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The Cl0p ransomware group, also known as Clop, has established itself as one of the more operationally disciplined and strategically opportunistic financially motivated threat actors. Active since at least 2019 and widely associated with the TA505 ecosystem, Cl0p is best known for its use of large-scale data theft, exploitation of zero-day vulnerabilities in enterprise software, and a preference for extortion over mass encryption in later campaigns. Understanding Cl0p indicators of compromise requires examining not only technical artifacts left on compromised systems, but also behavioral patterns, infrastructure choices, and post-exploitation tradecraft that consistently surface across incidents.
Identifying the operation
One of the most visible indicators of compromise associated with Cl0p operations is anomalous interaction with externally facing file transfer and managed file sharing platforms. Cl0p has repeatedly exploited vulnerabilities in products such as Accellion FTA, GoAnywhere MFT, MOVEit Transfer, and similar systems that often sit at the perimeter of enterprise environments. Indicators frequently manifest as unusual HTTP requests to these services, often leveraging newly disclosed or zero-day vulnerabilities, followed by the execution of unauthorized processes under the service account context. Logs may show unexpected file creation in application directories, particularly web shells or staging scripts placed in paths accessible by the application server. In many incidents, these artifacts appear shortly after exploitation and before any ransomware deployment, reflecting Cl0p’s focus on data theft and extortion rather than immediate disruption.
Recognising the signs on Windows
On compromised Windows systems, Cl0p activity often produces a recognizable set of execution patterns. Initial payloads or post-exploitation tools may be executed via common LOLbins such as PowerShell, rundll32.exe, mshta.exe, or wmic.exe. Command-line telemetry may reveal obfuscated PowerShell commands used to disable security controls, download secondary payloads, or enumerate domain resources. Cl0p affiliates are known to disable or tamper with endpoint protection by stopping services, modifying registry keys associated with Windows Defender, or deploying signed but abused binaries to evade detection. While none of these actions are unique to Cl0p, their sequencing and timing, especially when correlated with known vulnerable services, can serve as a strong composite indicator.
Persistence mechanisms associated with Cl0p compromises tend to be pragmatic rather than exotic. Registry Run keys, scheduled tasks with innocuous-looking names, and newly created local or domain accounts are common. In several investigations, attackers created backup administrator accounts shortly after gaining domain privileges, often naming them to blend into existing naming conventions. Event logs may reveal account creation followed by rapid assignment to privileged groups such as Domain Admins, particularly outside of normal administrative change windows. These actions typically precede lateral movement and large-scale data staging.
Recogising the signs at a network level
Network-level indicators of compromise play a critical role in identifying Cl0p activity, particularly because encryption is not always deployed. Exfiltration behavior is one of the most telling signals. Victim environments frequently exhibit sustained outbound data transfers to cloud storage providers, virtual private servers, or leased infrastructure hosted in jurisdictions commonly associated with bulletproof hosting. These transfers often occur over HTTPS on standard ports, but volume and duration distinguish them from normal business activity. In some cases, attackers compress data using tools such as 7-Zip or WinRAR before exfiltration, leaving behind large archive files in temporary directories or network shares. The presence of multi-gigabyte archives created shortly before unusual outbound traffic is a recurring forensic marker.
Identifying the worst case scenario
When ransomware is deployed, file-based indicators become more explicit. Cl0p has historically used extensions such as .clop or .CLOP appended to encrypted files, although extension usage has varied over time. Ransom notes are typically dropped in multiple directories and contain language directing victims to Tor-based leak sites. These notes often reference stolen data explicitly, reinforcing the group’s double-extortion model. Hash-based indicators for Cl0p binaries are of limited long-term value due to frequent recompilation and customization by affiliates, but code similarities, mutex names, and encryption routines can still aid in retrospective detection when combined with behavioral data.