CVE-2016-4456 in GnuTLS
Summary
by MITRE
The "GNUTLS_KEYLOGFILE" environment variable in gnutls 3.4.12 allows remote attackers to overwrite and corrupt arbitrary files in the filesystem.
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Analysis
by VulDB Data Team • 12/15/2022
The vulnerability identified as CVE-2016-4456 resides within the gnutls library version 3.4.12 and specifically targets the handling of the "GNUTLS_KEYLOGFILE" environment variable. This flaw represents a critical security weakness that enables remote attackers to manipulate file system contents through improper environment variable processing. The vulnerability stems from insufficient validation of the environment variable's value, allowing attackers to specify arbitrary file paths that can be overwritten or corrupted during normal library operations. The issue manifests when gnutls processes this environment variable during cryptographic operations, creating a path traversal scenario that can be exploited by remote adversaries.
The technical implementation of this vulnerability follows a classic insecure file handling pattern where the environment variable value is directly used to construct file paths without proper sanitization or validation. When gnutls initializes and processes the GNUTLS_KEYLOGFILE variable, it treats the specified path as a legitimate destination for key logging information without verifying whether the path points to a valid, secure location within the filesystem. This design flaw creates an opportunity for attackers to specify malicious file paths that can result in overwriting critical system files, log files, or other sensitive data. The vulnerability is particularly concerning because it can be triggered remotely, meaning an attacker does not need local access to exploit the issue, and the impact extends beyond simple file corruption to potentially system compromise.
From an operational perspective, this vulnerability presents significant risks to systems utilizing gnutls for secure communications, particularly those running web servers, email servers, or any service that relies on the library for TLS/SSL operations. The remote exploitation capability means that attackers can target vulnerable systems from anywhere on the network, making this a high-severity issue for organizations maintaining public-facing services. The potential for arbitrary file corruption can lead to service disruption, data loss, and in some cases, privilege escalation opportunities if the targeted files are system-critical. This vulnerability directly relates to CWE-22, which describes path traversal vulnerabilities, and aligns with ATT&CK technique T1059.007 for execution through environment variables, making it a multi-faceted threat that can be leveraged for various attack vectors.
Mitigation strategies for CVE-2016-4456 require immediate attention from system administrators and security teams. The most effective approach involves upgrading to gnutls version 3.4.13 or later, where the vulnerability has been patched through proper input validation of environment variables. Organizations should also implement strict environment variable controls, ensuring that the GNUTLS_KEYLOGFILE variable is either disabled or properly validated before use. System administrators should monitor for unauthorized changes to critical system files and implement file integrity monitoring solutions to detect potential exploitation attempts. Network segmentation and access controls can help limit the impact of exploitation, while regular security audits should verify that no systems are running vulnerable versions of the library. Additionally, organizations should consider implementing application whitelisting policies that restrict which processes can modify system files through environment variables, thereby reducing the attack surface for this particular vulnerability.