May 22, 2022

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A Look Into Purple Fox’s Server Infrastructure

5 min read

Working system execution through SQL Server

Purple Fox focuses on SQL servers as its goal versus regular computer systems for the previous’s cryptocurrency-mining actions. That is primarily due to the extra highly effective {hardware} configuration — for each CPU and reminiscence — that the servers would normally have. Extra particularly for SQL servers, the mix of CPU, reminiscence, and disk elements ought to scale with the database-related operations to keep away from bottlenecks in efficiency.

These machines usually possess a lot higher computing energy in comparison with regular desktops, as such servers are normally fitted with {hardware} such because the Intel Xeon line of CPUs that produces a considerably greater quantity of hash-based calculations (hash charges), making a server extra advantageous to coinmining in comparison with a typical desktop pc.

Since SQL databases help totally different vectors for executing working system instructions instantly, Purple Fox has leveraged the stealthiest technique of getting a binary inserted within the SQL server database that may be executed through TSQL instructions. The next interfaces can be found from the SQL parts for the malicious actors to make use of when focusing on an SQL server:




  • ShellExecute/ShellExecuteEx
  • xp_cmdshell 

COM objects

  • shell.utility
Desk 2. The out there interfaces from the SQL parts

Purple Fox opted to go along with the .NET technique utilizing CLR Assemblies, a bunch of DLLs that may be imported right into a SQL Server, in its an infection chain as a substitute of the extra in style xp_cmdshell, which is closely monitored by safety analysts. As soon as the DLLs have been imported, they are often linked to saved procedures that may be executed through a TSQL script. The affected variations for this vector begin from SQL Server 2008.

This technique, which requires a system administrator function by default, executes as an SQL Server service account. By leveraging this interface, an attacker is ready to compile a .NET meeting DLL after which have it imported into the SQL server. Additionally it is in a position to have an meeting saved within the SQL Server Desk, create a process that maps to the CLR technique, and at last, run the process.

The CLR Assemblies technique is reported to have been used earlier than by teams aside from Purple Fox, reminiscent of MrbMiner and Lemon Duck.

The C&C servers used within the communication schemes which have been described listed below are contaminated servers which might be a part of the botnet used to host the assorted payloads for Purple Fox. We deduced this through the next details:

  • The C&C servers are SQL Servers themselves.
  • The HTTP server header is mORMot, which is written in Delphi, the identical language used for the assorted parts.
  • There’s numerous servers (1,000+ in simply over per week).

Each preliminary DNS requests are CNAMEs to subdomains beneath kozow[.]com, which is a free dynamic area service offered by dynu[.]com. This service will be up to date with an API to make it level to totally different IP addresses — a method the attacker makes use of to vary the IP tackle at a daily interval.

Utilizing our telemetry, we discovered non-server programs contaminated with Purple Fox, indicating that there are different attainable preliminary entry strategies aside from the SQL Server brute-force assault to unfold the malware.

This exercise is much like those seen in Lemon Duck assaults and even shares some methods, like using PowerSploit for reflective PE loading and implementing the identical backdoor, evilclr.dll, for the SQL Server meeting. Each assaults additionally share the identical purpose of mining Monero.

Upon observing any suspicious actions associated to the Purple Fox botnet on a SQL server, we suggest the next steps to utterly take away all of the malicious remnants from the an infection.

  • Evaluate all of the SQL Server’s Saved Procedures and Assemblies for any suspicious assemblies not acknowledged by the DBAs. Take away any of those assemblies if detected.
  • Execute the next TSQL script to take away the next remnants of malicious CLR assemblies which might be inserted into the database:         
  • USE [master]
  • GO
  • DROP ASSEMBLY [fscbd]
  • GO
  • Disable all of the unknown accounts on the database server and alter all of the passwords.
  • As a defensive posture, don’t publish externally uncovered port TCP 1433 to an untrusted zone. As well as, safe the SQL server hosts through a fringe firewall in a DMZ zone with well-protected entry insurance policies.
  • Implement correct community microsegmentation and community zoning whereas additionally making use of a zero belief coverage through your community safety controls.
  • Limit the site visitors to and from SQL servers. These servers have a really particular operate; due to this fact, they need to solely be allowed to speak with different trusted hosts. Inbound and outbound web accessibility must also be managed.

Development Micro Imaginative and prescient One™️ with Managed XDR focuses on each the early levels of the assault kill chain (coated within the earlier analysis) and the ultimate payloads supposed to do the precise harm, thereby defending customers of this service in opposition to the harm brought on by the newest evolution of this botnet.  

Each the Imaginative and prescient One platform and Managed XDR menace specialists can correlate the suspicious actions noticed from the protected SQL servers. An surroundings that has any of the behavioral detections present in our Imaginative and prescient One heuristics guidelines would possibly imply that the SQL servers throughout the surroundings have already been affected by an assault. This  extends even to stealthy malware, reminiscent of Purple Fox, that doesn’t retailer majority of its information on the disk.

  • Since servers have a predictable community footprint and habits, uncommon or sudden community patterns may very well be an indication of botnet propagation.
  • The identical goes for uncommon and sudden SQL server utility login failures that appear like brute-force assaults . The principle propagation technique for Purple Fox when infecting SQL servers makes use of brute-force assaults fairly than performing as a worm that exploits solely the weak companies.
  • When a SQL server begins having uncommon site visitors associated to UDP and TCP, there ought to be an enormous surge in site visitors because it scans public IP addresses and the native community. This can create a domino impact inside an surroundings as a consequence of most organizations having multiple SQL server, reminiscent of standby or backup servers.
  • Uncommon community site visitors patterns and login failures on the SQL server are additionally a superb indicator for this menace.
  • A sudden and sudden spike in CPU utilization on the SQL server is also an indication of SQL bottlenecks or an an infection with the XMR Coinminer. Moreover, there is also uncommon quantities of community site visitors on the server because it joins the mining pool.


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