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In TRU Positives, eSentire’s Threat Response Unit (TRU) provides a summary of a recent threat investigation. We outline how we responded to the confirmed threat and what recommendations we have going forward.
Here’s the latest from our TRU Team…
What did we find?
We recently detected active exploitation attempts targeting the WinSock File Transfer Protocol (WS_FTP) vulnerability, identified as CVE-2023-40044. These observed attacks resulted in the attempted deployment of the Metasploit payload Meterpreter and the adversary simulation tool Cobalt Strike.
CVE-2023-40044 (CVSS: 10) is classified as a WS_FTP .NET Deserialization vulnerability within the Ad Hoc Transfer Module. Exploiting this vulnerability could allow an unauthorized threat actor to achieve remote command execution on the underlying operating system of the WS_FTP Server. The vulnerability was publicly disclosed, along with security patches, on September 27th, 2023. We first observed exploitation attempts beginning on September 30th.
As exploitation is currently ongoing, it is of utmost importance that organizations promptly apply the relevant security patches.
During the investigation, our Incident Handling team identified the initial command executed by the threat actor(s) under the w3wp.exe process:
We were not able to retrieve the executables at the time of analysis from the C2s. However, we found a similar binary on VirusTotal (MD5: 5bf7426379fe5eb5d0cd2ed13aa9a101). The binary is a Metasploit payload.
The payload iterates through the loaded modules listed in the Windows Process Environment Block (PEB), specifically the 'InMemoryOrderModuleList.' This list comprises all the dynamically linked libraries (DLLs) loaded by the current process.
The payload further enhances its obfuscation by converting the module names from lowercase to uppercase and subsequently incorporating each character into a hash computation using the ROR13 (Rotate Right by 13 bits) hashing algorithm.
It's important to note that the utilization of the ROR13 hashing algorithm for APIs and DLL names is a common obfuscation technique observed in both Cobalt Strike and Metasploit payloads. The binary eventually establishes a connection to the C2 server 103.163.187[.]12 over port 49900.
Figure 1: ROR13 hashing algorithm
On one of the affected machines, the threat actor(s) executed a PowerShell one-liner command, which was base64-encoded. Upon decoding this command, it becomes evident that it performs the decoding and execution of a block of XOR-encoded shellcode, based on Metasploit (MD5: 11939b0f83537612cab531630a981da7).
This shellcode is stored in the variable `$var_code`. The execution takes place by allocating memory, copying the shellcode into that allocated memory space, and then invoking it.
In Figure 2, we observe the presence of the C2 server at 95.214.24[.]238, along with the User-Agent (Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0; NP07; NP07)) used for communications. Further investigation reveals that this C2 server also functions as a Cobalt Strike server, hosting the Cobalt Strike payload (MD5: 9c313be764ad631490a8029e5a411523). For additional details, please refer to the Indicators of Compromise section for the Cobalt Strike configuration.
Moreover, upon pivoting through the C2 server, we discovered that this server previously operated as the C2 server for the Redline Stealer back in May 2023.
Figure 2: Base64-encoded PowerShell command that decodes to Metasploit-based shellcode
Another PowerShell one-liner command was observed executing on the host (Figure 3). Here's a breakdown of what the script does:
It starts by checking if the system's architecture is 32-bit or 64-bit.
If it's 32-bit, it sets the path for PowerShell to use the SysNative folder (a workaround to call 64-bit apps from a 32-bit process on a 64-bit OS).
If it's 64-bit, it simply uses `powershell.exe`.
Disable Script Logging: It checks if the major version of PowerShell is 3 or higher (PS 3.0 introduced more advanced logging).
If the version is valid, it tries to disable script logging and script block invocation logging. This is often done in malicious scripts to avoid detection because it prevents the logging of executed commands.
It also checks for the presence of AMSI (Anti-Malware Scan Interface) and tries to disable it. AMSI is a Windows feature that allows applications and services to integrate with any antimalware product present on a machine.
The script then decompresses a Base64-encoded, gzipped blob using .NET's `System.IO.Compression.GzipStream` class. After decompression, the resulting content, which is another Meterpreter shellcode, is executed in a new thread. The shellcode connects to 176.105.255[.]46 over port 51011.
Figure 3: Meterpreter payload
What did we do?
Our team of 24/7 SOC Cyber Analysts detected exploitation attempts and promptly notified the affected clients.
Subsequently, our Threat Response Unit (TRU) issued an advisory to clients regarding the active exploitation.
What can you learn from this TRU Positive?
We detected active exploitation attempts targeting WS_FTP's vulnerability, CVE-2023-40044. Exploitation attempts were first observed by our team on September 30, 2023.
Attacks involved the attempted deployment of Metasploit's Meterpreter and Cobalt Strike. The threat actor(s) ran a base64-encoded PowerShell command that decodes and executes a block of XOR-encoded shellcode. The shellcode connects to a Cobalt Strike server. This server also functioned as a C2 for the Redline Stealer in May 2023.
CVE-2023-40044 is a WS_FTP .NET Deserialization vulnerability. The vulnerability allows threat actors to achieve remote command execution on the WS_FTP Server's underlying OS. The vulnerability was publicly disclosed on September 27th, 2023, with security patches.
Recommendations from our Threat Response Unit (TRU):
After performing a business impact review, apply the relevant security patches.
If patching is not immediately possible, consider disabling the WS_FTP Server Ad hoc Transfer Module until patches are deployed.
The eSentire Threat Response Unit (TRU) is an industry-leading threat research team committed to helping your organization become more resilient. TRU is an elite team of threat hunters and researchers that supports our 24/7 Security Operations Centers (SOCs), builds threat detection models across the eSentire XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. By providing complete visibility across your attack surface and performing global threat sweeps and proactive hypothesis-driven threat hunts augmented by original threat research, we are laser-focused on defending your organization against known and unknown threats.
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