CVE-2021-39208 is a medium-severity (CVSS 4.3) Path Traversal vulnerability in SharpCompress. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for SharpCompress — see the affected versions and patch details below.
Partial path traversal in sharpcompress
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
SharpCompressReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
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);
...
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 thats begins as the destination directory one level up from the directory, i.e. /home/user/dir.sh.
Impact
Because of the file name and destination directory constraints the arbitrary file creation impact is limited and depends on the use case.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | SharpCompress | all versions | 0.29dotnet add package SharpCompress --version 0.29 |
Affected Products
sharpcompresssharpcompress_projectResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for SharpCompress, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update SharpCompress to 0.29 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39208 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.
Frequently Asked Questions
Is CVE-2021-39208 in your dependencies?
Find it across NuGet, including transitive dependencies.