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When setting access permissions, it is important to make sure that an attacker is not able to alter them. (See FIO15-C. Ensure that file operations are performed in a secure directory.)
Noncompliant Code Example (fopen())
The fopen() function does not allow the programmer to explicitly specify file access permissions. In this noncompliant code example, if the call to fopen() creates a new file, the access permissions are implementation-defined.
| Code Block | ||||
|---|---|---|---|---|
| ||||
char *file_name;
FILE *fp;
/* initialize file_name */
fp = fopen(file_name, "w");
if (!fp){
/* Handle error */
}
|
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On POSIX-compliant systems, the permissions may be restricted by the value of the POSIX umask() function [Open Group 2004].
The operating system modifies the access permissions by computing the intersection of the inverse of the umask and the permissions requested by the process [Viega 2003]. For example, if the variable requested_permissions contained the permissions passed to the operating system to create a new file, the variable actual_permissions would be the actual permissions that the operating system would use to create the file:
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For OpenBSD and Linux operating systems, any file created will have mode S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH (0666), as modified by the process's umask value. (See fopen(3) in the OpenBSD Manual Pages [OpenBSD].)
Compliant Solution (fopen_s(), ISO/IEC TR 24731-1)
The ISO/IEC TR 24731-1 function fopen_s() can be used to create a file with restricted permissions [ISO/IEC TR 24731-1:2007]:
If the file is being created, and the first character of the mode string is not 'u', to the extent that the underlying system supports it, the file shall have a file permission that prevents other users on the system from accessing the file. If the file is being created and the first character of the mode string is 'u', then by the time the file has been closed, it shall have the system default file access permissions.
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This example also violates EXP37-C. Call functions with the arguments intended by the APIcorrect number and type of arguments.
Compliant Solution (open(), POSIX)
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John Viega and Matt Messier also provide the following advice [Viega 2003]:
Do not rely on setting the umask to a "secure" value once at the beginning of the program and then calling all file or directory creation functions with overly permissive file modes. Explicitly set the mode of the file at the point of creation. There are two reasons to do this. First, it makes the code clear; your intent concerning permissions is obvious. Second, if an attacker managed to somehow reset the umask between your adjustment of the umask and any of your file creation calls, you could potentially create sensitive files with wide-open permissions.
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ISO/IEC 9899:2011 Section 7.21.5.3, "The fopen function"
ISO/IEC TR 24772 "XZN Missing or inconsistent access control"
ISO/IEC TR 24731-1:2007 Section 6.5.2.1, "The fopen_s function"
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MITRE CWE: CWE-732, "Incorrect permission assignment for critical resource"
Bibliography
[CVE]
[OpenBSD]
[Open Group 2004] "The open function" and "The umask function"
[Viega 2003] Section 2.7, "Restricting access permissions for new files on UNIX"
[Dowd 2006] Chapter 9, "UNIX 1: Privileges and Files"
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