CVE-2021-37417Network attack vector

CAPTCHA bypass via accepted exclusion parameter

Published Aug 30, 2021 · Updated Aug 4, 2024

CAPTCHA bypass in Zoho ManageEngine ADSelfService Plus 6103 and earlier allows remote attackers to evade login throttling controls. ADSelfService Plus accepts the EXCLUDE_CAPTCHA parameter at /j_security_check and skips CAPTCHA processing instead of rejecting the flag. The unauthenticated login endpoint is sufficient to trigger the bypass, enabling automated password-guessing attempts without solving CAPTCHA challenges.

CVSS severity9.8
Critical
EPSS probability4.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CAPTCHA bypass in Zoho ManageEngine ADSelfService Plus 6103 and earlier allows remote attackers to evade login throttling controls. ADSelfService Plus accepts the EXCLUDE_CAPTCHA parameter at /j_security_check and skips CAPTCHA processing instead of rejecting the flag. The unauthenticated login endpoint is sufficient to trigger the bypass, enabling automated password-guessing attempts without solving CAPTCHA challenges.

The record

CVE
CVE-2021-37417
Published
Aug 30, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-287, T1110
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 30, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record 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

5.0NIST NVDAV:N/AC:L/Au:N/C:P/I:N/A:N
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 exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication
T1110Brute Force

Public exploit references

No public exploit references are available in this record.

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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