On this publish we shall be discussing Group Coverage assaults, basing the risk hunt on a ransomware investigation undertaken by the Sophos X-Ops Incident Response crew earlier this yr. We’ll cowl malicious behaviors related to Energetic Listing and Group Coverage assaults, exhibiting you the way to examine and remediate a few of these threats.
A lot of the fabric on this publish can be coated within the video “Figuring out Group Coverage Assaults,” now exhibiting on our new Sophos X-Ops YouTube channel. The video exhibits a hunt and remediation (utilizing Sophos Stay Response — a key function of Sophos Intercept X Superior with XDR, our commonplace investigation device, although hunters can replicate these steps on any Home windows shell).
This publish walks by means of the identical materials, however offers the onscreen info 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 Trade server. 4 days after reaching preliminary entry, the risk actor started executing their assault utilizing encoded PowerShell instructions from the online shell on the Trade 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 take care of entry. A day later, the attacker used their community entry to exfiltrate knowledge to a number of cloud storage internet hosting suppliers.
After that, the attacker leveraged Energetic 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 process, executing the Cyclops ransomware binary, encrypting recordsdata, and leaving ransom notes.
Why goal Group Coverage?
Group Coverage assaults are a sign of a bigger Energetic Listing assault. In a Group Coverage assault, risk actors might leverage current Group Coverage Objects, resembling UNC path, to execute malicious payloads from less-secure areas preset on a GPO, or the interception of person passwords set through Group Coverage with the weak cpassword attribute.
As soon as a risk actor has escalated privileges, they typically create GPOs to perform targets at scale, resembling 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 providers to take care of persistence and execute malware.
Glad searching
Investigators start a ransomware investigation-and-remediation course of by amassing no matter sufferer testimonies and forensic knowledge can be found. Utilizing the instruments at hand, they seek for indicators of compromise in the usual forensic artifacts, resembling 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 process 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 aren’t 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 methods after being eliminated.
As soon as a Group Coverage assault is suspected, investigators ought to take a look at the Group Coverage objects on the area controller, utilizing the PowerShell command get-GPO -All to checklist all of them. Filtering these outcomes
Get-GPO -All | Kind-Object ModificationTime -Descending | Format-Desk DisplayName, ModificationTime, CreationTime
permits the investigator to see modification and creation occasions, looking for intersections with different details of the case. Sorting by the date on which recordsdata 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 are able to 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 process 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 could make sense for the attacker to do precisely that with malware, we instantly go to the native system to see if a duplicate remains to be obtainable, utilizing Get-ItemProperty “C:Windowsrook.exe”. Whether it is obtainable, 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 recognized to be malicious; this hash additionally offers the means to dam the file within the setting. It’s smart after all 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 potential.
Making it higher
As soon as malicious behaviors in your setting are recognized, containment and remediation can start through the Group Coverage Administration device on the Energetic Listing administration server.
First, deal with 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 world settings for your complete community. This can eradicate the malware’s potential 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 crucial matter for defenders to think about, nevertheless it lies barely outdoors the scope of this text.)
Lastly, remediate the malicious scheduled process named Pawn. Disabling this GPO prevents further deployments of the scheduled process 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 may itself be achieved at scale with GPOs or different machine administration platforms. An alternative choice, which some enterprises might choose, is rollback. When you select the latter, inspection of the archived materials for an infection or undesirable alteration is strongly really helpful.
Acknowledgements
Elida Leite and Rajat Wason contributed to this analysis.
