CVE-2025-40916Network attack vector

CAPTCHA bypass via predictable rand output

Published Jun 16, 2025 · Updated Jun 16, 2025

Weak randomness in Mojolicious::Plugin::CaptchaPNG 1.05 allows remote attackers to predict CAPTCHA text through Perl's built-in rand(). The CAPTCHA generator invokes rand() for both challenge text and image noise even though its 32-bit-seeded output is predictable. An application must expose plugin-generated CAPTCHAs; successful prediction defeats that anti-automation check, while downstream impact depends on the protected action.

CVSS severity9.1
Critical
EPSS probability0.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Weak randomness in Mojolicious::Plugin::CaptchaPNG 1.05 allows remote attackers to predict CAPTCHA text through Perl's built-in rand(). The CAPTCHA generator invokes rand() for both challenge text and image noise even though its 32-bit-seeded output is predictable. An application must expose plugin-generated CAPTCHAs; successful prediction defeats that anti-automation check, while downstream impact depends on the protected action.

The record

CVE
CVE-2025-40916
Published
Jun 16, 2025
Updated
Jun 16, 2025
Vendor
Gryphon Shafer
Product
Mojolicious::Plugin::CaptchaPNG
Classifications
CWE-804, CWE-338
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 16, 2025CVE publishedPublication date reported by the CVE source.
  2. Jun 16, 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=1.05

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Gryphon Shafer · Mojolicious::Plugin::CaptchaPNGversion=1.05

Published CVSS scores

9.1CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

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-804Guessable CAPTCHA
CWE-338Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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