
malware families to maintain access into this environment; impactful
remediation requires full scoping of an incident.
For effective Managed Detection &
Response services, having coverage of both Endpoint and Network is
critical for detecting and responding to targeted attacks.
Attackers may weaponize vulnerabilities
quickly after their release, especially if they are present within a
targeted environment. Patching of critical vulnerabilities ASAP is
crucial to deter active attackers.
Detecting the Techniques
FireEye detects this activity across our platform, including
detection for certutil usage, HIGHNOON, and China Chopper.

Detection
Signature Name

China Chopper FE_Webshell_JSP_CHOPPER_1

  FE_Webshell_Java_CHOPPER_1

  FE_Webshell_MSIL_CHOPPER_1

HIGHNOON.PASSIVE FE_APT_Backdoor_Raw64_HIGHNOON_2

  FE_APT_Backdoor_Win64_HIGHNOON_2

Certutil Downloader CERTUTIL.EXE DOWNLOADER (UTILITY)

  CERTUTIL.EXE DOWNLOADER A (UTILITY)

ACEHASH FE_Trojan_AceHash
Indicators

Type
Indicator
MD5 Hash (if applicable)

File test.jsp 84d6e4ba1f4268e50810dacc7bbc3935

File 64.dat 51e06382a88eb09639e1bc3565b444a6

File Ins64.exe e42555b218248d1a2ba92c1532ef6786

File c64.exe 846cdb921841ac671c86350d494abf9c

File F64.data a919b4454679ef60b39c82bd686ed141

IP Address 67.229.97[.]229 N/A
Looking for more? Join
us for a webcast on August 29, 2019 where we detail more of
APT41’s activities. You can also find a direct link to the public
APT41 report here.
Acknowledgements
Special thanks to Dan Perez, Andrew Thompson, Tyler Dean, Raymond
Leong, and Willi Ballenthin for identification and reversing of the
HIGHNOON.PASSIVE malware.

http://www.fireeye.com/blog/threat-research/2019/08/game-over-detecting-and-stopping-an-apt41-operation.html

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