CVE-2020-9399 in Avast
Summary
by MITRE
The Avast AV parsing engine allows virus-detection bypass via a crafted ZIP archive. This affects versions before 12 definitions 200114-0 of Antivirus Pro, Antivirus Pro Plus, and Antivirus for Linux.
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Analysis
by VulDB Data Team • 04/07/2024
The vulnerability identified as CVE-2020-9399 represents a critical flaw in the Avast antivirus parsing engine that enables adversaries to bypass virus detection mechanisms through specifically crafted malicious ZIP archives. This vulnerability specifically impacts Avast Antivirus Pro, Antivirus Pro Plus, and Antivirus for Linux versions prior to the 12 definitions update dated 200114-0, creating a significant security gap that could allow malware to evade protection. The issue stems from improper handling of ZIP archive structures within the antivirus engine's parsing logic, where malicious content can be concealed within archive files that appear benign to the detection system. This weakness directly relates to CWE-129, which addresses improper validation of array indices and buffer overflows, as the parsing engine fails to properly validate the structure and contents of ZIP archives before processing them. The vulnerability creates a pathway for attackers to deliver malware payloads that would otherwise be detected by standard antivirus scanning procedures.
The technical implementation of this vulnerability involves the manipulation of ZIP archive metadata and file structures that the Avast parsing engine does not properly validate. When the antivirus software encounters a crafted ZIP file, its parsing engine fails to recognize the malicious content embedded within the archive's structure, allowing the malware to bypass detection mechanisms. This behavior aligns with ATT&CK technique T1059.007, which covers the use of script-based execution through zip files, and demonstrates how adversaries can leverage archive manipulation to evade security controls. The flaw operates at the file format parsing layer where the antivirus engine should validate the integrity and structure of compressed files before analyzing their contents, but instead accepts malformed or specially constructed ZIP archives as legitimate.
The operational impact of CVE-2020-9399 extends beyond simple detection bypass, as it represents a fundamental weakness in the antivirus engine's ability to properly analyze compressed file formats. Organizations using affected Avast versions face increased risk of malware infiltration, as attackers can now craft ZIP archives containing malicious code that will not be flagged by the antivirus solution. This vulnerability particularly affects enterprise environments where ZIP files are commonly used for software distribution, file transfers, and legitimate business processes, creating multiple attack vectors for threat actors. The impact is further amplified by the fact that ZIP files are widely used across different operating systems and applications, making the attack surface for exploitation broad and accessible. Security teams must consider that this vulnerability could enable more sophisticated attack campaigns where malware delivery is concealed within seemingly harmless compressed files.
Mitigation strategies for CVE-2020-9399 focus primarily on updating to the patched versions of Avast antivirus software that include the 12 definitions update 200114-0 or later. Organizations should prioritize immediate deployment of these updates across all affected systems to eliminate the vulnerability. Additionally, network administrators should implement enhanced monitoring of ZIP file transfers and consider implementing additional security controls such as file type restrictions, content scanning, and behavioral analysis to detect anomalous ZIP file usage. The remediation process should include verification that all endpoints have received the update and that the antivirus definitions are current. Organizations should also consider implementing network segmentation and application whitelisting policies to limit the potential impact of any successful exploitation attempts, while maintaining compliance with industry standards such as those outlined in the NIST Cybersecurity Framework and ISO 27001 security controls. Security teams should conduct thorough testing of the updated antivirus definitions to ensure they properly detect the previously bypassed malware variants without introducing false positives that could disrupt legitimate business operations.