Citrix on Monday informed customers that it released firmware updates for its Application Delivery Controller (ADC) and Gateway products to prevent threat actors from abusing the appliances to launch and amplify distributed denial-of-service (DDoS) attacks.
Several people reported a few days before Christmas that they had started seeing DDoS attacks abusing their Citrix ADC and Gateway devices. Citrix confirmed that malicious actors had been targeting its products in an advisory published on December 24 and the company promised to release updates that would prevent attacks by January 12. However, it managed to release the updates one week sooner.
“As part of this attack, an attacker or bots can overwhelm the Citrix ADC DTLS network throughput, potentially leading to outbound bandwidth exhaustion. The effect of this attack appears to be more prominent on connections with limited bandwidth,” Citrix said in its advisory.
The company claimed “the scope of attack is limited to a small number of customers around the world, and further, there are no known Citrix vulnerabilities associated with this event.”
The Datagram Transport Layer Security (DTLS) feature targeted by the recent DDoS attacks is designed to secure communications between applications. Marius Sandbu, one of the first people to spot the Citrix DDoS attacks, has published a blog post explaining the role of DTLS and why such attacks are possible.
In order to prevent abuse of its products for DDoS attacks, Citrix on Monday introduced a “feature enhancement” for DTLS. Users can either enable this feature, or they can disable DTLS if it’s not needed.
Another mitigation recommended by several experts involves blocking UDP port 443 traffic on the firewall. This is the port targeted by the attacks.
It’s not uncommon for Citrix’s ADC and Gateway products to be targeted by hackers. A vulnerability disclosed in late 2019 has been exploited by both profit-driven cybercriminals and state-sponsored threat groups.