CVE-2020-27799 in UPX
Summary
by MITRE • 08/26/2022
A heap-based buffer over-read was discovered in the acc_ua_get_be32 function in miniacc.h in UPX 4.0.0 via a crafted Mach-O file.
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Analysis
by VulDB Data Team • 04/11/2025
The vulnerability identified as CVE-2020-27799 represents a critical heap-based buffer over-read flaw within the UPX 4.0.0 compression utility, specifically affecting the acc_ua_get_be32 function located in the miniacc.h header file. This issue manifests when processing specially crafted Mach-O binary files, which are commonly used on macOS and iOS operating systems. The vulnerability stems from inadequate input validation and boundary checking within the decompression routines that handle big-endian 32-bit integer extraction operations. When the malicious file triggers the acc_ua_get_be32 function, the code attempts to read beyond the allocated heap memory boundaries, potentially exposing sensitive data or causing application instability. The flaw falls under the CWE-125 vulnerability category, which specifically addresses out-of-bounds read conditions that can lead to information disclosure, system crashes, or potentially remote code execution depending on the execution context. This vulnerability directly impacts the integrity of the UPX compression utility's decompression process and represents a significant security risk for users who process untrusted binary files. The issue is particularly concerning in environments where UPX is used for software distribution or packaging, as attackers could exploit this weakness to craft malicious Mach-O files that trigger the buffer over-read during normal decompression operations. The vulnerability demonstrates a classic weakness in memory management where the decompression logic fails to properly validate the size and structure of input data before performing memory access operations.
The operational impact of CVE-2020-27799 extends beyond simple application instability, as it creates potential attack vectors for information disclosure and system compromise. When a user or automated system processes a malicious Mach-O file through UPX 4.0.0, the buffer over-read can expose adjacent memory contents that may contain sensitive information such as cryptographic keys, user credentials, or application state data. The heap-based nature of the vulnerability means that the over-read occurs within dynamically allocated memory regions, potentially allowing attackers to extract structured data from memory layouts that are not directly controlled by the application. This type of vulnerability aligns with ATT&CK technique T1059.007 for execution through Mach-O files, and could enable further exploitation pathways when combined with other vulnerabilities in the system. The exploitation requires a crafted Mach-O file that specifically triggers the acc_ua_get_be32 function with malformed data structures, making it a targeted attack vector rather than a general system vulnerability. Organizations using UPX for software packaging, distribution, or code obfuscation are particularly at risk, as they may unknowingly distribute malicious payloads that exploit this vulnerability during legitimate decompression processes.
Mitigation strategies for CVE-2020-27799 should prioritize immediate software updates to versions of UPX that have patched this vulnerability, as the official fix addresses the boundary checking issues within the acc_ua_get_be32 function. System administrators should implement strict file validation procedures for all binary inputs processed through UPX, including signature verification and sandboxed decompression environments. The vulnerability can be addressed through input sanitization techniques that validate Mach-O file structures before processing, implementing proper bounds checking, and using memory-safe programming practices that prevent heap-based over-read conditions. Organizations should also consider deploying network monitoring solutions to detect unusual UPX usage patterns that might indicate exploitation attempts, as well as implementing application whitelisting policies that restrict execution of UPX in sensitive environments. Additional defensive measures include regular security assessments of decompression workflows, deployment of intrusion detection systems that monitor for buffer over-read patterns, and maintaining updated threat intelligence feeds that track exploitation attempts targeting this specific vulnerability. The fix typically involves implementing proper validation of the input data size against expected buffer boundaries, ensuring that all memory access operations within the acc_ua_get_be32 function are properly bounded and validated before execution, thereby preventing unauthorized memory access that could lead to information disclosure or system compromise.