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    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
    

    using var zipToOpen = new FileStream(@"ZipBomb.zip", FileMode.Open);
using var archive = new ZipArchive(zipToOpen, ZipArchiveMode.Read);
foreach (ZipArchiveEntry entry in archive.Entries)
{
  entry.ExtractToFile("./output_onlyfortesting.txt", true); // Sensitive
}

    

    Compliant Solution
    

    int THRESHOLD_ENTRIES = 10000;
int THRESHOLD_SIZE = 1000000000; // 1 GB
double THRESHOLD_RATIO = 10;
int totalSizeArchive = 0;
int totalEntryArchive = 0;

using var zipToOpen = new FileStream(@"ZipBomb.zip", FileMode.Open);
using var archive = new ZipArchive(zipToOpen, ZipArchiveMode.Read);
foreach (ZipArchiveEntry entry in archive.Entries)
{
  totalEntryArchive ++;

  using (Stream st = entry.Open())
  {
    byte[] buffer = new byte[1024];
    int totalSizeEntry = 0;
    int numBytesRead = 0;

    do
    {
      numBytesRead = st.Read(buffer, 0, 1024);
      totalSizeEntry += numBytesRead;
      totalSizeArchive += numBytesRead;
      double compressionRatio = totalSizeEntry / entry.CompressedLength;

      if(compressionRatio > THRESHOLD_RATIO) {
        // ratio between compressed and uncompressed data is highly suspicious, looks like a Zip Bomb Attack
        break;
      }
    }
    while (numBytesRead > 0);
  }

  if(totalSizeArchive > THRESHOLD_SIZE) {
      // the uncompressed data size is too much for the application resource capacity
      break;
  }

  if(totalEntryArchive > THRESHOLD_ENTRIES) {
      // too much entries in this archive, can lead to inodes exhaustion of the system
      break;
  }
}

    

    See




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