What is a Zip Bomb (Decompression Bomb)? How Can You Protect Against It?

What is a zip bomb?

When downloading files from either emails or websites, it's critical to be aware of the risks. After all, this is how many viruses, trojans, and other types of cyberattacks are deployed, masking themselves as harmless files that are unknowingly downloaded. However, viruses and trojans are only the tip of the iceberg, and there are plenty of other threats out there that organizations need to protect against.

One such threat is that of zip bombs, a type of malicious archive file that exploits popular ZIP compression techniques, potentially causing significant damage to targeted systems. But what is a zip bomb, and how can your organization protect against them? To provide clarity to these questions, this article will explore zip bomb attacks, their potential impact, and practical strategies for protection. Read on to learn more.

Understanding zip bombs (decompression bombs)

A zip bomb, otherwise known as a decompression bomb or "zip of death," is a type of malicious file that leverages compression algorithms to create files significantly smaller than their decompressed counterparts. Its purpose is to deceive a system or application into allocating excessive resources during decompression, leading to system instability, crashes, or denial of service.

Zip bombs exploit the compression techniques employed in the most widely used file archiving formats, such as ZIP or RAR. These formats use algorithms like DEFLATE, which remove redundancy and reduce file size; however, when compression is applied recursively, with files containing other compressed files in a nested manner, the resulting archive can become incredibly large. Below, we detail exactly how zip bombs work and how to understand their potential impact on your organization.

How zip bombs work

Zip bombs rely on what is known as recursive compression, where files are compressed multiple times within an archive—exploiting existing compression algorithms used in ZIP applications. Each iteration compounds the compression, leading to exponential growth in the compressed size. For example, a file may start with a small size of just a few kilobytes, but after several rounds of compression, it can expand to hundreds of gigabytes or even terabytes.

To further increase the impact, zip bombs employ infinite decompression loops that trick decompression tools by creating structures that never end, consuming excessive system resources and causing the decompression process to continue indefinitely. This means that when a decompression tool encounters such a loop in a compressed file, it continuously attempts to decompress the contents, leading to an infinite cycle of extraction attempts.

Potential impact and risks of zip bomb attacks

Any zip bomb attack will negatively affect an organization's network and applications. However, the type and extent of the damage will depend on the uncompressed size of the eventual file, as well as whether the zip bomb has been "weaponized". Typical risks and impacts include:

System Resource Exhaustion

1. CPU and Memory Overload — When a system attempts to decompress a zip bomb, it needs to allocate significant CPU and memory resources to handle the decompression process. The excessive resource consumption can overwhelm the system, resulting in freezing, slowdowns, or even crashes.
2. Denial of Service (DoS) — Zip bombs can be weaponized to launch denial-of-service attacks. This means it will target a network or server infrastructure with massive archives, consuming available bandwidth or overwhelming processing capabilities, leading to service unavailability for legitimate users.

Disruption of Operations

1. Downtime and Loss of Productivity — Recovering from a zip bomb attack can be time-consuming and resource-intensive, and systems affected by zip bombs may require extensive analysis, clean-up, and restoration—resulting in significant downtime and loss of productivity for individuals or organizations.
2. Data Loss and Corruption — During the decompression process, zip bombs can pose a risk to files and databases. The sheer size of the decompressed files can overload storage capacity or corrupt existing data, leading to potential data loss or irreparable damage to critical information.

Protecting against zip bomb attacks

To mitigate the risks posed by zip bomb attacks, it is crucial to implement robust security measures focusing on proactive prevention and detection. This can be achieved by introducing the following elements into a broader cybersecurity program:

Implement robust security measures against zip bombs

Employing robust antivirus and anti-malware solutions is essential to detect and prevent the execution of malicious files, including zip bombs. Regularly scanning files and archives for known malware signatures helps identify potential threats before they can cause harm, allowing users to recognize an attack before downloading the associated files.

In addition, signature-based scanning should be implemented to compare files against a database of known malicious signatures. This technique allows users and admins to identify recognized zip bombs and prevent their execution.

Limit decompression resources

Decompression tools often provide options to limit resource allocation during the decompression process, and these should be implemented across the board. Configuring these tools to enforce resource limits, such as restricting CPU usage or memory allocation, can help prevent excessive resource consumption by zip bombs.

Further risk mitigation can be achieved by restricting the maximum allowed file size for decompression attempts can help prevent the extraction of exceptionally large and potentially malicious archives. By implementing file size restrictions, systems can block or alert users when they encounter files that exceed predefined limits.

User education and awareness to idenfity zip bombs

As with any cyberattack, user education and awareness are critical elements of risk mitigation. Educating users about the characteristics of suspicious files, such as unusually small sizes or unexpected file extensions, can help them identify potential zip bombs. And since most cyberattacks are deployed thanks to user error, education, and awareness provide significant risk mitigation.

Encouraging users to exercise caution when handling files from unknown or untrusted sources is crucial to avoid inadvertently executing malicious archives. However, promoting safe file handling practices, such as verifying file sources, scanning attachments before opening, and using strong password protection for sensitive archives, will help your organization reduce the risk of zip bomb attacks.

Future mitigation strategies

As cyber threats evolve, mitigation strategies must continually adapt, incorporating advanced technologies and collaborative defense mechanisms. This means staying up to date with the latest developments in cybersecurity, paying attention to emergent threats, and introducing automated AI and machine learning technologies. Here, we look a little deeper into these strategies.

Advanced zip bomb threat detection

1. Behavior-Based Detection — Analyzing the behavior of file decompression processes can aid in detecting potential zip bombs. Unusual patterns, such as excessive resource consumption or infinite loops, can trigger alerts or preventive actions, enabling systems to identify and mitigate zip bomb attacks.
2. Machine Learning and AI — Leveraging machine learning and artificial intelligence techniques can enhance threat detection and classification capabilities. Training models on large datasets of known zip bombs and non-malicious archives can improve the accuracy and efficiency of identifying emerging threats.

Collaborative Defense Mechanisms

1. Sharing Threat Intelligence— Establishing platforms and channels for sharing threat intelligence among security professionals, organizations, and communities is crucial. Timely dissemination of information about new zip bombs, attack vectors, and mitigation strategies enables a collective defense approach against these threats.
2. Industry Standards and Practices — Developing industry standards and best practices specific to preventing zip bomb attacks can foster a proactive defense environment. Collaboration between security experts, researchers, and relevant stakeholders can lead to the establishment of guidelines, frameworks, and defensive mechanisms that help organizations safeguard their systems.

Conclusion: zip bombs

Zip bombs, like other types of malware, pose significant risks to an organization's networks and applications and can cause disruption if not adequately addressed. However, through a combination of education and robust security measures, your organization can mitigate the threat. Understanding the mechanisms behind zip bombs and their potential impact is crucial when introducing measures such as limiting decompression resources, educating users, and investing in future mitigation strategies. Remember, as with all other types of malware, proactive protection is the best kind of protection.

Zip bomb FAQs

What happens if you open a zip bomb?

When you open a zip bomb, your system starts decompressing the archive, which can rapidly consume all available system resources. This can lead to the system freezing, crashing, or becoming unresponsive due to the excessive CPU and memory usage. In severe cases, it can cause a denial-of-service condition, making your computer or network services unavailable. Additionally, if the zip bomb is designed to be particularly malicious, it might cause data corruption or loss during the decompression process.

How to detect a zip bomb download?

Detecting a zip bomb download involves several strategies. Firstly, robust antivirus and anti-malware solutions can scan and flag suspicious files based on known signatures and behaviours. Secondly, monitoring network traffic for unusual activity, such as large data downloads that don't match the apparent file size, can help identify potential zip bombs. Additionally, setting up alerts for files with unusually high compression ratios or those that trigger repeated decompression attempts can also indicate a zip bomb presence.

How do I recognize a zip bomb if I come across one?

Recognizing a zip bomb can be challenging because it often appears as an innocuous compressed file. However, there are some key indicators and best practices that can help you identify a potential zip bomb:

1. File Size Discrepancy: One of the primary indicators of a zip bomb is a significant discrepancy between the compressed file size and the expected size of the uncompressed content. For example, a zip file that is only a few kilobytes in size but contains several gigabytes of data when decompressed is highly suspicious.
2. Unexpected Source: If you receive a zip file from an unknown or untrusted source, especially if it was unsolicited, treat it with caution. Verify the sender's identity and the legitimacy of the file before opening it.
3. Multiple Levels of Compression: Zip bombs often use multiple layers of compression to hide their true size. Be wary of zip files that contain other compressed files nested within them. This recursive compression can be a red flag.
4. File Structure: Examine the file structure within the zip file if possible. If you see an unusual number of small, similarly named files or directories, it could indicate a zip bomb. Normal zip files usually contain a reasonable number of files with varying sizes and names.
5. Antivirus Warnings: Keep your antivirus and anti-malware software up to date. Many modern security tools can detect zip bombs and warn you before you attempt to decompress them. Pay attention to any warnings or alerts from your security software.
6. Unusual Context: Consider the context in which you received the zip file. If it arrived unexpectedly, without any prior communication or justification, be extra cautious. Contact the sender directly to confirm the file's legitimacy if you have any doubts.
7. Safe Decompression Practices: Use decompression tools that offer safe extraction modes. Some tools allow you to view the contents of a zip file without fully extracting it. This can help you assess the file structure and size before deciding to decompress it.