CVE-2018-18531
📋 TL;DR
This vulnerability in kaptcha 2.3.2 uses insecure Random instead of SecureRandom for CAPTCHA generation, allowing attackers to predict or brute-force CAPTCHA values. This enables bypassing CAPTCHA-based access restrictions on authentication forms, registration pages, or rate-limited endpoints. Any application using the vulnerable kaptcha library for CAPTCHA protection is affected.
💻 Affected Systems
- kaptcha
📦 What is this software?
Kaptcha by Kaptcha Project
⚠️ Risk & Real-World Impact
Worst Case
Complete bypass of CAPTCHA protection leading to automated account takeover, credential stuffing, spam registration, or denial of service through rate limit evasion.
Likely Case
Automated bots bypass CAPTCHA on login/registration forms, enabling credential stuffing attacks, spam account creation, or scraping protected content.
If Mitigated
With additional authentication layers and monitoring, impact is limited to increased bot activity that can be detected and blocked.
🎯 Exploit Status
Exploitation requires only predictable random number generation analysis or brute-force attacks against CAPTCHA values.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.3.3
Vendor Advisory: https://github.com/penggle/kaptcha/issues/3
Restart Required: Yes
Instructions:
1. Update kaptcha dependency to version 2.3.3 or later. 2. Replace Random with SecureRandom in CAPTCHA generation code. 3. Restart application server.
🔧 Temporary Workarounds
Implement custom SecureRandom CAPTCHA generator
allCreate custom text producer classes using SecureRandom instead of Random for CAPTCHA generation.
// Java code: Use SecureRandom.getInstanceStrong() instead of new Random()
🧯 If You Can't Patch
- Implement additional authentication factors beyond CAPTCHA
- Deploy WAF with CAPTCHA bypass detection rules
🔍 How to Verify
Check if Vulnerable:
Check if kaptcha version is 2.3.2 in pom.xml or build.gradle, and inspect if CAPTCHA generation uses Random class.Check Version:
grep -r "kaptcha" pom.xml build.gradle || check Maven/Gradle dependenciesVerify Fix Applied:
Verify kaptcha version is 2.3.3+ and CAPTCHA generation uses SecureRandom instead of Random.📡 Detection & Monitoring
Log Indicators:
- Unusual CAPTCHA bypass patterns
- High volume of successful CAPTCHA attempts from single IP
- Multiple failed then successful CAPTCHA attempts
Network Indicators:
- Rapid CAPTCHA submission patterns
- Automated tool user-agents with successful CAPTCHA
SIEM Query:
source="application.logs" AND ("CAPTCHA bypass" OR "Random" AND "kaptcha")