https://arstechnica.com/information-technology/2020/01/unpatched-citrix-vulnerability-now-exploited-patch-weeks-away/

On December 16, 2019, Citrix revealed a vulnerability in the company's Application Delivery Controller and Gateway products—commercial virtual-private-network gateways formerly marketed as NetScaler and used by tens of thousands of companies. The flaw, discovered by Mikhail Klyuchnikov of Positive Technologies, could give an attacker direct access to the local networks behind the gateways from the Internet without the need for an account or authentication using a crafted Web request.

Citrix has published steps to reduce the risk of the exploit. But these steps, which simply configure a responder to handle requests using the text that targets the flaw, breaks under some circumstances and might interfere with access to the administration portal for the gateways by legitimate users. A permanent patch will not be released until January 20. And as of January 12, over 25,000 servers remain vulnerable, based on scans by Bad Packets.