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 Each entry As ZipArchiveEntry in archive.Entries
        ' entry.FullName could contain parent directory references ".." and the destinationPath variable could become outside of the desired path
        string destinationPath = Path.GetFullPath(Path.Combine(path, entry.FullName))
        entry.ExtractToFile(destinationPath) ' Sensitive, extracts the entry to a file

        Dim stream As Stream
        stream = entry.Open() ' Sensitive, the entry is about to be extracted
    Next

## Compliant Solution

    Const ThresholdRatio As Double = 10
    Const ThresholdSize As Integer = 1024 * 1024 * 1024 ' 1 GB
    Const ThresholdEntries As Integer = 10000
    Dim TotalSizeArchive, TotalEntryArchive, TotalEntrySize, Cnt As Integer
    Dim Buffer(1023) As Byte
    Using ZipToOpen As New FileStream("ZipBomb.zip", FileMode.Open), Archive As New ZipArchive(ZipToOpen, ZipArchiveMode.Read)
        For Each Entry As ZipArchiveEntry In Archive.Entries
            Using s As Stream = Entry.Open
                TotalEntryArchive += 1
                TotalEntrySize = 0
                Do
                    Cnt = s.Read(Buffer, 0, Buffer.Length)
                    TotalEntrySize += Cnt
                    TotalSizeArchive += Cnt
                    If TotalEntrySize / Entry.CompressedLength > ThresholdRatio Then Exit Do    ' Ratio between compressed And uncompressed data Is highly suspicious, looks Like a Zip Bomb Attack
                Loop While Cnt > 0
            End Using
            If TotalSizeArchive > ThresholdSize Then Exit For       ' The uncompressed data size Is too much for the application resource capacity
            If TotalEntryArchive > ThresholdEntries Then Exit For   ' Too much entries in this archive, can lead to inodes exhaustion of the system
        Next
    End Using

## See

- OWASP - Top 10 2021 Category A1 - Broken Access Control
- OWASP - Top 10 2021 Category A5 - Security Misconfiguration
- OWASP - Top 10 2017 Category A5 - Broken Access Control
- OWASP - Top 10 2017 Category A6 - Security Misconfiguration
- CWE - CWE-409 - Improper Handling of Highly Compressed Data (Data Amplification)
- bamsoftware.com - A better Zip Bomb