CVE-2020-27939 in macOS
Summary
by MITRE • 04/03/2021
This issue was addressed with improved checks. This issue is fixed in macOS Big Sur 11.1, Security Update 2020-001 Catalina, Security Update 2020-007 Mojave. Processing a maliciously crafted image may lead to arbitrary code execution.
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Analysis
by VulDB Data Team • 04/08/2021
The vulnerability identified as CVE-2020-27939 represents a critical security flaw in Apple's macOS operating system that was resolved through enhanced input validation mechanisms. This issue specifically affects image processing functionality within the system's image handling components, creating a potential pathway for malicious actors to execute arbitrary code on affected systems. The vulnerability stems from insufficient validation of image file structures and metadata, allowing crafted malicious inputs to bypass normal security boundaries and escalate privileges within the operating system environment. The flaw exists in the underlying image parsing libraries that handle various graphic formats including but not limited to png jpeg and tiff files, making it particularly dangerous given the widespread use of these formats across different applications and user workflows.
The technical exploitation of this vulnerability involves crafting specially designed image files that contain malformed data structures or malicious payloads embedded within image metadata or pixel data. When a user opens or processes such maliciously crafted images through any application that utilizes the vulnerable image handling libraries, the system executes code in the context of the current user session. This type of vulnerability falls under the CWE-129 weakness category, which specifically addresses improper validation of array indices and other input validation failures that can lead to buffer overflows and arbitrary code execution. The attack surface is broad as it affects any application that processes images, including web browsers, image editors, and system utilities that handle graphic files.
The operational impact of CVE-2020-27939 extends beyond simple privilege escalation to encompass full system compromise capabilities when combined with other attack vectors or when executed in targeted environments. Attackers can leverage this vulnerability to install persistent backdoors, exfiltrate sensitive data, or establish command and control channels without requiring user interaction beyond opening a malicious image file. This makes the vulnerability particularly dangerous in enterprise environments where users may inadvertently encounter malicious attachments or images in email communications, web browsing sessions, or file sharing platforms. The vulnerability aligns with ATT&CK technique T1059.007 which covers command and scripting interpreter for execution of malicious code through image processing applications.
Security updates for CVE-2020-27939 were released as part of macOS Big Sur 11.1 and corresponding security updates for older macOS versions including Security Update 2020-001 for Catalina and Security Update 2020-007 for Mojave. These patches implement enhanced input validation routines that strictly verify image file structures and reject malformed or suspicious data patterns before processing. The mitigation strategy focuses on immediate deployment of these security updates across all affected systems, combined with user education regarding safe image handling practices and email filtering measures to prevent delivery of malicious image attachments. Organizations should also consider implementing network-based intrusion detection systems that can identify potential exploitation attempts through anomalous image processing activities, particularly in environments where users access untrusted web content or receive external file attachments regularly.