CVE-2014-0016 in stunnel
Summary
by MITRE
stunnel before 5.00, when using fork threading, does not properly update the state of the OpenSSL pseudo-random number generator (PRNG), which causes subsequent children with the same process ID to use the same entropy pool and allows remote attackers to obtain private keys for EC (ECDSA) or DSA certificates.
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Analysis
by VulDB Data Team • 05/08/2026
The vulnerability identified as CVE-2014-0016 affects stunnel versions prior to 5.00 when operating in fork threading mode, representing a critical weakness in cryptographic security implementation. This flaw specifically targets the OpenSSL pseudo-random number generator PRNG which is fundamental to generating secure cryptographic keys and certificates. The issue manifests when multiple child processes are spawned through forking, creating a scenario where the PRNG state is not properly maintained across process boundaries, leading to predictable cryptographic outputs.
The technical flaw stems from improper state management within the stunnel application's interaction with OpenSSL's cryptographic libraries. When stunnel operates in fork mode, it creates child processes that inherit the parent's memory space including the PRNG state. However, the application fails to properly reseed or reset the PRNG for each new process, causing subsequent children with identical process IDs to utilize the same entropy pool. This creates a deterministic cryptographic environment where attackers can potentially predict or reconstruct the random number sequences used in key generation, ultimately compromising the security of ECDSA and DSA certificates.
The operational impact of this vulnerability is severe as it directly undermines the confidentiality and integrity of encrypted communications. Remote attackers can exploit this weakness to obtain private keys for EC and DSA certificates, effectively breaking the encryption that protects sensitive data transmission. This compromise allows attackers to decrypt communications, impersonate legitimate services, and potentially gain unauthorized access to systems protected by these vulnerable certificates. The vulnerability affects any system running stunnel versions before 5.00 in fork threading mode, making it particularly dangerous in environments where secure communication is critical.
Mitigation strategies should focus on immediate patching of affected stunnel installations to version 5.00 or later, which includes proper PRNG state management. Organizations should also implement comprehensive monitoring to detect any potential exploitation attempts and consider certificate revocation for any compromised certificates. The vulnerability aligns with CWE-330 weakness category, specifically addressing insufficient entropy in cryptographic operations, and maps to attack techniques in the MITRE ATT&CK framework under credential access and defense evasion categories. System administrators should also review their stunnel configurations to ensure proper threading model selection and implement additional security controls such as certificate pinning and regular security assessments to prevent similar vulnerabilities from emerging in other cryptographic implementations.