SharpCompress is a fully managed C# library to deal with many compression types and formats. In 0.47.4 and earlier, a path traversal vulnerability in IArchive.WriteToDirectory() allows a malicious archive to create directories outside the intended extraction root. For TAR archives, this can be escalated to arbitrary file writes by chaining with a symlink entry, giving a full write primitive on the target filesystem subject to the permissions of the running process.

Affected: NuGet: SharpCompress

### Summary

A path traversal vulnerability in `IArchive.WriteToDirectory()` allows a malicious archive to create directories outside the intended extraction root. For TAR archives, this can be escalated to arbitrary file writes by chaining with a symlink entry, giving a full write primitive on the target filesystem subject to the permissions of the running process.

### Details

The vulnerable code is in the directory-entry branch of `WriteToDirectoryInternal` (sync, `IArchiveExtensions.cs:48–61`) and `WriteToDirectoryAsyncInternal` (async, `IAsyncArchiveExtensions.cs:70–84`):

```csharp var dirPath = Path.Combine(destinationDirectory, entry.Key); Directory.CreateDirectory(Path.GetDirectoryName(dirPath + "/")); ```

No `Path.GetFullPath()` normalisation and no bounds check are applied before the `Directory.CreateDirectory` call. Two .NET `Path.Combine` behaviours make this exploitable:

- **Relative traversal**: `Path.Combine("/safe/extract", "../../evil")` → the OS resolves `..` segments on the raw path, placing the directory outside the extraction root. - **Absolute path override**: `Path.Combine("/safe/extract", "/tmp/evil")` → returns `"/tmp/evil"` — the base is discarded entirely for rooted paths.

File entries are **not** directly affected — they route through `ExtractionMethods.WriteEntryToDirectory` which applies the correct guard (`GetFullPath` + `StartsWith`, see `ExtractionMethods.cs:54–65`). The directory-entry branch is a separate fast-path that was added witho…