CVE-2021-41435Public exploitNetwork attack vector

Unlimited login attempts via CAPTCHA bypass

Published Nov 19, 2021 · Updated Jul 9, 2026

CAPTCHA protection bypass in affected ASUS router firmware allows remote attackers to make unlimited login attempts through the web interface. A specially formed HTTP request bypasses the CAPTCHA-based restriction that should throttle repeated authentication failures. No credentials or user interaction are required to reach the management service; the direct consequence is unrestricted password guessing, while successful account compromise still depends on guessing valid credentials.

CVSS severity9.8
Critical
EPSS probability6.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CAPTCHA protection bypass in affected ASUS router firmware allows remote attackers to make unlimited login attempts through the web interface. A specially formed HTTP request bypasses the CAPTCHA-based restriction that should throttle repeated authentication failures. No credentials or user interaction are required to reach the management service; the direct consequence is unrestricted password guessing, while successful account compromise still depends on guessing valid credentials.

The record

CVE
CVE-2021-41435
Published
Nov 19, 2021
Updated
Jul 9, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-307, CAPEC-49, T1110.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 19, 2021CVE publishedPublication date reported by the CVE source.
  2. Jul 9, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-307Improper Restriction of Excessive Authentication Attempts
CAPEC-49Password Brute Forcing
T1110.001Password Guessing

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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