CVE-2020-27801 in UPX
Summary
by MITRE • 08/26/2022
A heap-based buffer over-read was discovered in the get_le64 function in bele.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 CVE-2020-27801 represents a critical heap-based buffer over-read flaw in the UPX 4.0.0 compression utility that affects Mach-O binary files. This issue resides within the get_le64 function located in the bele.h header file, which is responsible for handling byte order conversions during the decompression process. The vulnerability manifests when UPX processes malformed Mach-O files that contain crafted data structures designed to trigger the buffer over-read condition. This flaw falls under the Common Weakness Enumeration category CWE-125, which specifically addresses out-of-bounds read vulnerabilities that occur when a program reads data past the end of a valid buffer. The attack vector involves an adversary constructing a malicious Mach-O file that exploits the improper bounds checking in the get_le64 function, potentially allowing for information disclosure or system instability. When the vulnerable UPX version attempts to decompress such a crafted file, the function reads beyond allocated memory boundaries, creating opportunities for attackers to extract sensitive information from adjacent memory regions or potentially cause denial of service conditions.
The technical implementation of this vulnerability demonstrates a classic buffer over-read scenario where the get_le64 function fails to properly validate input parameters before performing memory access operations. The function likely assumes a minimum size for incoming data structures without verifying that sufficient data exists in the buffer before attempting to read 64-bit values. This particular flaw affects the Mach-O file format processing within UPX, which is commonly used on macOS and iOS platforms. The vulnerability is particularly concerning because UPX is widely used for binary compression and packaging, making it a potential attack surface for adversaries who might leverage this weakness to gain unauthorized access to system information or disrupt normal operations. The heap-based nature of the over-read indicates that the memory corruption occurs in dynamically allocated memory regions, which can be particularly challenging to detect and exploit reliably. From an operational perspective, this vulnerability creates a significant risk for systems that process untrusted Mach-O binaries through UPX, as the attack requires only the delivery of a specially crafted file rather than complex exploitation techniques.
The impact of CVE-2020-27801 extends beyond simple information disclosure, as it represents a potential pathway for more sophisticated attacks within the broader ATT&CK framework. The vulnerability could enable adversaries to perform reconnaissance activities by extracting memory contents that might contain sensitive data, cryptographic keys, or system information. When considering the ATT&CK matrix, this flaw aligns with techniques such as T1059.001 (Command and Scripting Interpreter: PowerShell) and T1068 (Exploitation for Privilege Escalation) through potential memory corruption that could be leveraged in subsequent attack phases. Organizations using UPX for software distribution, packaging, or deployment are at risk of exposure, particularly those that do not properly validate or sanitize input files before processing them through the compression utility. The vulnerability also intersects with T1553 (Subvert Trust Controls) as it exploits the trust placed in legitimate compression tools to execute malicious code or extract sensitive information. The attack surface is broadened by the fact that UPX is frequently used in software supply chain operations, making it a potential target for attackers seeking to compromise legitimate software distribution channels.
Mitigation strategies for CVE-2020-27801 should prioritize immediate patching of UPX installations to version 4.0.1 or later, which contains the necessary fixes for the buffer over-read condition. Security administrators should implement strict file validation procedures for Mach-O binaries processed through UPX, including signature verification and content scanning before compression operations. Network security controls should be configured to monitor for suspicious file transfers or decompression activities that might indicate exploitation attempts. Organizations should also consider implementing sandboxing mechanisms when processing untrusted Mach-O files to isolate potential exploitation attempts from core systems. The fix implemented in subsequent UPX versions addresses the root cause by adding proper bounds checking to the get_le64 function and ensuring that all memory access operations are validated against available buffer sizes. Additionally, security teams should establish monitoring protocols to detect unusual decompression patterns or memory access behaviors that might indicate exploitation attempts. Regular security assessments of software supply chain tools and compression utilities should be conducted to identify similar vulnerabilities that might exist in other third-party components. The vulnerability serves as a reminder of the critical importance of proper input validation and bounds checking in security-critical code, particularly in tools that process untrusted binary data from diverse sources.