Threat Assessment: Clop Ransomware

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Executive Summary

Unit 42 researchers have observed an uptick in Clop ransomware activity affecting the wholesale and retail, transportation and logistics, education, manufacturing, engineering, automotive, energy, financial, aerospace, telecommunications, professional and legal services, healthcare and high tech industries in the U.S., Europe, Canada, Asia Pacific and Latin America. Clop also leverages double extortion practices and hosts a leak site, where the number of victims has grown significantly since its launch in March 2020. Clop has been commonly observed being delivered as the final-stage payload of a malicious spam campaign carried out by the financially motivated actor TA505. This ransomware has also been linked to threat actors behind the recent global zero-day attacks on users of the Accellion File Transfer Appliance (FTA) product.

Due to the surge of this malicious activity, we’ve created this threat assessment for overall threat awareness. Full visualization of the techniques observed and their relevant courses of action can be viewed in the Unit 42 ATOM Viewer.

Clop Ransomware Overview

Clop ransomware is a variant of a previously known strain called CryptoMix. In 2019, Clop was delivered as the final payload of a phishing campaign associated with the financially motivated actor TA505. The threat actors would send phishing emails that would lead to a macro-enabled document that would drop a loader named Get2. This loader can download different tools used by this group, such as SDBot, FlawedAmmy or FlawedGrace. After the threat actors obtain the initial foothold on the system, they start employing reconnaissance, lateral movement and exfiltration techniques to prepare the ransomware deployment. SDBot has been observed delivering Clop as the final payload.

After the ransomware is executed, Clop appends the .clop extension to the victim's files. We have observed different variants using different extensions, such as “.CIIp”, “.Cllp” and “.C_L_O_P”. Different versions of the ransom note have also been observed after encryption. Depending on the variant, any of these ransom text files could drop: “ClopReadMe.txt”, “README_README.txt”, “Cl0pReadMe.txt“ and “READ_ME_!!!.TXT”.

This ransomware includes various features to avoid detection. Observed Clop samples try to kill several processes and services related to backups and security solutions. It won’t execute if it detects it’s running in a virtual environment. Clop also leverages Code Signing to evade detection. We observed the use of two signers during our research, as shown below in Figure 1.

Clop ransomware leverages code signing to evade detection. The image shows two signers we observed in our research.
Figure 1. Observed Clop digital signers.

Clop went from being ransomware delivered through malicious spam to being used in targeted campaigns against high-profile companies. In recent events, Clop has been linked to threat actors who have been exploiting Accellion File Transfer Appliance (FTA) vulnerabilities: CVE-2021-27101, CVE-2021-27102, CVE-2021-27103 and CVE-2021-27104. The exploitation of these vulnerabilities led to the compromise of high-profile companies starting in February. Additionally, there has been evidence of an affiliate using a webshell named DEWMODE that was being used to steal data from Accellion FTA devices. Not long after compromise, victims affected by DEWMODE began receiving emails from threat actors announcing the breach with an unique URL per victim to start negotiation efforts. If ignored, the threat actors would reach out again with an ultimatum of releasing the data to “Cl0p^_-Leaks”.

Clop didn't have a leak site when it was first sighted back in February 2019. It was in March 2020 when the threat actors decided to launch a leak site titled, “Cl0p^_- Leaks” (Figure 2). This website is a Tor-based blog site, where victims who don’t pay the ransom or ignore threats have their confidential data publicly exposed. The threat actors behind Clop also leverage a variety of extortion techniques, such as targeting workstations of top executives, “doxxing” employees and advertising their breaches to reporters.

The left side of the screenshot shows the appearance of Clop's leak site. The right side shows an example of a message encouraging victims to learn how to improve security posture and close security holes - for a price.
Figure 2. Clop leak site and sample instructions delivered by Clop operators detailing how to improve security posture and close security holes – for a price.

More information on ransomware and victimology can be found in the 2021 Unit 42 Ransomware Threat Report.

Courses of Action

This section documents relevant tactics, techniques and procedures (TTPs) used with Clop and maps them directly to Palo Alto Networks product(s) and service(s). It also further instructs customers on how to ensure their devices are configured correctly.

Product / Service Course of Action
Initial Access, Exfiltration, Defense Evasion, Execution
Exploit Public-Facing Application [T1190], Exfiltration Over C2 Channel [T1041], Spearphishing Attachment [T1566.001], Code Signing [T1553.002], Windows Command Shell [T1059.003]
NGFW Ensure application security policies exist when allowing traffic from an untrusted zone to a more trusted zone
Ensure 'Service setting of ANY' in a security policy allowing traffic does not exist
Ensure 'Security Policy' denying any/all traffic to/from IP addresses on Trusted Threat Intelligence Sources Exists
Set up File Blocking
Threat Prevention Ensure a Vulnerability Protection Profile is set to block attacks against critical and high vulnerabilities, and set to default on medium, low and informational vulnerabilities
Ensure a secure Vulnerability Protection Profile is applied to all security rules allowing traffic
Ensure that antivirus profiles are set to block on all decoders except 'imap' and 'pop3'
Ensure a secure antivirus profile is applied to all relevant security policies
Ensure an anti-spyware profile is configured to block on all spyware severity levels, categories and threats
Ensure DNS sinkholing is configured on all anti-spyware profiles in use
Ensure passive DNS monitoring is set to enabled on all anti-spyware profiles in use
Ensure a secure anti-spyware profile is applied to all security policies permitting traffic to the internet
DNS Security Enable DNS Security in Anti-Spyware profile
URL Filtering Ensure that URL Filtering is used
Ensure that URL Filtering uses the action of ‘block’ or ‘override’ on the <enterprise approved value> URL categories
Ensure that access to every URL is logged
Ensure all HTTP Header Logging options are enabled
Ensure secure URL Filtering is enabled for all security policies allowing traffic to the internet
WildFire Ensure that WildFire file size upload limits are maximized
Ensure forwarding is enabled for all applications and file types in WildFire file blocking profiles
Ensure a WildFire Analysis profile is enabled for all security policies
Ensure forwarding of decrypted content to WildFire is enabled
Ensure all WildFire session information settings are enabled
Ensure alerts are enabled for malicious files detected by WildFire
Ensure 'WildFire Update Schedule' is set to download and install updates every minute
Cortex XSOAR Deploy XSOAR Playbook - Isolate Endpoint - Generic
Deploy XSOAR Playbook - Block IP
Deploy XSOAR Playbook - Block URL
Deploy XSOAR Playbook - Hunting and Threat Detection Playbook
Deploy XSOAR Playbook - PAN-OS Query Logs for Indicators
Deploy XSOAR Playbook - Phishing Investigation - Generic V2
Deploy XSOAR Playbook - Endpoint Malware Investigation
Cortex XDR Enable Anti-Exploit Protection
Enable Anti-Malware Protection
File and Directory Discovery [T1083], Process Discovery [T1057]
Cortex XDR Look for the following BIOCs alerts to detect activity*:

Cortex XDR Analytics - Multiple Discovery Commands

Data Encrypted for Impact [T1486], Inhibit System Recovery [T1490], Service Stop [T1489]
Cortex XSOAR Deploy XSOAR Playbook - Ransomware Manual
Cortex XDR Look for the following BIOCs alerts to detect activity*:

Manipulation of Volume Shadow Copy configuration

Cortex XDR Agent - Behavioral Threat Detected

Table 1. Courses of Action for Clop ransomware.
†These capabilities are part of the NGFW security subscriptions service.
* These analytic detectors will trigger automatically for Cortex XDR Pro customers.


Clop ransomware is a high-profile ransomware family that has compromised industries globally. Organizations should be aware of SDBot, used by TA505, and how it can lead to the deployment of Clop ransomware. Like many other current ransomware families, Clop hosts a leak site to create additional pressure and shame victims into paying the ransom.

Indicators associated with this Threat Assessment are available on GitHub, have been published to the Unit 42 TAXII feed and are viewable via the ATOM Viewer.

In addition to the above courses of action, AutoFocus customers can review additional activity by using the tag Clop.

Additional Resources

Palo Alto Networks has shared our findings, including file samples and indicators of compromise, in this report with our fellow Cyber Threat Alliance members. CTA members use this intelligence to rapidly deploy protections to their customers and to systematically disrupt malicious cyber actors. For more information on the Cyber Threat Alliance, visit www.cyberthreatalliance.org.

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