Introduction

In this article, we’ll examine the security risks posed by ZIP file uploads, explore real-world attack techniques like ZIP bombs and double-extension spoofing, and outline how to detect and mitigate hidden threats with deep content verification using the Cloudmersive Virus Scan API.

The Compressed Convenience

ZIP files are extremely popular for bundling and sharing content. They reduce file sizes and preserve folder structures, facilitating the transportation of numerous files in a single upload package. From sharing resume and image sets to storing legal archives and app bundles, utilizing ZIP uploads is standard practice for countless platforms.

The convenience ZIP offers with compression has a darker side, however – and this makes them an extremely attractive attack vector. ZIP uploads are like any sealed box; if you simply pass them along without thoroughly inspecting their contents, you’re accepting a very high degree of risk.

Why Attackers Love ZIP Uploads

Simply put: ZIP files are a nightmare from a security standpoint, and a dream for attackers.

Not only are ZIP files excellent at hiding content via mass compression, but they can carry anything – including scripts, executables, and seemingly legitimate documents (or other archives) with their own nested subfile trees. They’re easy to manipulate, too; attackers can spoof ZIP filenames, use misleading folder paths within them, or insert deeply nested structures to avoid threat detection.

A threat actor could, for example, upload an innocuous ZIP file labeled “Client_Contracts.zip” through an application portal, and that archive could contain anything from a DOCX laced with malicious OLE objects, an EXE file with a misleading name or spoofed extension, or a chain of files designed to work together in executing a payload once unpacked.

Each individual file within a malicious ZIP archive requires its own undivided attention to mitigate disaster. Threat actors are counting on weakly configured security policies to admit their payload without thorough inspection.

Real Incidents, Real Damage

One infamous ZIP attack tactic involves hiding a malicious executable inside a ZIP archive with a double extension (invoice.pdf.exe) and a PDF icon for good measure. When the “PDF” is extracted and launched, the user believes they’re opening a harmless document — when they’re actually triggering malware execution.

Another common attack method involves the use of ZIP bombs – archives designed to decompress into massive volumes of data and crash the system (or weakly configured scanning engine) they were opened in. This type of ZIP-based attack can crash systems and serve as a smokescreen for follow-up attacks.

In many high-profile network perimeter breaches, the attacker’s initial system access originated from a ZIP file uploaded to an application with insufficient inspection protocols.

The Limits of Standard File Upload Filters

Common approaches to upload validation tend to fall short when it comes to investigating ZIP files. File extension checks, MIME type validation, and basic signature-based antivirus (AV) scans aren’t sufficient for uprooting dozens of potentially malicious files or identifying a ZIP bomb.

A traditional scanner might inspect the ZIP archive’s outer shell and stop there — completely ignoring what’s inside, how deep it goes, or whether the structure itself is dangerous.

Unpacking ZIP Archives with the Cloudmersive Advanced Virus Scan API

Cloudmersive’s Advanced Virus Scan API doesn’t treat ZIP files as static, independent documents. It treats them like dynamic collections of files, each requiring independent content verification.

Deep verification of ZIP files includes recursively unpacking archive contents, no matter how deeply nested they are. It also involves scanning each individual file inside the archive, using a combination of signature-based and behavioral detection techniques.

Overloaded archives are analyzed in-depth to prevent decompression-based denial-of-service attacks, and malicious subfiles are identified based on their content types and behaviors (this roots out disguised .exe, .js, .scr, .py, and other high risk spoofed content) rather than the extension or icon they present.

The Advanced Scan API doesn’t just look — it understands what’s inside.

Bottom Line: Open Carefully

ZIP files are too common — and too dangerous — to trust without proper inspection. Letting users upload compressed archives without scanning them is like accepting a sealed package from a stranger and tossing it straight into your server room.

With Cloudmersive’s Advanced Virus Scan API, you gain the ability to open those packages safely — inspect everything, reject what’s harmful, and protect your users and infrastructure without slowing things down.

If you're interested in learning more about file upload protection with the Advanced Virus Scan API, please feel free to contact a member of our team.