FIO42-C. Ensure files are properly closed when they are no longer needed

Failing to close files when they are no longer needed may allow attackers to exhaust, and possibly manipulate, system resources. This phenomenon is typically referred to as file descriptor leakage, although file pointers may also be used as an attack vector. To prevent file descriptor leaks, file pointers and file descriptors should be closed when they are no longer needed.

Non-Compliant Code Example

In this non-compliant example inspired by a vulnerability in OpenBSD's chpass program [Openbsd 98], a file containing sensitive data is opened for reading. Before closing this file, the program retrieves the registered editor from the environment and executes it using the system() command.

FILE* f;
char *editor;

f = fopen("Sensitive.txt", "r");
if (fd == NULL) {
  /* Handle fopen() error */
}
/* ... */
editor = getenv("EDITOR");
if (editor == NULL) {
  /* Handle getenv() error */
}
system(editor);

If, the system() command is implemented in a way that spawns a child process to run the editor, then the child process inherits the file descriptors opened by the parent process. As a result, the editor will be able to access the contents of Sensitive.txt.

Compliant Solution

To correct this example, Sensitive.txt should be closed before launching the editor.

ILE* f;
char *editor;

f = fopen("Sensitive.txt", "r");
if (fd == NULL) {
  /* Handle fopen() error */
}
/* ... */
fclose(f);
editor = getenv("EDITOR");
if (editor == NULL) {
  /* Handle getenv() error */
}

Risk Assessment

Failing to properly close files may allow unintended access to, or exhaustion of, system resources.

References

[[Dowd 06]] Chapter 10, "UNIX Processes" (File Descriptor Leaks 582-587)
[CWE 403] UNIX file descriptor leaks