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 and TMPDIR.
-
/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
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
- Use a dedicated sub-folder with tightly controlled permissions
- Use secure-by-design APIs to create temporary files. Such API will make sure:
- The generated filename is unpredictable
- The file is readable and writable only by the creating user ID
- The file descriptor is not inherited by child processes
- The file will be destroyed as soon as it is closed
Sensitive Code Example
Examples of sensitive file creation:
file, _ = os.Create("/tmp/tempfile.txt") // Sensitive
file, _ = os.Create(os.TempDir()+"/tempfile.txt") // Sensitive
file, _ := os.OpenFile("/tmp/tempfile.txt", os.O_CREATE, 0755) // Sensitive
os.WriteFile("/tmp/tempfile.txt", []byte{"sensitive"}, 0755) // Sensitive
Example of sensitive directory creation:
tempdir := "/tmp/tempdir/"
os.Mkdir(tempdir, 0755) // Sensitive
file, _ := os.Create("/tmp/tempdir/tempfile.txt")
Compliant Solution
Compliant temporary file creation:
file, _ := os.CreateTemp("", "example-pattern")
Compliant temporary directory creation:
dir, _ := os.MkdirTemp("", "example-directory")
filename := filepath.Join(dir, "tempfile.txt")
file, _ := os.Create(filename)
See
