Zip bomb


A zip bomb, also known as a zip of death or decompression bomb, is a malicious archive file designed to crash or render useless the program or system reading it. It is often employed to disable antivirus software, in order to create an opening for more traditional viruses.
Rather than hijacking the normal operation of the program, a zip bomb allows the program to work as intended, but the archive is carefully crafted so that unpacking it requires inordinate amounts of time, disk space or memory.
Most modern antivirus programs can detect whether a file is a zip bomb, to avoid unpacking it.

Details and use

A zip bomb is usually a small file for ease of transport and to avoid suspicion. However, when the file is unpacked, its contents are more than the system can handle.
One example of a zip bomb is the file 42.zip, which is a zip file consisting of 42 kilobytes of compressed data, containing five layers of nested zip files in sets of 16, each bottom layer archive containing a 4.3-gigabyte file for a total of 4.5 petabytes of uncompressed data.
This file is available for download on various websites across the Internet. In many anti-virus scanners, only a few layers of recursion are performed on archives to help prevent attacks that would cause a buffer overflow, an out-of-memory condition, or exceed an acceptable amount of program execution time.
Zip bombs often rely on repetition of identical files to achieve their extreme compression ratios. Dynamic programming methods can be employed to limit traversal of such files, so that only one file is followed recursively at each level, effectively converting their exponential growth to linear.
There are also zip files that, when uncompressed, yield identical copies of themselves. A sophisticated form of zip bombs exploits the specifications of zip files and the Deflate compression algorithm to create bombs without the use of nested layers as used in 42.zip.