In the not-so-distant past, malware authors, mirroring their software development counterparts, strived for compact code, aiming to keep their executables as small as possible. However, contemporary computer environments, characterized by abundant storage, bandwidth, and processing power, have witnessed a paradigm shift. This article explores the emergence of massive malware binaries on macOS, their prevalence, the reasons behind their adoption, and the challenges they pose for detection and analysis.
I. The Evolution of Malware Size: A Historical Perspective
In the bygone era of small disk drives, slow network connections, and underpowered chips, compact malware binaries had a tactical advantage. Their inconspicuous nature facilitated easy concealment within other files, attachments, or even images. Small file sizes were not just a matter of stealth; they also facilitated quicker transfers over the network and imposed less strain on host CPUs during execution.
II. The Contemporary Landscape: The Era of Oversized Binaries
In today’s computing landscape, where storage, bandwidth, and processor power are abundant resources, both legitimate programs and malware have undergone significant size inflation. Malicious executables, once measured in kilobytes, have now ballooned to several megabytes. This section explores the phenomenon of oversized binaries, delving into their prevalence and the unique challenges they present.
A. The Shift in Malware Binaries: From Compact to Colossal
While malware binaries exceeding several megabytes are now commonplace, recent malicious programs have pushed the boundaries of size to new extremes. macOS, in particular, has witnessed the proliferation of binaries surpassing 50MB, with some even crossing the 100MB threshold, notably in campaigns involving cryptominers. The implications of such colossal file sizes extend beyond mere detection challenges.
B. Magnitude of the Issue: A Glimpse into Public Repositories
To gauge the prevalence of large malicious binaries, a dive into public malware repositories like VirusTotal provides insights. Filtering for Mach-O binaries recognized as malware by multiple vendors and exceeding 35MB yields hundreds of hits. Notably, the search unveils an increasing number of samples, particularly those associated with Atomic Stealer, weighing in at 50MB or more.
III. Unpacking the Reasons Behind Supersized Binaries
Understanding why threat actors opt for oversized binaries is crucial in navigating this evolving landscape. Several factors contribute to the adoption of larger file sizes, each revealing unique insights into the tactics and motivations of malware authors.
A. Cryptominers and Emulation Environments
Certain large binaries, such as those associated with cryptominers like BirdMiner (LoudMiner), result from bundling emulation environments, such as QEMU, within the malware. This approach introduces a layer of complexity and evasion, making detection and analysis more challenging.
B. Cross-Platform Programming Languages
The use of cross-platform programming languages like Go and Rust plays a pivotal role in the size inflation of binaries. To ensure compatibility across platforms, these languages compile runtime, libraries, and dependencies into the final payload, contributing to the overall file size.
C. Apple’s Architecture Transition: ARM vs. Intel
The transition from Intel to ARM architecture by Apple has reintroduced the Universal/FAT binary format. This format incorporates two architectures into a single binary, ensuring compatibility with both Intel and Apple silicon Macs. Consequently, binaries compiled in this format effectively double in size.
D. Bloating for Evasion and Anti-Analysis
Some threat actors deliberately inflate file sizes with junk code to circumvent file size limits imposed by scanners and malware repositories like VirusTotal. This tactic highlights a strategic move by adversaries to impede detection and analysis efforts.
IV. Detection Challenges and Analysis Implications
The surge in massive binaries poses significant challenges for traditional antivirus (AV) scanners and malware analysts. This section explores the issues associated with detecting and analyzing these outsized binaries, shedding light on the performance implications and hurdles faced by security professionals.
A. Scanning Woes: AV Performance and File Size Limits
Massive binaries strain traditional AV scanners that rely on hash computation or pre-execution content scanning. The larger the binary, the more time-consuming the scanning process becomes. This leads to performance degradation and system sluggishness, especially when scanning across numerous files.
B. File Size Limits: A Double-Edged Sword
To address scanning performance issues, many AV scanners impose file size limits. However, this approach, rooted in an era where few legitimate programs surpassed 20MB, is rendered obsolete by today’s bloated binaries. Threat actors, cognizant of these limitations, exploit file size restrictions to ensure their malware goes undetected.
C. Analysis Hurdles: Navigating Tens of Megabytes
Massive binaries not only challenge detection software but also impede the efforts of researchers, reverse engineers, and malware analysts. Analyzing tens of megabytes of code, a substantial portion of which may be benign or part of standard runtimes, becomes a daunting task. This hampers the identification of malicious components and increases the risk of undetected malware using similar code.
V. Adaptive Solutions: Navigating the Era of Massive Binaries
Amidst the challenges posed by massive binaries, defenders and analysts need adaptive strategies to effectively deal with this evolving threat landscape. This section explores solutions and techniques that can enhance detection capabilities and streamline the analysis of oversized macOS malware binaries.
A. Behavioral Detection: A Paradigm Shift
Traditional file scanning limitations necessitate a paradigm shift in detection mechanisms. Behavioral detection, which observes a binary’s actions during execution, emerges as a viable solution. Solutions like SentinelOne, incorporating behavioral engines, can detect and neutralize malware, irrespective of file size or packaging.
B. Beyond File Scanning: Multifaceted Detection Approaches
Modern security software integrates multiple detection mechanisms, combining behavioral and machine learning engines. This multifaceted approach addresses the limitations of traditional file scanning, ensuring comprehensive protection in the face of evolving malware tactics.
C. Triage Techniques for Analysts: Leveraging YARA and Radare2
For analysts faced with the challenge of triaging massive macOS malware samples, leveraging tools like YARA and radare2 proves invaluable. YARA rulesets can be crafted to identify indicators of compromise (IoCs) and tactics, techniques, and procedures (TTPs) associated with malware behavior. Running YARA within a radare2 session enables efficient analysis and identification of suspicious code segments.
VI. Conclusion: Adapting to the New Normal
As macOS malware binaries continue to swell in size, organizations, security professionals, and analysts must adapt to this new normal. The tactical shift from compact to colossal binaries requires a proactive approach to detection and analysis. Behavioral detection, multifaceted security solutions, and advanced analysis techniques empower defenders to navigate the challenges posed by massive macOS malware binaries effectively.
In conclusion, the era of oversized binaries demands strategic vigilance, collaborative industry efforts, and continuous adaptation to stay ahead in the cat-and-mouse game between defenders and threat actors.
