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. Therefore, 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 filenames.
Non-Compliant Code Example: fopen()
The following non-compliant code creates some_file in the /tmp directory. The name is hard-coded and is thus not unique or unpredictable.
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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()
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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 \[[Open Group 04|AA. C References#Open Group 04]\] 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()}}. |
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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 \[[Callaghan 95|AA. C References#Callaghan 95]\], in particular the {{EXCLUSIVE}} value to the {{mode}} argument of {{CREATE}}. |
Non-Compliant Code Example: tmpnam()
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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 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]. 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|BB. Definitions#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. |
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/* ... */
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 */
}
}
/* ... */
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Non-Compliant Code Example: tmpnam_s() (ISO/IEC TR 24731-1)
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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 \[[ISO/IEC TR 24731-1-2007|AA. C References#SO/IEC TR 24731-1-2007]\]. 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. |
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/* ... */
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 */
}
/* ... */
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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.
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The mktemp() function was 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.
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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 |
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TMP30-C | 3 (high) | 2 (probable) | 1 (high) | P6 | L2 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Sections 7.19.4.4, "The {{tmpnam}} function," 7.19.4.3, "The {{tmpfile}} function," and 7.19.5.3, "The {{fopen}} function"
\[[ISO/IEC TR 24731-1-2007|AA. C References#ISO/IEC TR 24731-1-2007]\] 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"
\[[Open Group 04|AA. C References#Open Group 04]\] [mktemp()|http://www.opengroup.org/onlinepubs/000095399/functions/mktemp.html], [mkstemp()|http://www.opengroup.org/onlinepubs/009695399/functions/mkstemp.html], [open()|http://www.opengroup.org/onlinepubs/009695399/functions/open.html]
\[[Seacord 05a|AA. C References#Seacord 05a]\] Chapter 3, "File I/O"
\[[Wheeler 03|AA. C References#Wheeler 03]\] [Chapter 7, "Structure Program Internals and Approach"|http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/avoid-race.html#TEMPORARY-FILES]
\[[Viega 03|AA. C References#Viega 03]\] Section 2.1, "Creating Files for Temporary Use"
\[[Kennaway 00|AA. C References#Kennaway 00]\]
\[[HP 03|AA. C References#HP 03]\] |
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