Fortra Cobalt Strike: cross-site scripting flaw under attack

What is affected

Fortra Cobalt Strike is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

Fortra Cobalt Strike contains a cross-site scripting (XSS) vulnerability in Teamserver that would allow an attacker to set a malformed username in the Beacon configuration, allowing them to execute code remotely.

What an attacker can do

This is a cross-site scripting flaw. It lets an attacker plant a script that runs in the browser of anyone who opens the affected page, which lets them steal that person's session or act on their behalf.

How it is reached

Over the internet or network

Access the attacker needs

No account needed

Does the victim have to do something?

Yes, the victim must open or click something

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer Fortra Cobalt Strike at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

Apply updates per vendor instructions.

Vendor advisories and fixes

• cobaltstrike.comwww.cobaltstrike.com/blog/out-of-band-update-cobalt-strike-4-7-1/

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on March 30, 2023, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by April 20, 2023, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 46% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99% of all scored vulnerabilities.

Its CVSS severity score is 6.1 out of 10 (medium), as recorded in the US National Vulnerability Database.

Technical description

An XSS (Cross Site Scripting) vulnerability was found in HelpSystems Cobalt Strike through 4.7 that allowed a remote attacker to execute HTML on the Cobalt Strike teamserver. To exploit the vulnerability, one must first inspect a Cobalt Strike payload, and then modify the username field in the payload (or create a new payload with the extracted information and then modify that username field to be malformed).