Operating systems have global directories where any user has write access. Those folders are mostly used as temporary storage areas like /tmp in Linux based systems. An application manipulating files from these folders is exposed to race conditions on filenames: a malicious user can try to create a file with a predictable name before the application does. A successful attack can result in other files being accessed, modified, corrupted or deleted. This risk is even higher if the application runs with elevated permissions.

In the past, it has led to the following vulnerabilities:

This rule raises an issue whenever it detects a hard-coded path to a publicly writable directory like /tmp (see examples bellow). It also detects access to environment variables that point to publicly writable directories, e.g., TMP, TMPDIR and TEMP.

Ask Yourself Whether

There is a risk if you answered yes to any of those questions.

Recommended Secure Coding Practices

Out of the box, .NET is missing secure-by-design APIs to create temporary files. To overcome this, one of the following options can be used:

Sensitive Code Example

Using Writer As New StreamWriter("/tmp/f") ' Sensitive
' ...
End Using

Dim Tmp As String = Environment.GetEnvironmentVariable("TMP") ' Sensitive

Compliant Solution

Dim RandomPath = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName())

' Creates a new file with write, non inheritable permissions which is deleted on close.
Using FileStream As New FileStream(RandomPath, FileMode.CreateNew, FileAccess.Write, FileShare.None, 4096, FileOptions.DeleteOnClose)
    Using Writer As New StreamWriter(FileStream) ' Sensitive
    ' ...
    End Using
End Using

See