The security community has recently discovered a severe format string vulnerability (CVE-2023-39240) in ASUS RT-AX56U V2's iperf client function API. This vulnerability is caused by the lack of proper validation in the set_iperf3_cli.cgi module for a specific user-input value, allowing an unauthenticated remote attacker to execute arbitrary code, perform unauthorized system operations, or disrupt service without any privilege. In this post, we will dive deep into the vulnerability's technical details, discuss its possible exploit scenarios, and provide mitigation measures to safeguard your network.

Technical Details

The ASUS RT-AX56U V2 is a popular, high-performance router that has built-in support for the widely-used iperf network performance testing tool. Researchers found a format string vulnerability in the router's iperf client function API, which stems from the set_iperf3_cli.cgi module's improper handling of user input values.

Here's a snippet of the vulnerable code within the set_iperf3_cli.cgi module

// set_iperf3_cli.cgi: iperf client function API
int set_iperf3_cli_cgi(void)
    char iperf_args[MAX_IPERF_ARG_LEN];
    snprintf(iperf_args, sizeof(iperf_args), "iperf3 " USER_INPUT);
    return ;

This code is vulnerable because it uses the snprintf() function to format an iperf command string with user-supplied input (USER_INPUT). The snprintf() function is easily exploitable if the attacker can control the format string. By injecting a crafted format string through the USER_INPUT parameter, an attacker can achieve remote code execution, unauthorized system operations, or even crash the router's software, leading to a denial of service (DoS) attack.

Exploit Scenarios

1. Remote arbitrary code execution: An attacker can craft a malicious format string, injecting shellcode into the router's memory and executing it. This could potentially allow the attacker to gain control over the device and access to the victim's network.

2. Arbitrary system operation: By injecting a crafted format string, an attacker can manipulate the router's memory and potentially perform unauthorized system operations, such as exfiltrating sensitive data or tampering with the system's settings.

3. Denial of service (DoS): If an attacker injects a malformed format string that causes the router's software to crash, it could lead to a denial of service for the victims, rendering their network inaccessible.

To protect your network from this vulnerability, we recommend taking the following actions

1. Apply the latest firmware update from ASUS: ASUS has been made aware of this vulnerability, and they have released a firmware update to address the issue. You can download the update from the official ASUS website ( and follow the instructions provided.

2. Limit access to the router's management interface: Restrict access to the router's web-based management interface from the internet by implementing access control lists, firewall rules, or VPN connections.

3. Monitor network traffic for suspicious activity: Keep an eye on your network traffic, and be vigilant in detecting any anomalies that could point to exploitation attempts.


CVE-2023-39240 is a critical format string vulnerability in ASUS RT-AX56U V2's iperf client function API. If left unpatched, it can lead to severe consequences, including remote code execution, unauthorized system operations, or denial of service. Network administrators and users must act promptly to apply the necessary patches and implement the best security practices to protect their networks from potential exploitation.


Published on: 09/07/2023 08:15:00 UTC
Last modified on: 09/12/2023 20:09:00 UTC