C# Zip Slip demo

Snapshot

Use this snapshot of PowerShell.

Introduction

The "Zip Slip" vulnerability was announced on June 5th 2018, by Snyk. You can see on this page some sample code that shows the vulnerable code.

Microsoft immediately wanted to search their codebase to see if any of their own code was vulnerable. Within a few days, they had written a basic query and run it against a number of critical codebases, turning up multiple valuable results. Because Semmle has a close working relationship with Microsoft, we then helped Microsoft to refine that query further and submit it as a pull request against our open source QL repository.

It was deployed to the now deprecated LGTM website within 2 weeks where it was run over thousands of open source C# projects.

The CodeQL ZipSlip query found a vulnerability in Microsoft PowerShell.

As a result of this query, a senior Microsoft engineer fixed this vulnerability in November 2018.

So how did they do it?

Query 1-4: Exploring sources and sinks

Open the snapshot in QL4E, and show QL as a simple query language for identifying sources and sinks. Look for sinks (calls to ExtractToFile), noting that we actually want to identify the vulnerable argument.

Sources are identified by for example the FullName property of ZipArchiveEntry. If we omit the name of the declaring type, we get too many results, and the full query would use the qualified name of the property.

Already we've found the vulnerable code.

Query 5

This query uses local dataflow to find data flow from the source to the sink. We actually need to use taint tracking due to the use of Path.Combine.

Query 6

This uses a global taint tracking configuration.

Final query

The final query below includes query help, and identifies various other sources and sinks, but uses the same general structure.

using System.IO;
using System.IO.Compression;
class Good
{
    public static void WriteToDirectory(ZipArchiveEntry entry,
                                        string destDirectory)
    {
        string destFileName = Path.GetFullPath(Path.Combine(destDirectory, entry.FullName));
        string fullDestDirPath = Path.GetFullPath(destDirectory + Path.DirectorySeparatorChar);
        if (!destFileName.StartsWith(fullDestDirPath)) {
            throw new System.InvalidOperationException("Entry is outside the target dir: " +
                                                                                 destFileName);
        }
        entry.ExtractToFile(destFileName);
    }
}