As you might already know Cobalt Strike is a legitimate penetration testing tool that can be used as an attack framework by red teams. Red Teams are groups of security professionals that try to attack their own organization’s infrastructure in order to discover security gaps and vulnerabilities.

Unfortunately, Cobalt Strike is used also by threat actors for post-exploitation purposes after the deployment of so-called beacons that are able to provide them with persistent remote access to compromised devices. The beacons are very important as they allow the attackers to later access the breached servers and easily harvest data or deploy second-stage malware payloads.

The researchers at SentinelLabs discovered the DoS vulnerabilities that were collectively tracked as CVE-2021-36798 (and dubbed Hotcobalt) in the latest versions of the Cobalt Strike’s server.

They discovered that a user is able to register fake beacons with the server of a particular Cobalt Strike installation and that by sending fake tasks to the server, can crash it by exhausting the available memory.

This lets a malicious actor cause memory exhaustion on the machine the Cobalt’s server (the ‘Teamserver’) runs on, which makes the server unresponsive until it’s restarted.

This means that live beacons cannot communicate to their C2 until the operators restart the server. Restarting, however, won’t be enough to defend against this vulnerability as it is possible to repeatedly target the server until it is patched or the beacon’s configuration is changed.


The good news is that since Cobalt Strike is heavily used by threat actors, the law enforcement and security researchers are also able to employ the Hotcobalt vulnerabilities in order to take down the malicious infrastructure.

Disclosure Timeline:

04/20/2021 – Initial contact with HelpSystems for issue disclosure.
04/22/2021 – Issue details disclosed to HelpSystems.
04/23/2021 – HelpSystems confirmed the issue and asked for an extension until August 3rd.
04/28/2021 – SentinelOne accepted the extension.
07/18/2021 – Submitted CVE request to MITRE.
07/19/2021 – CVE-2021-36798 was assigned and reserved for the specified issue.
08/02/2021 – SentinelOne shared the publication date and post for review.
08/02/2021 – HelpSystems reviewed and confirmed the post for publication.
08/04/2021 – HelpSystems released Cobalt Strike 4.4, which contains a fix for CVE-2021-36798.


The researchers at SentinelLabs disclosed the vulnerabilities HelpSystems, the parent company of Cobalt Strike in April. The vulnerabilities were addressed in Cobalt Strike 4.4.

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