TMP30-C. Temporary files must be created with unique and unpredictable file names

Programs that create files in shared directories may be exploited to overwrite protected files. For example, 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 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 file referenced by the symbolic link can be overwritten when the program is executed. To ensure that the name of the temporary file does not conflict with a preexisting file and that it cannot be guessed before the program is run, temporary files must be created with unique and unpredictable file names.

Whenever working with temporary files, it is important to keep in mind [[TMP33-C. Temporary files must be removed before the program exits]] for implementations where the temporary file is not removed if the program terminates abnormally

Non-Compliant Code Example: fopen()

The following non-compliant code creates some_file in the /tmp directory. The name is hard coded and consequently is neither unique nor unpredictable.

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 whether 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]] 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 secure, but a temporary file is presumably still required. One approach that can be used with open() is to generate random filenames 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 filename and that is not the same as the name of an existing file [[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.

char filename[L_tmpnam];

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

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 filename and that is not the same as the name of an existing file [[ISO/IEC TR 24731-1-2007]]. It is almost identical to the tmpnam function above except with an added maxsize argument for the supplied buffer.

Non-normative text in TR 24731-1 also recommends the following:

Implementations should take care in choosing the patterns used for names returned by tmpnam_s. For example, making a thread id part of the names avoids the race condition and possible conflict when multiple programs run simultaneously by the same user generate the same temporary file names.

If implemented, this reduces the space for unique names and increases the predictability of the resulting names.  But in general, TR 24731-1 does not establish any criteria for predictability of names.

char filename[L_tmpnam_s];

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

/* A TOCTOU race condition exists here */

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

Implementation Details

For Microsoft Visual Studio 2005, the name generated by tmpnam_s consists of a program-generated filename and, after the first call to tmpnam_s(), a file extension of sequential numbers in base 32 (.1-.1vvvvvu, when TMP_MAX_S in stdio.h is INT_MAX).

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

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

char filename[] = "/tmp/temp-XXXXXX";

if (mktemp(filename) == NULL) {
  /* Handle Error */
}

/* A TOCTOU race condition exists here */

if ((fd = open(filename, O_WRONLY | O_CREAT | O_EXCL | O_TRUNC, 0600)) == -1) {
  /* Handle Error */
}

The mktemp() function has been marked LEGACY in the Open Group Base Specifications Issue 6.

Non-Compliant Code Example: tmpfile()

The C99 tmpfile() function creates a temporary binary file that is different from any other existing file and that is automatically removed when it is closed or at program termination.

It should be possible to open at least TMP_MAX temporary files during the lifetime of the program (this limit may be shared with tmpfile()). The value of the macro TMP_MAX is only required to be 25 by the C99 standard.

Most historic implementations provide only a limited number of possible temporary filenames (usually 26) before filenames are recycled.

if ((fp = tmpfile()) == NULL) {
  /* Handle Error */
}

Compliant Solution: mkstemp() (POSIX)

A reasonably secure solution for generating random file names is to use the mkstemp() function. The mkstemp() function is available on systems that support the Open Group Base Specifications Issue 4, Version 2 or later.

A call to mkstemp() replaces the six Xs in the template string with six randomly selected characters and returns a file descriptor for the file (opened for reading and writing):

char template[] = "/tmp/temp-XXXXXX";
if ((fd = mkstemp(template)) == -1) {
   /* Handle Error */
}

The mkstemp() algorithm for selecting filenames has proven to be immune to attacks.

char sfn[] = "/tmp/temp-XXXXXX";
FILE *sfp;
int fd = -1;

if ((fd = mkstemp(sfn)) == -1) {
  /* Handle Error */
}

unlink(sfn); /* unlink immediately to allow name to be recycled */

if ((sfp = fdopen(fd, "w+")) == NULL) {
  close(fd);
  /* Handle Error */
}

/* use temporary file */

fclose(sfp); /* also closes fd */

The Open Group Base Specification Issue 6 [[Open Group 04]] does not specify the permissions the file is created with, so these are implementation-defined. However, Issue 7 (that is, POSIX.1-2008) specifies them as S_IRUSR|S_IWUSR (0600).

Implementation Details

For glibc versions 2.0.6 and earlier, the file is created with permissions 0666; for glibc versions 2.0.7 and later, the file is created with permissions 0600. On NetBSD, the file is created with permissions 0600.

In many older implementations, the name is a function of process ID and time, so it is possible for the attacker to predict the name and create a decoy in advance. FreeBSD has recently changed the mk*temp() family to eliminate the PID component of the filename and replace the entire field with base-62 encoded randomness. This raises the number of possible temporary files for the typical use of 6 Xs significantly, meaning that even mktemp() with 6 Xs is reasonably (probabilistically) secure against guessing, except under frequent usage [[Kennaway 00]] .

Compliant Solution: tmpfile_s() (ISO/IEC TR 24731-1 )

The ISO/IEC TR 24731-1 function tmpfile_s() creates a temporary binary file that is different from any other existing file and that is automatically removed when it is closed or at program termination. If the program terminates abnormally, whether an open temporary file is removed is implementation-defined.

The file is opened for update with "wb+" mode, which means "truncate to zero length or create binary file for update." To the extent that the underlying system supports the concepts, the file is opened with exclusive (non-shared) access and has a file permission that prevents other users on the system from accessing the file.

It should be possible to open at least TMP_MAX_S temporary files during the lifetime of the program (this limit may be shared with tmpnam_s()). The value of the macro TMP_MAX_S is only required to be 25 by ISO/IEC TR 24731-1.

The tmpfile_s() function is available on systems that support ISO/IEC TR 24731-1.

TR 24731-1 notes the following regarding the use of tmpfile_s() instead of tmpnam_s():

After a program obtains a file name using the tmpnam_s function and before the program creates a file with that name, the possibility exists that someone else may create a file with that same name. To avoid this race condition, the tmpfile_s function should be used instead of tmpnam_s when possible. One situation that requires the use of the tmpnam_s function is when the program needs to create a temporary directory rather than a temporary file.

if (tmpfile_s(&fp)) {
  /* Handle Error */
}

The tmpfile_s() function may not be compliant with [[TMP33-C. Temporary files must be removed before the program exits]] for implementations where the temporary file is not removed if the program terminates abnormally.

Risk Assessment

Failure to create unique, unpredictable temporary file names can make it possible for an attacker to access or modify privileged files.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899-1999]] Sections 7.19.4.4, "The tmpnam function," 7.19.4.3, "The tmpfile function," and 7.19.5.3, "The fopen function"
[[ISO/IEC PDTR 24772]] "EWR Path Traversal"
[[ISO/IEC TR 24731-1-2007]] Sections 6.5.1.2, "The tmpnam_s function," 6.5.1.1, "The tmpfile_s function," and 6.5.2.1, "The fopen_s function"
[[Open Group 04]] mktemp(), mkstemp(), open()
[[Seacord 05a]] Chapter 3, "File I/O"
[[Wheeler 03]] Chapter 7, "Structure Program Internals and Approach"
[[Viega 03]] Section 2.1, "Creating Files for Temporary Use"
[[Kennaway 00]]
[[HP 03]]

TMP00-A. Do not create temporary files in shared directories      10. Temporary Files (TMP)       TMP32-C. Temporary files must be opened with exclusive access