Fox-IT writes about 1900 Citrix Netscaler devices being backdoored due to CVE-2023-3519
„A patched NetScaler can still contain a backdoor. It is recommended to perform an Indicator of Compromise check on your NetScalers, regardless of when the patch was applied.“
◦ Fox-IT https://github.com/fox-it/citrix-netscaler-triage
◦ Mandiant https://github.com/mandiant/citrix-ioc-scanner-cve-2023-3519/