SUNZIP

Introduction

Why are we doing this?

According to Cara Marie, an archive bomb a.k.a. A zip bomb is often employed to disable antivirus software, in order to create an opening for more traditional viruses. In addition, various kinds of pitfalls may occur during decompression.

Decompression pitfalls

The extraction in zipfile module might fail against some pitfalls listed below.

From file itself

Decompression may fail due to incorrect password / CRC checksum / ZIP format or unsupported compression method / decryption.

File System limitations

Exceeding limitations on different file systems can cause decompression failed. Such as allowable characters in the directory entries, length of the file name, length of the pathname, size of a single file, and number of files, etc.

Resources limitations

The lack of memory or disk volume would lead to decompression failed. For example, decompression bombs (aka ZIP Bomb) apply to zipfile library that can cause disk volume exhaustion.

Interruption

Interruption during the decompression, such as pressing control-C or killing the decompression process may result in incomplete decompression of the archive.

Default behaviors of extraction

Not knowing the default extraction behaviors can cause unexpected decompression results. For example, when extracting the same archive twice, it overwrites files without asking.

What is zip bomb?

It often appeared as a relatively small size zip file. And the unzipped file will be much larger than the zipped one. This would probably cause a problem when your disk volume or memory is relatively small than the unzipped one.

How do we defense zip bomb?

• Defense Layer 1 - checks perform on the server side.

    1. Check if it's a nested zip file. (i.e. 42.zip)
    2. Check if the compression ratio (Uncompressed Content/Compressed Content) 
       is greater than the threshold?
    3. Check if the file format is expected for context.
    4. Upload file size does not exceed the maximum limit. 

• Defense Layer 2 - limit the number of resources available to the process and its children.

    1. Check if CPU time is greater than the threshold.
    2. Check if the extracted part in memory is oversized. (memory usage)

• Defense Layer 3 - filetype-specific mitigations.

  Filetype: Archives

    1. Restrict output file size and number of extracted files 
       to ensure the total doesn't exceed the maximum limit.

How do we set thresholds?

Defense Layer 1:
  Uncompressed content size:  200 MB (vt)
  Compression ratio:          https://youtu.be/IXkX2ojrKZQ?t=553

Defense Layer 2:
  CPU time:                   2 seconds(vt)
  Memoery oversized:

Defense Layer 3:
  Output file size:
  Number of extracted files:

Useful resources

Bomb Codes
https://bomb.codes/

Mitigation Summary
https://youtu.be/IXkX2ojrKZQ?t=1296

Defense layers
https://bomb.codes/mitigations

Install

$ pip3 install sunzip

Usage

import sunzip

f = sunzip("archive.zip")

Customize your resource limit.

Maximum compression ratio threshold

f.threshold = 50

Maximum CPU time

f.cpu = 1

Maximum memory usage

f.memory = 1024

Maximum file size

f.filesize = 1024

If there is no setting, the default value will be used.

extract() would perform a series of the above checks before decompression. If all pass, the zip file will be decompressed.

import sunzip

f = sunzip("archive.zip")

f.extract()