CVE-2021-20231 in GnuTLS
Summary
by MITRE • 03/13/2021
A flaw was found in gnutls. A use after free issue in client sending key_share extension may lead to memory corruption and other consequences.
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Analysis
by VulDB Data Team • 05/16/2025
The vulnerability identified as CVE-2021-20231 represents a critical use-after-free flaw within the gnutls library implementation that affects the handling of key_share extensions during client-side TLS communication. This issue manifests when a client application utilizing gnutls attempts to send key_share extensions as part of the TLS handshake process, creating a scenario where memory management becomes compromised. The root cause stems from improper memory deallocation followed by subsequent access to freed memory regions, which violates fundamental memory safety principles and creates opportunities for arbitrary code execution or system instability. The vulnerability specifically impacts the TLS client-side implementation where key_share extensions are processed, making it particularly concerning for applications that rely heavily on secure communication protocols.
The technical exploitation of this vulnerability occurs during the TLS handshake phase when the client sends key_share information to establish cryptographic parameters with the server. When the gnutls library processes these extensions, it fails to properly manage the memory lifecycle of key_share data structures, leading to a situation where freed memory locations are accessed by subsequent operations. This memory corruption can result in unpredictable behavior including application crashes, data corruption, or more critically, potential code execution in the context of the vulnerable application. The flaw falls under the CWE-416 use-after-free vulnerability category, which is classified as a memory safety issue that has been consistently identified as one of the most dangerous classes of vulnerabilities in software systems. The vulnerability is particularly dangerous because it can be triggered through normal TLS handshake operations without requiring special privileges or complex attack vectors.
The operational impact of CVE-2021-20231 extends across numerous applications and systems that depend on gnutls for secure communication, including web browsers, email clients, network monitoring tools, and server applications. Any system that utilizes gnutls as its underlying TLS library for secure connections becomes vulnerable to potential exploitation, making this issue particularly widespread in the security landscape. The memory corruption resulting from this flaw can lead to denial of service conditions where applications crash or become unresponsive, or more severe scenarios where attackers could potentially execute arbitrary code on affected systems. This vulnerability directly impacts the integrity and availability of secure communication channels, potentially compromising sensitive data transmission and authentication mechanisms that rely on TLS protocols. Organizations using affected versions of gnutls should consider this vulnerability as a high-priority risk that could affect their entire network infrastructure, particularly in environments where TLS-based communication is prevalent.
Mitigation strategies for CVE-2021-20231 primarily involve updating to patched versions of the gnutls library where the memory management issues have been resolved through proper implementation of memory deallocation and access controls. System administrators should prioritize patching affected systems and applications that utilize gnutls, particularly those handling sensitive data or serving as critical network components. The remediation process should include comprehensive testing of patched versions to ensure that the vulnerability is fully addressed without introducing regressions in functionality. Additional defensive measures include implementing network monitoring to detect anomalous TLS handshake behaviors that might indicate exploitation attempts, and maintaining updated security configurations that limit the exposure of vulnerable systems. Organizations should also consider implementing intrusion detection systems that can identify potential exploitation attempts targeting this specific memory corruption vulnerability, as the attack surface is significant due to the widespread use of gnutls across various applications and platforms. The ATT&CK framework categorizes this vulnerability under the T1059 command and control techniques, as exploitation could enable adversaries to establish persistent access through compromised secure communication channels.