https://www.theregister.co.uk/2020/01/17/hackers_patch_citrix_vulnerability/

Hackers exploiting the high-profile Citrix CVE-2019-19781 flaw to compromise VPN gateways are now patching the servers to keep others out.

Researchers at FireEye report finding a hacking group (dubbed NOTROBIN) that has been bundling mitigation code for NetScaler servers with its exploits. In effect, the hackers exploit the flaw to get access to the server, kill any existing malware, set up their own backdoor, then block off the vulnerable code from future exploit attempts by mitigation.