CVE-2023-1539Public exploitNetwork attack vector

CAPTCHA bypass via reusable solved challenge

Published Mar 21, 2023 · Updated Feb 27, 2025

Guessable CAPTCHA in Apache Answer before 1.0.6 allows remote attackers to reuse one solved challenge for repeated requests. CaptchaService.VerifyCaptcha reads the keyed solution from cache but leaves it stored after comparing the submitted value, so a valid captcha_id and captcha_code remain reusable. Unauthenticated network access is sufficient to replay the pair against CAPTCHA-gated operations, while published assessments disagree on whether the bounded effect is integrity or availability.

CVSS severity5.3
Medium
EPSS probability0.61%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Guessable CAPTCHA in Apache Answer before 1.0.6 allows remote attackers to reuse one solved challenge for repeated requests. CaptchaService.VerifyCaptcha reads the keyed solution from cache but leaves it stored after comparing the submitted value, so a valid captcha_id and captcha_code remain reusable. Unauthenticated network access is sufficient to replay the pair against CAPTCHA-gated operations, while published assessments disagree on whether the bounded effect is integrity or availability.

The record

CVE
CVE-2023-1539
Published
Mar 21, 2023
Updated
Feb 27, 2025
Vendor
The Apache Software Foundation
Product
Apache Answer
Classifications
CWE-307, CWE-804, T1110
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 21, 2023CVE publishedPublication date reported by the CVE source.
  2. Feb 27, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <1.0.6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache Answerversion=unspecified <1.0.6

Published CVSS scores

5.3GitHub Security AdvisoryCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
5.3CVE Program CNA recordCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 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
CWE-804Guessable CAPTCHA
T1110Brute Force

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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