Cobalt Strike is a security tool, used by penetration testers to simulate network attackers. But it’s also used by attackers — from criminals to governments — to automate their own attacks. Researchers have found a vulnerability in the product.

The main components of the security tool are the Cobalt Strike client — also known as a Beacon — and the Cobalt Strike team server, which sends commands to infected computers and receives the data they exfiltrate. An attacker starts by spinning up a machine running Team Server that has been configured to use specific “malleability” customizations, such as how often the client is to report to the server or specific data to periodically send.

Then the attacker installs the client on a targeted machine after exploiting a vulnerability, tricking the user or gaining access by other means. From then on, the client will use those customizations to maintain persistent contact with the machine running the Team Server.

The link connecting the client to the server is called the web server thread, which handles communication between the two machines. Chief among the communications are “tasks” servers send to instruct clients to run a command, get a process list, or do other things. The client then responds with a “reply.”

Researchers at security firm SentinelOne recently found a critical bug in the Team Server that makes it easy to knock the server offline. The bug works by sending a server fake replies that “squeeze every bit of available memory from the C2’s web server thread….”

It’s a pretty serious vulnerability, and there’s already a patch available. But — and this is the interesting part — that patch is available to licensed users, which attackers often aren’t. It’ll be a while before that patch filters down to the pirated copies of the software, and that time window gives defenders an opportunity. They can simulate a Cobolt Strike client, and leverage this vulnerability to reply to servers with messages that cause the server to crash.