CVE-2021-3692Public exploitNetwork attack vector

Security-value prediction via mt_rand usage

Published Aug 10, 2021 · Updated Aug 3, 2024

Insufficient randomness in Yii 2 before 2.0.43 allows remote attackers to predict security-sensitive values over the network. CaptchaAction::generateVerifyCode and other critical paths use mt_rand, a noncryptographic generator, for values that must remain unpredictable. Exposure depends on an application reaching these paths; predictable CAPTCHA values weaken anti-automation controls, while the published assessment bounds impact to limited confidentiality loss.

CVSS severity5.3
Medium
EPSS probability1.70%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficient randomness in Yii 2 before 2.0.43 allows remote attackers to predict security-sensitive values over the network. CaptchaAction::generateVerifyCode and other critical paths use mt_rand, a noncryptographic generator, for values that must remain unpredictable. Exposure depends on an application reaching these paths; predictable CAPTCHA values weaken anti-automation controls, while the published assessment bounds impact to limited confidentiality loss.

The record

CVE
CVE-2021-3692
Published
Aug 10, 2021
Updated
Aug 3, 2024
Vendor
Yiisoft
Product
Yii 2
Classifications
CWE-330, CWE-1241
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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?

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

  1. Affected versionversion=unspecified <=2.0.42.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Yiisoft · Yii 2version=unspecified <=2.0.42.1

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:P/I:N/A:N
8.1huntr.dev CNACVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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 exploitPublished

Weakness, pattern, technique

CWE-330Use of Insufficiently Random Values
CWE-1241Use of Predictable Algorithm in Random Number Generator

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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