Successful Zip Bomb attacks occur when an application expands untrusted archive files without controlling the size of the expanded data, which can
lead to denial of service. A Zip bomb is usually a malicious archive file of a few kilobytes of compressed data but turned into gigabytes of
uncompressed data. To achieve this extreme compression ratio, attackers will
compress irrelevant data (eg: a long string of repeated bytes).
Ask Yourself Whether
Archives to expand are untrusted and:
- There is no validation of the number of entries in the archive.
- There is no validation of the total size of the uncompressed data.
- There is no validation of the ratio between the compressed and uncompressed archive entry.
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- Define and control the ratio between compressed and uncompressed data, in general the data compression ratio for most of the legit archives is
1 to 3.
- Define and control the threshold for maximum total size of the uncompressed data.
- Count the number of file entries extracted from the archive and abort the extraction if their number is greater than a predefined threshold, in
particular it's not recommended to recursively expand archives (an entry of an archive could be also an archive).
Sensitive Code Example
For tarfile module:
import tarfile
tar = tarfile.open("attachment.tar.gz") # Sensitive
tar.extractall()
tar.close()
See