Privileged programs that create files in world-writable directories can overwrite protected system files. An attacker who can predict the name of a file created by a privileged program can create a symbolic link (with the same name as the file used by the program) to point to a protected system file. Unless the privileged program is coded securely, the program will follow the symbolic link instead of opening or creating the file that it is supposed to be using. As a result, the protected system file referenced by the symbolic link can be overwritten when the program is executed.

Non-Compliant Code Example: fopen()

The following statement creates some_file in the /tmp directory.

FILE *fp = fopen("/tmp/some_file", "w");

If /tmp/some_file already exists then that file is opened and truncated. If /tmp/some_file is a symbolic link, then the target file referenced by the link is truncated.

To exploit this coding error, an attacker need only create a symbolic link called /tmp/some_file before execution of this statement.

Non-Compliant Code Example: open()

The {{fopen()}} function does not indicate if an existing file has been opened for writing or a new file has been created. However, the {{open()}} function as defined in the Open Group Base Specifications Issue 6 \[[Open Group 04|AA. C References#Open Group 04]\] provides such a mechanism.  If the {{O_CREAT}} and {{O_EXCL}} flags are used together, the {{open()}} function fails when the file specified by {{file_name}} already exists. To prevent an existing file from being opened and truncated, include the flags {{O_CREAT}} and {{O_EXCL}} when calling {{open()}}. 

int fd = open("/tmp/some_file", O_WRONLY | O_CREAT | O_EXCL | O_TRUNC, 0600);

This call to open() fails whenever /tmp/some_file already exists, including when it is a symbolic link. This is a good thing, but a temporary file is presumably still required. One approach that can be used with open() is to generate random file names and attempt to open() each until a unique name is discovered. Luckily, there are predefined functions that perform this function.

Care should be observed when using O_EXCL with remote file systems as it does not work with NFS version 2. NFS version 3 added support for O_EXCL mode in open(); see IETF RFC 1813 Callaghan 95, in particular the EXCLUSIVE value to the mode argument of CREATE.

Non-Compliant Code Example: tmpnam() 

The C99 {{tmpnam()}} function generates a string that is a valid file name and that is not the same as the name of an existing file \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]. Files created using strings generated by the {{tmpnam()}} function are temporary in that their names should not collide with those generated by conventional naming rules for the implementation.  The function is potentially capable of generating {{TMP_MAX}} different strings, but any or all of them may already be in use by existing files.  If the argument is not a null pointer, it is assumed to point to an array of at least {{L_tmpnam}} chars; the {{tmpnam()}} function writes its result in that array and returns the argument as its value.

...
if (tmpnam(temp_file_name)) {
  /* temp_file_name may refer to an existing file */
  t_file = fopen(temp_file_name,"wb+");
  if (!t_file) {
     /* Handle Error */
  }
}
...


Unfortunately, this solution is still non-compliant because it violates \[[FIO32-C|FIO32-C. Temporary file names must be unique when the file is created]\], \[[FI040-C|FI040-C. Temporary files must be opened with exclusive access]\], \[[FI041-C|FI041-C. Temporary files must have an unpredictable name]\], \[[FI042-C|FI042-C. Temporary files must be removed before the program exits]\].

Non-Compliant Code Example: tmpnam_s() (ISO/IEC TR 24731-1)

The TR 24731-1 {{tmpnam_s()}} function generates a string that is a valid file name and that is not the same as the name of an existing file \[[ISO/IEC TR 24731-2006|AA. C References#SO/IEC TR 24731-2006]\]. The function is potentially capable of generating {{TMP_MAX_S}} different strings, but any or all of them may already be in use by existing files and thus not be suitable return values.  The lengths of these strings must be less than the value of the {{L_tmpnam_s}} macro.

...
FILE *file_ptr;
char filename[L_tmpnam_s];

if (tmpnam_s(filename, L_tmpnam_s) != 0) {
  /* Handle Error */
}

if (!fopen_s(&file_ptr, filename, "wb+")) {
  /* Handle Error */
}
...


This solution is also non-compliant because it violates \[[FIO32-C|FIO32-C. Temporary file names must be unique when the file is created]\] and \[[FI042-C|FI042-C. Temporary files must be removed before the program exits]\].

Non-Compliant Code Example: mktemp() (POSIX)

The POSIX function mktemp() takes a given file name template and overwrites a portion of it to create a file name. The template may be any file name with some number of 'X's appended to it, for example /tmp/temp.XXXXXX. The trailing 'X's are replaced with the current process number and/or a unique letter combination. The number of unique file names mktemp() can return depends on the number of 'X's provided.

...
FILE *temp_ptr;
char temp_name[] = "/tmp/temp-XXXXXX";

if (mktemp(temp_name) == NULL) {
  /* Handle Error */
}
temp_ptr = fopen(temp_name,"w+");
if (temp_ptr == NULL) {
  /* Handle Error */
}
...


This solution is also non-compliant because it violates \[[FIO32-C|FIO32-C. Temporary file names must be unique when the file is created]\] and \[[FI042-C|FI042-C. Temporary files must be removed before the program exits]\].

Risk Assessment

A protected system file to which the symbolic link points can be overwritten when a vulnerable program is executed.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

FIO39-C

2 (high)

2 (probable)

2 (medium)

P8

L2

References