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December 8, 2021

GHSL-2021-082: Path traversal in SharpCompress - CVE-2021-39208

Jaroslav Lobacevski

Coordinated Disclosure Timeline


WriteEntryToDirectory used for an archive extraction is vulnerable to partial path traversal.



Tested Version



Issue: Extraction directory path is not enforced to be slash terminated

SharpCompress recreates a hierarchy of directories under destinationDirectory if ExtractFullPath is set to true in options. In order to prevent extraction outside the destination directory the destinationFileName path is verified to begin with fullDestinationDirectoryPath. However it is not enforced that fullDestinationDirectoryPath ends with slash:

public static void WriteEntryToDirectory(IEntry entry,
                                         string destinationDirectory,
                                         ExtractionOptions? options,
                                         Action<string, ExtractionOptions?> write)
    string destinationFileName;
    string file = Path.GetFileName(entry.Key);
    string fullDestinationDirectoryPath = Path.GetFullPath(destinationDirectory);
    destinationFileName = Path.GetFullPath(destinationFileName);

    if (!destinationFileName.StartsWith(fullDestinationDirectoryPath, StringComparison.Ordinal))
        throw new ExtractionException("Entry is trying to write a file outside of the destination directory.");

If the destinationDirectory is not slash terminated like /home/user/dir it is possible to create a file with a name that begins with the destination directory, i.e. /home/user/


Because of the file name and destination directory constraints the arbitrary file creation impact is limited and depends on the use case.



This issue was discovered and reported by GHSL team member @JarLob (Jaroslav Lobačevski).


You can contact the GHSL team at, please include a reference to GHSL-2021-082 in any communication regarding this issue.