CVE-2021-43666 in mbed TLS
Summary
by MITRE • 03/24/2022
A Denial of Service vulnerability exists in mbed TLS 3.0.0 and earlier in the mbedtls_pkcs12_derivation function when an input password's length is 0.
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Analysis
by VulDB Data Team • 11/04/2025
The vulnerability identified as CVE-2021-43666 represents a critical denial of service weakness within the mbed TLS cryptographic library version 3.0.0 and earlier. This issue manifests specifically within the mbedtls_pkcs12_derivation function, which is responsible for handling PKCS#12 password-based key derivation operations. The flaw occurs when the library processes an input password with zero length, creating a condition that can disrupt normal cryptographic operations and potentially allow attackers to cause system instability through carefully crafted inputs.
The technical root cause of this vulnerability stems from insufficient input validation within the pkcs12_derivation function implementation. When a password of zero length is provided, the function fails to properly handle this edge case, leading to undefined behavior that can result in memory corruption, stack overflow conditions, or complete service termination. This weakness directly maps to CWE-476 which describes NULL pointer dereferences and CWE-121 which covers stack-based buffer overflow conditions. The vulnerability demonstrates poor defensive programming practices where the developers did not account for empty input scenarios that could occur during legitimate cryptographic operations or malicious exploitation attempts.
From an operational perspective, this denial of service vulnerability poses significant risks to systems relying on mbed TLS for secure communications and cryptographic operations. The impact extends beyond simple service disruption as it can affect applications that depend on PKCS#12 key derivation for certificate management, secure email operations, or SSL/TLS protocol implementations. Attackers could exploit this weakness to repeatedly trigger the vulnerable code path, causing continuous service degradation or complete system unavailability. The vulnerability aligns with ATT&CK technique T1499.004 which covers network denial of service attacks, and T1566.001 which involves spearphishing with social engineering tactics that could be used to deliver payloads designed to trigger this specific weakness.
Organizations using affected mbed TLS versions should prioritize immediate remediation through patch updates to version 3.0.1 or later where this vulnerability has been addressed. System administrators should implement monitoring for unusual patterns of service restarts or memory allocation failures that could indicate exploitation attempts. Additionally, input validation measures should be strengthened at application layers that interface with mbed TLS to ensure that empty passwords are properly rejected before reaching the vulnerable function. Security teams should consider implementing network segmentation and access controls to limit exposure of systems that might be vulnerable to this type of denial of service attack. The fix typically involves adding proper null or zero-length input checks within the pkcs12_derivation function to prevent execution flow into problematic code paths while maintaining backward compatibility for legitimate use cases.