CVE-2021-3689
📋 TL;DR
CVE-2021-3689 is a vulnerability in Yii2 PHP framework where predictable algorithms in random number generation could allow attackers to guess generated values like CSRF tokens or session IDs. This affects all Yii2 applications using the vulnerable random number generator. Attackers could potentially bypass security mechanisms that rely on randomness.
💻 Affected Systems
- Yii2 PHP Framework
📦 What is this software?
Yii by Yiiframework
⚠️ Risk & Real-World Impact
Worst Case
Attackers could predict CSRF tokens, session IDs, or cryptographic nonces, leading to CSRF attacks, session hijacking, or bypassing security controls that depend on randomness.
Likely Case
CSRF token prediction enabling cross-site request forgery attacks against authenticated users.
If Mitigated
Limited impact if additional security layers like proper session management and input validation are implemented.
🎯 Exploit Status
Exploitation requires understanding of the predictable algorithm and ability to observe or predict generated values.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.0.43
Vendor Advisory: https://github.com/yiisoft/yii2/commit/13f27e4d920a05d53236139e8b07007acd046a46
Restart Required: No
Instructions:
1. Update Yii2 to version 2.0.43 or later via Composer: composer require yiisoft/yii2:"^2.0.43" 2. Run composer update 3. Clear any cached files if using caching mechanisms.
🔧 Temporary Workarounds
Override Random Generator
allManually override the random number generator with a cryptographically secure alternative
Implement custom random generator using PHP's random_bytes() or openssl_random_pseudo_bytes()
🧯 If You Can't Patch
- Implement additional CSRF protection layers like double-submit cookies or custom token validation
- Use external session management that doesn't rely on Yii2's random generation
🔍 How to Verify
Check if Vulnerable:
Check composer.json or Yii2 version: grep -r "yiisoft/yii2" composer.jsonCheck Version:
php -r "echo Yii::getVersion();" or check composer.lock for yiisoft/yii2 versionVerify Fix Applied:
Verify installed version is 2.0.43+: php -r "echo Yii::getVersion();"📡 Detection & Monitoring
Log Indicators:
- Multiple failed CSRF token validations from same source
- Unusual session creation patterns
Network Indicators:
- Predictable token values in requests
- Repeated requests with sequential token values
SIEM Query:
source="web_logs" AND (csrftoken MATCHES "predictable_pattern" OR sessionid SEQUENTIAL)