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.
-
/tmp
-
/var/tmp
-
/usr/tmp
-
/dev/shm
-
/dev/mqueue
-
/run/lock
-
/var/run/lock
-
/Library/Caches
-
/Users/Shared
-
/private/tmp
-
/private/var/tmp
-
\Windows\Temp
-
\Temp
-
\TMP
-
%USERPROFILE%\AppData\Local\Temp
Ask Yourself Whether
- Files are read from or written into a publicly writable folder
- The application creates files with predictable names into a publicly writable folder
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:
- Use a dedicated sub-folder with tightly controlled permissions
- Created temporary files in a publicly writable folder and make sure:
- Generated filename is unpredictable
- File is readable and writable only by the creating user ID
- File descriptor is not inherited by child processes
- File is destroyed as soon as it is closed
Sensitive Code Example
using var writer = new StreamWriter("/tmp/f"); // Sensitive
var tmp = Environment.GetEnvironmentVariable("TMP"); // Sensitive
Compliant Solution
var randomPath = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName());
// Creates a new file with write, non inheritable permissions which is deleted on close.
using var fileStream = new FileStream(randomPath, FileMode.CreateNew, FileAccess.Write, FileShare.None, 4096, FileOptions.DeleteOnClose);
using var writer = new StreamWriter(fileStream);
See
