Zyxel patches critical vulnerability in NAS devices (CVE-2023-27992)

Zyxel NAS326, NAS540, and NAS542 network-attached storage devices contain a pre-authentication command injection flaw (CWE-78) that lets an unauthenticated attacker execute operating system commands by sending a crafted HTTP request to the device. Because the flaw is network-facing and requires no credentials or user interaction, a remote attacker gains the ability to run arbitrary OS commands on the device, effectively full compromise. Affected firmware is NAS326 versions prior to V5.21(AAZF.14)C0, NAS540 versions prior to V5.21(AATB.11)C0, and NAS542 versions prior to V5.21(ABAG.11)C0. The flaw scores 9.8 (critical) on CVSS 3.1, carries a very high 83.8% probability of exploitation within 30 days per EPSS, and CISA added it to the Known Exploited Vulnerabilities catalog on 2023-06-23. News reporting indicates a Mirai-like botnet is already exploiting the flaw in the wild, so defenders should treat it as an actively exploited, internet-exposable issue and patch immediately.

Do: Upgrade NAS326 to V5.21(AAZF.14)C0, NAS540 to V5.21(AATB.11)C0, and NAS542 to V5.21(ABAG.11)C0 per Zyxel's security advisories. Until patched, keep the NAS web administration interface off the public internet or restrict access with firewall rules. Because a Mirai-like botnet is actively exploiting this flaw, inspect patched and unpatched devices for signs of compromise, such as unfamiliar processes or unexpected outbound traffic; organizations covered by CISA's KEV requirements must apply the vendor updates by the required deadline.