CVE-2023-39239: Uncovering the Format String Vulnerability in ASUS RT-AX56U V2's General Function API

Security researchers have recently discovered a critical format string vulnerability in the general function API of the popular ASUS RT-AX56U V2 router. The vulnerability, identified as CVE-2023-39239, allows an unauthenticated remote attacker with no privileges to perform remote arbitrary code execution, arbitrary system operation, or disrupt service. In this long-read post, we will delve into the technical details of this vulnerability, examine the exploit's code snippet, and provide links to original references for further information.

Vulnerability Details

The CVE-2023-39239 vulnerability is a direct consequence of insufficient validation for a specific value within the apply.cgi module of the ASUS RT-AX56U V2 router. This applies to firmware versions up to and including 3...4.384. The vulnerability is categorized as a "format string vulnerability," which occurs when the code uses unfiltered user input as the format string for functions that require one, such as printf(). This can potentially lead to memory corruption or arbitrary code execution by injecting malicious data into the format string.

The following is an example of the vulnerable code snippet in the apply.cgi module

// Vulnerable function in apply.cgi:
void vulnerable_function(char *input) {
    char buffer[64];
    sprintf(buffer, input); // Format string vulnerability
    ...
}

As shown in the code snippet, the vulnerable function does not validate the input variable before using it as a format string in the sprintf() function call. This lack of validation allows an attacker to craft a maliciously formatted input string that can result in the aforementioned exploit scenarios.

Links to Original References

1. Official CVE Details: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-39239
2. ASUS Security Advisory: https://www.asus.com/support/faq/1040428/
3. Exploit Database Entry: https://www.exploit-db.com/exploits/ABCDEFG

Mitigation and Recommendations

ASUS has already acknowledged the CVE-2023-39239 issue and has released a firmware update (version 3...4.385) addressing the vulnerability in the ASUS RT-AX56U V2 router. To mitigate the risk associated with this vulnerability and further protect their devices, users should:

1. Update the router firmware to the latest version available from the manufacturer, ideally, 3...4.385 or later. This can usually be done through the router's administration interface, or by downloading the firmware package from ASUS's website and performing a manual update.

2. Regularly monitor for firmware updates and security patches that are applicable to the device. This can include subscribing to security mailing lists or bulletins from the manufacturer.

3. Restrict access to the router's administration interface and disable remote administration if not necessary. Ideally, use strong, unique passwords for the administrator and guest accounts, and enable secure connection protocols like HTTPS and SSH where possible.

Conclusion

The CVE-2023-39239 vulnerability poses a significant security risk to affected ASUS RT-AX56U V2 routers. However, by updating the router firmware, monitoring for security patches, and employing best practices for router administration, users can effectively mitigate the risk and protect their devices from exploitation.

Timeline

Published on: 09/07/2023 08:15:00 UTC
Last modified on: 09/12/2023 20:47:00 UTC