Citrix has begun issuing patches for a serious vulnerability in its Application Delivery Controller (ADC) product which experts have warned is being exploited in the wild.

The tech giant revealed the CVE-2019-19781 bug in ADC and its Citrix Gateway back in December. If successfully exploited, it could allow an unauthenticated attacker to perform arbitrary code execution.

Although the firm announced a series of mitigations to help protect customers as it readied a permanent fix, researchers claimed to have discovered tens of thousands of users that were still exposed, including high value targets across verticals including finance, government and healthcare.

Part of the problem appeared to be that not all of these mitigations worked as intended. The Dutch authorities urged businesses to disable Citrix systems altogether.

With proof-of-concept exploits appearing online in recent days and reports of active attacks, Citrix appeared to accelerate the process of readying patches.

Permanent fixes for ADC versions 11.1 and 12.0 are now ready and it has “moved forward” availability dates for other versions 12.1, 13 and 10.5 to January 24. Its Citrix SD-WAN WANOP product will also be patched on the same day.

The news comes as FireEye warned it had spotted “dozens of successful exploitation attempts” against ADC deployments that had not put in place temporary pre-patch mitigations.

One particular payload, which it named “NotRobin,” appears to be hoarding access to exposed Citrix systems.

“FireEye believes that the actor behind NotRobin has been opportunistically compromising NetScaler devices, possibly to prepare for an upcoming campaign. They remove other known malware, potentially to avoid detection by administrators that check into their devices after reading Citrix security bulletin CTX267027,” FireEye explained.

“NotRobin mitigates CVE-2019-19781 on compromised devices but retains a backdoor for an actor with a secret key. While we haven’t seen the actor return, we’re skeptical that they will remain a Robin Hood character protecting the internet from the shadows.”