CVE-2021-39208 is a medium-severity (CVSS 4.3) Path Traversal vulnerability in SharpCompress. 1 public exploit reference exists, 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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-39208 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 |
Research 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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-39208 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-39208. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-39208 in your dependencies?
O3 Security finds CVE-2021-39208 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.