Citrix has issued an emergency advisory warning its customers of a security issue affecting its NetScaler application delivery controller (ADC) devices that attackers are abusing to launch amplified distributed denial-of-service (DDoS) attacks against several targets.

"An attacker or bots can overwhelm the Citrix ADC [Datagram Transport Layer Security] network throughput, potentially leading to outbound bandwidth exhaustion," the company noted. "The effect of this attack appears to be more prominent on connections with limited bandwidth."

ADCs are purpose-built networking appliances whose function is to improve the performance, security, and availability of applications delivered over the web to end-users.

The desktop virtualization and networking service provider said it's monitoring the incident and is continuing to investigate its impact on Citrix ADC, adding "the attack is limited to a small number of customers around the world."

The issue came to light after multiple reports of a DDoS amplify attack over UDP/443 against Citrix (NetScaler) Gateway devices at least since December 19, according to Marco Hofmann, an IT administrator for a German software firm ANAXCO GmbH.

Datagram Transport Layer Security or DTLS is based on the Transport Layer Security (TLS) protocol that aims to provide secure communications in a way that's designed to thwart prevent eavesdropping, tampering, or message forgery.

Since DTLS uses the connectionless User Datagram Protocol (UDP) protocol, it's easy for an attacker to spoof an IP packet datagram and include an arbitrary source IP address.

Thus when the Citrix ADC is flooded with an overwhelming flux of DTLS packets whose source IP addresses are forged to a victim IP address, the elicit responses lead to an oversaturation of bandwidth, creating a DDoS condition.

Citrix is currently working to enhance DTLS to eliminate the susceptibility to this attack, with an expected patch to be released on January 12, 2021.

To determine if a Citrix ADC equipment is targeted by the attack, Cisco recommends keeping an eye on the outbound traffic volume for any significant anomaly or spikes.

Customers impacted by the attack, in the meantime, can disable DTLS while a permanent fix from Citrix is pending by running the following command on the Citrix ADC: "set vpn vserver <vpn_vserver_name> -dtls OFF."