Zyxel Devices Hit by Active Exploits Targeting CVE-2023-28771 Vulnerability

A serious security vulnerability, tracked as CVE-2023-28771, is affecting Zyxel networking devices. Security researchers at GreyNoise noticed a sudden sharp rise, and a concentrated effort by attackers to exploit this flaw on June 16^th.

The vulnerability allows for remote code execution, which means attackers can run their own programs on vulnerable devices from a distance. This particular weakness is found in how Zyxel devices handle specific internet messages, called Internet Key Exchange (IKE) packets coming through the UDP port 500.

While attacks targeting this Zyxel flaw had been minimal, June 16th brought a significant spike in activity. GreyNoise recorded 244 different internet addresses trying to exploit the issue within a single day.

These attacks are aimed at devices in various countries, with the following being the most targeted:

• India
• Spain
• Germany
• United States
• United Kingdom

Interestingly, a review of these 244 attacking addresses showed they had not been involved in any other suspicious network activity in the two weeks leading up to this sudden burst.

An investigation into the attacking internet addresses revealed they were all registered under Verizon Business infrastructure and appeared to originate from the United States. However, because the attacks use UDP port 500, which allows for spoofing (faking the sender’s address), the true source might be hidden, noted GreyNoise researchers in their blog post shared with Hackread.com.

Further analysis by GreyNoise, supported by checks from VirusTotal, found signs that these attacks might be linked to variants of the Mirai botnet, a type of malicious software that takes over devices.

In response to these active threats, security experts are urging immediate action. It is advised to block all 244 identified malicious IP addresses and to check if any internet-connected Zyxel devices have the necessary security patches for CVE-2023-28771.

Device owners should also watch for any unusual activity after an exploit attempt, as this could lead to further compromise or the device being added to a botnet. Finally, it’s recommended to limit any unnecessary exposure of IKE/UDP port 500 by applying network filters.

It’s important to note that Zyxel devices have faced security challenges in the past. For instance, Hackread.com reported in June 2024, about Zyxel NAS devices being targeted by a Mirai-like botnet exploiting a different recent vulnerability (CVE-2024-29973), highlighting a recurring pattern of issues for the company’s products.

“This was added to the CISA Known Exploited vulnerabilities list on May 31, 2023, requiring agencies to have it resolved before June 21 that same year. The activity observed appears to be the Mirai botnet activity,” said Martin Jartelius, CISO at cybersecurity company Outpost24.

“As the vulnerability has been extensively targeted before, for someone to fall victim now, they would have had to obtain a vulnerable device, deploy it without updates, and expose it to the internet, even though it’s in a known vulnerable state,” explained Martin.

“One would almost say that the chain of incompetence needed to be victimized at this point is borderline impressive, but of course, it can happen. This, however, is not the vulnerability we should all wake up and worry about today. In fact, if you were worried about it, you would have fixed it years ago.”