External risk intelligence

ASUS Router CAPTCHA Bypass Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2021-41435

This CVE affects consumer-grade wireless routers. These devices often feature web-based administration interfaces that, while intended for local network management, are frequently exposed to the internet via misconfiguration or enabling remote management features, making them a common target for internet-reachable access.

Asus Gt Ax11000 Firmware

before 3.0.0.4.386.45898before 3.0.0.4.386.45911

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in the CAPTCHA protection of various ASUS Wi-Fi router models. This issue allows attackers to bypass security measures, potentially enabling them to attempt unlimited login attempts without being blocked, which could lead to unauthorized access or further compromise of the network. The main concern is confirming the relevance and exposure of these devices within the organization.

Bypasses security on ASUS Wi-Fi routers.
Allows unlimited, unblocked login attempts.
Confirm relevance and exposure for affected devices.

Attack Path

How an attacker could exploit the issue

An attacker can exploit this by sending a specially crafted HTTP request to the device's web interface. This bypasses the CAPTCHA protection, allowing an unlimited number of login attempts, which could lead to unauthorized access to the router and potentially compromise the entire network.

Network access required to reach the router.
Bypasses CAPTCHA to allow unlimited login attempts.
Enables unauthorized access and network compromise.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an unauthenticated remote attacker to bypass CAPTCHA protection on affected ASUS routers, enabling them to attempt an unlimited number of login attempts. This could be used to try and gain unauthorized access to the router's administrative interface.

Router administrative access.
Bypass CAPTCHA via specific HTTP request.
Unauthorized access to network settings.

Operational Fix

Recommended remediation, mitigation, and detection steps

Real-world responsibility for this vulnerability likely falls to infrastructure or platform teams managing network devices, with potential involvement from security teams for exposure assessment. The initial practical step is to inventory all affected ASUS router models, confirm their internet reachability and business criticality, and identify the designated owner for each device before planning remediation.

Network infrastructure teams own the issue.
Verify internet exposure and criticality first.
Plan phased firmware updates by risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is the ASUS equipment affected by CVE-2021-41435?

This vulnerability impacts a wide range of ASUS wireless routers and mesh systems, including popular models like the ROG Rapture GT-AX11000, various RT-AX series units (such as the RT-AX86U and RT-AX88U), and ZenWiFi systems. These devices function as home or small-office network hubs. The flaw specifically resides in the firmware managing the device's administrative web interface.

What is the security weakness behind CVE-2021-41435?

This issue is classified as CWE-307: Improper Restriction of Excessive Authentication Attempts. In plain terms, the router's CAPTCHA mechanism—designed to prevent automated 'brute-force' password guessing—is flawed. Because this protection can be bypassed, the router fails to lock out attackers after multiple failed login attempts, potentially allowing them to keep guessing passwords until they gain entry.

How does an attacker trigger this vulnerability?

An attacker triggers this by sending a specially crafted HTTP request to the router's web management interface. This request forces the device to ignore or bypass the CAPTCHA step. Simply interacting with the router's interface normally does not trigger the bug; the attacker must intentionally send this specific request to neutralize the security control.

Is my router at risk if it is not reachable from the internet?

According to Halo Surface Signal, this vulnerability is most concerning for devices exposed to the internet. If your router is managed only from within your local network, the risk is lower but not zero. Devices with remote management enabled or those misconfigured to be accessible via public IP addresses are primary targets, as attackers can reach the administrative interface directly from anywhere.

What is the recommended first step to address this?

The most effective action is to update your router's firmware to the latest version provided by ASUS. Start by checking your model against the manufacturer's support site to ensure you have version 3.0.0.4.386.45898 or later (or 3.0.0.4.386.45911 for the RT-AX68U). Additionally, ensure that remote administration features are disabled if you do not strictly require them for your network configuration.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.