CVE-2021-23979 in Firefox
Summary
by MITRE • 02/26/2021
Mozilla developers reported memory safety bugs present in Firefox 85. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 86.
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Analysis
by VulDB Data Team • 08/17/2025
The vulnerability identified as CVE-2021-23979 represents a critical memory safety issue within Mozilla Firefox browser versions prior to 86. This flaw stems from multiple memory corruption bugs that were discovered during routine security auditing of the browser's codebase. The presence of memory safety issues in Firefox 85 indicates a fundamental weakness in how the browser handles memory allocation and deallocation processes, creating potential attack vectors for malicious actors. These memory corruption vulnerabilities are particularly dangerous because they can lead to arbitrary code execution when successfully exploited, making them high-value targets for cybercriminals and nation-state actors seeking to compromise user systems.
The technical nature of this vulnerability falls under the category of memory safety flaws that are commonly classified as CWE-119 (Improper Access to Memory) and CWE-121 (Stack-based Buffer Overflow) within the Common Weakness Enumeration framework. These bugs typically manifest when the browser fails to properly validate memory boundaries during operations such as array indexing, string manipulation, or object allocation. The vulnerability affects Firefox versions less than 86, indicating that Mozilla's security team identified these issues during the development cycle and implemented fixes in the subsequent release. The memory corruption patterns suggest that attackers could potentially manipulate memory addresses or overwrite critical data structures to gain control over the browser process.
From an operational perspective, this vulnerability poses significant risks to end users who may unknowingly encounter malicious content while browsing the web. The ability to execute arbitrary code through memory corruption means that attackers could potentially install malware, steal sensitive data, or establish persistent access to compromised systems. The attack surface is broad since web browsers are frequently targeted due to their privileged position in user environments and the variety of content they process. Security analysts have noted that these types of memory safety bugs are particularly challenging to exploit reliably, but when successful, they can provide attackers with complete control over the affected browser and potentially the underlying operating system. The vulnerability's impact extends beyond individual users to enterprise environments where browser-based attacks are commonly used in phishing campaigns and supply chain attacks.
Organizations and individuals should prioritize immediate remediation by upgrading to Firefox version 86 or later, which contains the necessary patches to address these memory safety issues. System administrators should implement automated update mechanisms to ensure all browser installations remain current with security patches. Additional mitigations include deploying web application firewalls, implementing strict content security policies, and utilizing sandboxing technologies to limit the potential impact of successful exploitation attempts. The vulnerability demonstrates the critical importance of continuous security auditing and timely patch management in maintaining secure computing environments. According to ATT&CK framework, this vulnerability could be leveraged during the initial access phase of an attack, potentially enabling adversaries to establish a foothold through malicious web content or drive-by downloads, making it a significant concern for cybersecurity professionals responsible for defending against browser-based threats.