October 3, 2023


Three critical-severity remote code execution vulnerabilities impact ASUS RT-AX55, RT-AX56U_V2, and RT-AC86U routers, potentially allowing threat actors to hijack devices if security updates are not installed.

These three WiFi routers are popular high-end models within the consumer networking market, currently available on the ASUS website, favored by gamers and users with demanding performance needs.

The flaws, which all have a CVSS v3.1 score of 9.8 out of 10.0, are format string vulnerabilities that can be exploited remotely and without authentication, potentially allowing remote code execution, service interruptions, and performing arbitrary operations on the device.

Format string flaws are security problems arising from unvalidated and/or unsanitized user input within the format string parameters of certain functions. They can lead to various issues, including information disclosure and code execution.

Attackers exploit these flaws using specially crafted input sent to the vulnerable devices. In the case of the ASUS routers, they would target certain administrative API functions on the devices.

The flaws

The three vulnerabilities that were disclosed earlier today by the Taiwanese CERT are the following:

  1. CVE-2023-39238: Lack of proper verification of the input format string on the iperf-related API module ‘ser_iperf3_svr.cgi’.
  2. CVE-2023-39239: Lack of proper verification of the input format string in the API of the general setting function.
  3. CVE-2023-39240: Lack of proper verification of the input format string on the iperf-related API module ‘ser_iperf3_cli.cgi’.

The above issues impact ASUS RT-AX55, RT-AX56U_V2, and RT-AC86U in firmware versions,, and respectively.

The recommended solution is to apply the following firmware updates:

ASUS released patches that address the three flaws in early August 2023 for RT-AX55, in May 2023 for AX56U_V2, and in July 2023 for RT-AC86U.

Users who haven’t applied security updates since then should consider their devices vulnerable to attacks and prioritize the action as soon as possible.

Furthermore, as many consumer router flaws target the web admin console, it is strongly advised to turn off the remote administration (WAN Web Access) feature to prevent access from the internet.

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