What is a Zip Bomb attack and how to prevent it?

Overview

A zip bomb is a malicious file characterized by its compressed size being significantly smaller than its uncompressed size , which can consume substantial disk space or memory , causing a target system to crash or become unresponsive. There are various types of zip bombs, including recursive zip bombs, nested zip bombs, and infinite zip bombs. This article will explain the principles, examples, risks, and prevention methods associated with zip bombs.

Contents 

What is a Zip Bomb?

Types of Zip Bombs

How Does a Zip Bomb Work?

An Example of a Zip Bomb

Is a Zip Bomb Dangerous?

How to Prevent Zip Bombs?

What is a Zip Bomb?

A zip bomb is a malicious file that utilizes the characteristics of compression algorithms to create highly asymmetric file size ratios when compressed and uncompressed. Compression algorithms typically reduce file size by identifying and eliminating duplicate content in files. The Zip bomb took advantage of this behavior by creating a file containing a large amount of duplicate content, which was then compressed using advanced compression algorithms, resulting in a compressed file size much smaller than the original file. For example, a file containing 1 billion zero bytes can be compressed into dozens of kilobytes.

Types of Zip Bombs

Zip bombs come in different types based on their construction and attack methods. They can be categorized as follows:

① Recursive Zip Bomb

These zip bombs consist of multiple layers of compressed files, with each compressed file containing additional compressed files, and so on. When the target system attempts to decompress this zip bomb, it continually generates more uncompressed files, eventually depleting disk space or memory.

② Nested Zip Bomb

These zip bombs comprise one or more compressed files, with each compressed file containing other types of compressed files (e.g., .rar, .7z, .gz). When the target system attempts to decompress this zip bomb, it encounters different compression formats, necessitating various decompression tools, thereby increasing decompression time and complexity.

③ Infinite Zip Bomb

These zip bombs are constructed using a specially crafted compressed file that exploits vulnerabilities or features of certain decompression software. This causes the uncompressed file size to grow infinitely when extracted. For example, one infinite zip bomb consists of a 42KB compressed file that, when extracted using specific versions of WinZip, generates an uncompressed file of approximately 4.5PB (around 4500TB) in size.

How Does a Zip Bomb Work?

The operation of a zip bomb is relatively straightforward, relying on the target system's trust and handling of compressed files. Typically, when receiving an attachment or downloading a file, users first consider its size and format. If the file is small and in a common format (e.g., .zip), users tend to assume it is safe and legitimate, proceeding to open or extract it. However, if this seemingly innocuous .zip file is, in fact, a zip bomb, it will unleash its concealed enormous uncompressed files, occupying disk space or memory and causing the system to become sluggish or unresponsive. If users do not promptly identify and halt this process, the system may eventually crash or sustain damage.

An Example of a Zip Bomb

One well-known example of a zip bomb is "42.zip," a compressed file with a size of only 42KB. However, upon extraction, it generates an uncompressed file size of 4.5PB. This zip bomb was created by Philippe Teuwen in 2001 with the goal of demonstrating the limits of compression algorithms and the potential danger of compressed files. He humorously named this zip bomb "42.zip," referencing a meme from "The Hitchhiker's Guide to the Galaxy" where "42" is considered the ultimate answer to the universe. ".zip" denotes the common compressed file format.

Is a Zip Bomb Dangerous?

While zip bombs may appear intriguing, they are genuinely dangerous malicious files that can be employed for attacks on target systems or networks, resulting in severe consequences. For example:

① Disk Space Consumption

Zip bombs can be disseminated as email attachments or web download links, deceiving users into opening or extracting them, thereby consuming users' disk space or memory and disrupting normal work or leisure activities.

② Network Attacks

Zip bombs can serve as tools for network attacks by sending numerous zip bombs to target servers or websites. This forces them to continually decompress these zip bombs, depleting their resources and bandwidth, rendering them incapable of providing regular services or functions.

③ Destructive Weapons

Zip bombs can be utilized as destructive weapons by implanting them into target systems. They can be triggered or scheduled to decompress, thereby damaging the target system's data or files, causing irreparable losses.

How to Prevent Zip Bombs?

To safeguard against zip bomb threats, it is essential to adopt effective measures to prevent and mitigate their potential harm. Here are some common and straightforward methods:

① Avoid Suspicious Files

Refrain from casually opening or extracting compressed files of uncertain or suspicious origins, especially those that are small but claim to contain substantial content.

② Use Security Software

Employ reliable and up-to-date antivirus software and firewalls to scan and intercept files or network traffic that may contain zip bombs.

③ Regular Data Backup and Cleanup

Periodically back up and clean your system and data to guard against potential zip bomb attacks or damage.

④ Use MetroVPN

Utilize MetroVPN to protect your online privacy and security, shielding you from monitoring or tracking. MetroVPN encrypts your network traffic, preventing data theft or leakage. It conceals your real IP address and location, safeguarding against tracking or tracing. Additionally, MetroVPN can bypass any network censorship or blocks, ensuring your unrestricted access to various internet content.