On this put up we might be discussing Group Coverage assaults, basing the risk hunt on a ransomware investigation undertaken by the Sophos X-Ops Incident Response group earlier this yr. We are going to cowl malicious behaviors related to Lively Listing and Group Coverage assaults, displaying you how you can examine and remediate a few of these threats.
A lot of the fabric on this put up can also be lined within the video “Figuring out Group Coverage Assaults,” now displaying on our new Sophos X-Ops YouTube channel. The video exhibits a hunt and remediation (utilizing Sophos Dwell Response — a key function of Sophos Intercept X Superior with XDR, our normal investigation instrument, although hunters can replicate these steps on any Home windows shell).
This put up walks by means of the identical materials, however offers the onscreen data in a reader-friendly format.
The case
Within the Cyclops ransomware case below dialogue, the risk actor gained preliminary entry to the setting by leveraging a ProxyShell vulnerability to breach an unpatched Alternate server. 4 days after attaining preliminary entry, the risk actor started executing their assault utilizing encoded PowerShell instructions from the net shell on the Alternate server.
The attacker proceeded to disable endpoint safety as a protection evasion approach, and to clear Home windows occasion logs and web browser historical past. The attacker then leveraged Distant Desktop Protocol (RDP) to carry out lateral motion to further machines on the community. Each Cobalt Strike command-and-control malware in addition to AnyDesk distant entry software program had been put in on a number of machines to keep up entry. A day later, the attacker used their community entry to exfiltrate information to a number of cloud storage internet hosting suppliers.
After that, the attacker leveraged Lively Listing Group Coverage to distribute the Cyclops ransomware binary to machines on the area, additionally making a Group Coverage to execute the ransomware binary utilizing scheduled duties. Within the remaining stage of the assault, the attacker deleted quantity shadow copy backups. Machines on the area ran the scheduled activity, executing the Cyclops ransomware binary, encrypting information, and leaving ransom notes.
Why goal Group Coverage?
Group Coverage assaults are a sign of a bigger Lively Listing assault. In a Group Coverage assault, risk actors could leverage current Group Coverage Objects, comparable to UNC path, to execute malicious payloads from less-secure places preset on a GPO, or the interception of consumer passwords set by way of Group Coverage with the susceptible cpassword attribute.
As soon as a risk actor has escalated privileges, they typically create GPOs to perform objectives at scale, comparable to disabling of core safety software program and options together with firewalls, antivirus, safety updates, and logging. They might additionally use GPOs for deployment of malicious instruments by means of the creation of scheduled duties, startup or login scripts, or companies to keep up persistence and execute malware.
Pleased searching
Investigators start a ransomware investigation-and-remediation course of by accumulating no matter sufferer testimonies and forensic information can be found. Utilizing the instruments at hand, they seek for indicators of compromise in the usual forensic artifacts, comparable to Home windows occasion logs, PowerShell historical past, startup objects, shellbags, scheduled duties, shim cache, and so forth.
When performing an evaluation, if synchronized or reoccurring proof is discovered, it could be a key indication of a Group Coverage assault. For instance, when a scheduled activity or file execution is seen on a number of machines, it signifies distant execution or using Group Coverage. When system logs indicating using software program deployment instruments or Home windows Administration Instrumentation will not be current, it serves as a sign that Group Coverage was possible compromised. This use of malicious synchronizing is particularly evident throughout triage, when persistent scheduled duties reappear on programs after being eliminated.
As soon as a Group Coverage assault is suspected, investigators ought to have a look at the Group Coverage objects on the area controller, utilizing the PowerShell command get-GPO -All to listing all of them. Filtering these outcomes
Get-GPO -All | Type-Object ModificationTime -Descending | Format-Desk DisplayName, ModificationTime, CreationTime
permits the investigator to see modification and creation instances, looking for intersections with different information of the case. Sorting by the date on which information had been final modified can result in any GPOs created or modified by the risk actor. At this level, it’s helpful for the investigator to generate a GPO report for additional investigation.
Get-GPOReport -All -ReportType Html -Path “C:WindowsTempSophos_GPOReport.html”
Analyzing the GPO report we will discern the aim of any Group Coverage objects with suspicious names. Within the Cyclops case anonymized for our video, we recognized three suspicious-looking GPOs, which for anonymization functions we name “Pawn,” “Rook,” and “Queen.”
Within the case of Pawn, the attacker used the GPO to put in a scheduled activity on area computer systems to run this system rook.exe.
The Rook GPO is used to repeat the rook.exe file to domain-joined machines from an administrative share on the file server. Since it might make sense for the attacker to do precisely that with malware, we instantly go to the native system to see if a duplicate continues to be out there, utilizing Get-ItemProperty “C:Windowsrook.exe”. Whether it is out there, an investigator can get the hash worth for this file (utilizing Get-FileHash “C:Windowsrook.exe”) and examine it in opposition to VirusTotal to see if it’s identified to be malicious; this hash additionally offers the means to dam the file within the setting. It’s smart in fact to retain a pattern of the malware for additional forensic evaluation.
The Queen GPO configures Home windows Firewall states to Off. It additionally seems that Queen disables Home windows Defender’s antimalware protections, together with real-time scanning capacity.
Making it higher
As soon as malicious behaviors in your setting are recognized, containment and remediation can start by way of the Group Coverage Administration instrument on the Lively Listing administration server.
First, tackle the Queen, which is undermining Home windows Firewall and Home windows Defender operations. Disabling this coverage will forestall these settings from overriding the default native Home windows settings.
Subsequent it’s Rook’s flip to be taken off the board. Disabling this coverage will forestall the malware rook.exe from being copied to any further machines on the community. The malware executable must also be blacklisted within the international settings for your entire community. It will remove the malware’s capacity to be executed sooner or later – kind a brand new attacker try, for example, or in case an contaminated backup makes an attempt to re-load the executable. (Good backup hygiene is a vital subject for defenders to think about, nevertheless it lies barely outdoors the scope of this text.)
Lastly, remediate the malicious scheduled activity named Pawn. Disabling this GPO prevents further deployments of the scheduled activity to computer systems on the area. Following these remediation steps will assist forestall the unfold of malicious exercise all through the community.
All three of those steps contain disabling malicious GPOs, however that’s not sufficient; correct remediation will contain taking steps that may carry out the alternative motion(s) as these taken by the malicious GPOs. This could itself be completed at scale with GPOs or different gadget administration platforms. Another choice, which some enterprises could favor, is rollback. In the event you select the latter, inspection of the archived materials for an infection or undesirable alteration is strongly advisable.
Acknowledgements
Elida Leite and Rajat Wason contributed to this analysis.