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Cobalt Strike XSS to RCE (CVE-2022-39197)

Cobalt Strike 4.7.1 is now available. This is an out of band update to fix an issue discovered in the 4.7 release that was reported to be impacting users, and for which there was no workaround. We also took the opportunity to address a vulnerability that was reported shortly after the 4.7 release, along with mitigations for potential denial-of-service attacks.

CVE-2022-39197

An independent researcher identified as “Beichendream” reached out to inform us about an XSS vulnerability that they discovered in the teamserver. This would allow an attacker to set a malformed username in the Beacon configuration, allowing them to remotely execute code. We created a CVE for this issue (which at the time of writing is still pending approval), which has been fixed.

As part of this fix, a new property has been added to the TeamServer.prop file (located in the home folder of the teamserver):

limits.beacons_xssvalidated specifies whether XSS validation is performed on selected Beacon metadata. By default, this is set to true.

Denial-of-Service Mitigations

We were also made aware of the potential to conduct a denial-of-service attack against the teamserver itself. While this can be mitigated by good OPSEC (using a redirector, turning staging off and so on), we have made updates to mitigate this type of attack.

A number of new properties have been added to the TeamServer.prop file as part of the mitigations:

limits.beacons_max sets a limit on the total number of Beacons that the teamserver will support. The default is 500. To turn this off (support an unlimited number of Beacons), use 0.

Three additional settings allow you to set a threshold rate for adding new Beacons (how many new Beacons can be added in a specific time period):

limits.beacon_rate_period specifies the time period (in milliseconds) during which the number of Beacons added is monitored and limited.

limits.beacon_rate_maxperperiod specifies how many new Beacons can be added in the specified time period.

limits.beacon_rate_disableduration specifies how long the teamserver will ignore additional new Beacons for if the number of new Beacons exceeds the limit in the given time period.

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