CVE-2021-43666
📋 TL;DR
This vulnerability in mbed TLS allows attackers to cause a Denial of Service by providing an empty password to the mbedtls_pkcs12_derivation function. It affects systems using mbed TLS 3.0.0 and earlier for PKCS#12 operations. The vulnerability can crash applications that process malicious PKCS#12 data.
💻 Affected Systems
- mbed TLS
- Applications using mbed TLS library
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Application crashes leading to complete service unavailability, potentially affecting multiple services if mbed TLS is used in critical infrastructure.
Likely Case
Targeted DoS attacks against specific services using mbed TLS for PKCS#12 operations, causing temporary service disruption.
If Mitigated
Minimal impact with proper input validation and updated libraries, though some performance degradation may occur from validation overhead.
🎯 Exploit Status
Exploitation is straightforward - send PKCS#12 data with empty password. Public proof-of-concept exists in GitHub issues.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: mbed TLS 3.1.0 and later
Vendor Advisory: https://github.com/ARMmbed/mbedtls/issues/5136
Restart Required: Yes
Instructions:
1. Update mbed TLS to version 3.1.0 or later. 2. Recompile applications using mbed TLS. 3. Restart affected services. 4. Test PKCS#12 functionality after update.
🔧 Temporary Workarounds
Input validation wrapper
allAdd input validation to check password length before calling mbedtls_pkcs12_derivation
// In application code, add: if(password_length == 0) { return MBEDTLS_ERR_PKCS12_PASSWORD_MISMATCH; }
Disable PKCS#12 functionality
allRemove or disable PKCS#12 support if not required
# Recompile mbed TLS with MBEDTLS_PKCS12_C disabled in config.h
🧯 If You Can't Patch
- Implement network-level filtering to block PKCS#12 data with empty passwords
- Deploy application-level input validation to reject empty passwords before reaching mbed TLS
🔍 How to Verify
Check if Vulnerable:
Check mbed TLS version and verify if mbedtls_pkcs12_derivation is used in codebaseCheck Version:
grep MBEDTLS_VERSION_STRING mbedtls/include/mbedtls/version.hVerify Fix Applied:
Test PKCS#12 operations with empty password - should return error instead of crashing📡 Detection & Monitoring
Log Indicators:
- Application crashes with segmentation faults in mbed TLS code
- Error logs showing PKCS#12 processing failures
Network Indicators:
- Unusual PKCS#12 data transfers to services
- Repeated connection attempts with malformed cryptographic data
SIEM Query:
source="application.logs" AND ("segmentation fault" OR "mbedtls_pkcs12" OR "PKCS12")