CVE-2021-40556

Name: CVE-2021-40556
Creator: NVD / NIST
Published: 2022-10-06T18:15:50.453+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.34%

Last modified Jun 17, 2026

CVE-2021-40556 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. A stack overflow vulnerability exists in the httpd service in ASUS RT-AX56U Router Version 3.0.0.4.386.44266. This vulnerability is caused by the strcat function called by "caupload" input handle function allowing the user to enter 0xFFFF bytes into the stack. EPSS estimates a 1.34% chance of exploitation in the next 30 days.

Description

A stack overflow vulnerability exists in the httpd service in ASUS RT-AX56U Router Version 3.0.0.4.386.44266. This vulnerability is caused by the strcat function called by "caupload" input handle function allowing the user to enter 0xFFFF bytes into the stack. This vulnerability allows an attacker to execute commands remotely. The vulnerability requires authentication.

Metrics

CVSS 3.1

8.8/10

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

EPSS Probability

1.34%

67.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-787

Affected Software

Vendor	Product	Versions
Asus	Rt-Ax56u Firmware	3.0.0.4.386.44266

References

https://www.asus.com/tw/Networking-IoT-Servers/WiFi-Routers/ASUS-WiFi-Routers/RT-AX56U/HelpDesk_BIOS/Product, Vendor Advisory
https://x1ng.top/2021/10/14/ASUS%E6%A0%88%E6%BA%A2%E5%87%BA%E6%BC%8F%E6%B4%9E%E5%88%86%E6%9E%90/Exploit, Third Party Advisory
https://www.asus.com/tw/Networking-IoT-Servers/WiFi-Routers/ASUS-WiFi-Routers/RT-AX56U/HelpDesk_BIOS/Product, Vendor Advisory
https://x1ng.top/2021/10/14/ASUS%E6%A0%88%E6%BA%A2%E5%87%BA%E6%BC%8F%E6%B4%9E%E5%88%86%E6%9E%90/Exploit, Third Party Advisory

Timeline

Published: Oct 6, 2022
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-40556?

How severe is CVE-2021-40556?

CVE-2021-40556 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 1.34% probability of exploitation in the next 30 days.

How do I fix CVE-2021-40556?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-40556?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST