CVE-2021-20232 in GnuTLS
Summary
by MITRE • 03/13/2021
A flaw was found in gnutls. A use after free issue in client_send_params in lib/ext/pre_shared_key.c may lead to memory corruption and other potential consequences.
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Analysis
by VulDB Data Team • 12/03/2025
The vulnerability identified as CVE-2021-20232 represents a critical use-after-free flaw within the gnutls library implementation, specifically within the client_send_params function located in lib/ext/pre_shared_key.c. This issue arises from improper memory management during the processing of pre-shared key extensions in the tls protocol implementation, creating a scenario where freed memory regions may be accessed or modified by subsequent operations. The flaw manifests during the client-side tls handshake process when handling pre-shared key extensions, making it particularly dangerous in environments where gnutls is used for secure communications.
The technical exploitation of this vulnerability occurs when the gnutls library processes client hello messages containing pre-shared key extensions, leading to a situation where memory allocated for cryptographic parameters is freed but subsequently accessed by the client_send_params function. This memory corruption vulnerability can be triggered through maliciously crafted tls handshake messages that manipulate the pre-shared key extension parameters. The use-after-free condition creates potential for arbitrary code execution, data corruption, or denial of service scenarios, as the freed memory may contain sensitive cryptographic data or control structures that are accessed after deallocation. According to CWE-416, this vulnerability maps directly to use-after-free conditions that occur when memory is freed but not properly nullified, allowing subsequent access to freed memory regions.
The operational impact of CVE-2021-20232 extends across numerous systems and applications that rely on gnutls for secure communications, including web servers, email clients, and network applications implementing tls protocols. Attackers can exploit this vulnerability by crafting malicious tls handshake messages that cause the client to trigger the use-after-free condition during pre-shared key processing. The attack vector typically involves network-based exploitation where a remote adversary can send specially crafted tls handshake packets to a vulnerable gnutls implementation. This vulnerability directly impacts the integrity and availability of secure communications, potentially allowing attackers to execute arbitrary code on affected systems or cause service disruption through memory corruption. The vulnerability aligns with attack techniques described in the mitre attack framework under initial access and execution phases, particularly targeting the tls protocol implementation as a means to gain system access.
Mitigation strategies for CVE-2021-20232 primarily involve immediate patching of gnutls library installations to versions that address the memory management issues in the pre-shared key extension handling. System administrators should prioritize updating gnutls to versions that contain fixes for the use-after-free condition in client_send_params, typically released as part of security updates from upstream maintainers or distribution vendors. Additionally, network administrators should consider implementing network segmentation and monitoring to detect potential exploitation attempts targeting this vulnerability. Organizations using gnutls in production environments should conduct thorough vulnerability assessments to identify systems running vulnerable versions and implement proper patch management procedures. The fix typically involves proper memory management practices including nullifying pointers after freeing memory, implementing proper reference counting, or using safer memory allocation patterns that prevent access to freed memory regions. Organizations should also consider implementing intrusion detection systems to monitor for tls handshake anomalies that may indicate exploitation attempts.