Attackers target Zyxel RCE vulnerability CVE-2023
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2023-28771 | Unauthenticated OS Command Injection in Zyxel ATP, USG FLEX, VPN, and ZyWALL Firewalls CVE-2023-28771 is an unauthenticated OS command injection flaw (CWE-78) in Zyxel firewall firmware, caused by improper error message handling in the IKE packet decoder. A remote attacker triggers it by sending crafted packets to an affected device, with no credentials or user interaction required (CVSS 3.1: 9.8, network vector, low complexity). Successful exploitation lets the attacker execute operating-system commands on the firewall, which typically means full device compromise of these perimeter/VPN gateway appliances. Organizations running Zyxel ZyWALL/USG, VPN, USG FLEX, or ATP series firewalls on the affected firmware ranges are exposed, especially where IKE/VPN traffic is reachable from the internet. Exploitation is confirmed in the wild: the flaw was added to CISA's KEV on 2023-05-31, carries a 99.3% EPSS score (100th percentile), is reportedly used by DDoS botnets, and was reportedly exploited in the May 2023 coordinated attacks against nearly two dozen Danish energy companies. Do: Apply Zyxel's patched firmware to all affected devices per the vendor advisory - releases newer than 4.73 for ZyWALL/USG and newer than 5.35 for ATP, USG FLEX, and VPN series - prioritizing internet-facing and VPN gateway appliances, as this is a KEV required-action vulnerability. Where immediate patching is not possible, restrict IKE traffic (UDP 500/4500) to trusted peers or disable unneeded IPsec VPN termination. After patching, review device logs and configurations for signs of command execution or unexpected changes, given confirmed botnet and targeted-attack use. | 9.8 | 99% | KEV PoC |
| largetens of thousands of internet-exposed Zyxel firewall/VPN gateways (order of 10,000-100,000 devices/sites); estimate |
Full article361 words · extracted from securityaffairs.com · click to collapse
GreyNoise researchers have observed exploit attempts targeting the remote code execution vulnerability CVE-2023-28771 in Zyxel devices.
On June 16, GreyNoise researchers detected exploit attempts targeting CVE-2023-28771 (CVSS score 9.8), a remote code execution flaw impacting Zyxel IKE decoders over UDP port 500.
“Exploitation attempts against CVE-2023-28771 were minimal throughout recent weeks. On June 16, GreyNoise observed a concentrated burst of exploit attempts within a short time window, with 244 unique IPs observed attempting exploitation.” reads the alert published by GreyNoise.
The main targets of the CVE-2023-28771 exploit were the U.S., U.K., Spain, Germany, and India. Interestingly, the attacking IPs showed no other suspicious activity in the two weeks before June 16, the attempts focused solely on this specific vulnerability.
All 244 IP addresses linked to the exploit attempts appear to come from Verizon Business in the U.S., but because the attack uses UDP (port 500), the researchers warn that the IP addresses could be spoofed and may not reveal the real source. GreyNoise linked the attempts to Mirai botnet variants, which VirusTotal later confirmed.
Below are mitigations provided by GreyNoise:
- Block malicious IPs: While spoofing is possible, GreyNoise has classified all 244 IPs as malicious. Defenders should immediately block these IPs while monitoring for related activity.
- Review Zyxel device exposure: Verify that any internet-exposed Zyxel devices are patched for CVE-2023-28771.
- Monitor for post-exploitation activity: Exploit attempts may lead to botnet enlistment or additional compromise. Monitor affected devices for anomalies.
- Limit unnecessary IKE/UDP port 500 exposure: Apply network filtering where possible to reduce unnecessary exposure.
In April 2023, Zyxel addressed the critical vulnerability CVE-2023-28771 in its firewall devices. The company promptly advised customers to install the provided patches in order to mitigate the vulnerability. At the time, the vendor reported that threat actors are actively attempting to exploit the command injection vulnerability CVE-2023-28771 impacting Zyxel firewalls. Their objective was to leverage this vulnerability to deploy and install malware on the affected systems. At the end of May 2025, US CISA added the vulnerability to its Known Exploited Vulnerability to Catalog based on evidence of active exploitation.
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