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The tmpfile() family of functions construct a unique filename and open it in the same way that fopen() would if passed the flags wb+, as a binary file in read/write mode If the file already exists, tmpfile() truncates it to size zero, possibly in an attempt to assuage the security risk mentioned earlier that might allow an attacker to inject malicious data However, this behavior does not solve the function s security problems An attacker can precreate the file with relaxed access permissions that will be retained when tmpfile() opens the file, leaving the resulting file vulnerable If the program operates on the attacker-supplied file, any data that it writes to the file will be accessible to the attacker and any data that it reads from the file will have been susceptible to manipulation by the attacker Furthermore, if the attacker precreates the file as a link to another file, the application could use its privileges to truncate that file, thereby doing damage on behalf of the attacker Finally, if tmpfile() does create a new file, the access permissions applied to that file vary from one operating system to another, which can leave application data vulnerable even if an attacker cannot predict the filename to be used in advance On most platforms, mkstemp() is a reasonably safe way to create temporary files It attempts to create and open a unique file based on a filename template provided by the user combined with a series of random characters If it is unable to create such a file, it fails and returns -1 On modern systems (GNU C Library Versions 207 and later), the file is opened using mode 0600, which means that the file cannot be tampered with unless the user explicitly changes its access permissions However, as with the other functions, mkstemp() uses predictable filenames and can leave an application vulnerable to denial-of-service attacks if an attacker predicts and precreates the filenames the function tries to open Also, on older systems, the mkstemp() function creates files using mode 0666 modified by the current umask, potentially making the temporary file accessible to all users and leaving the program vulnerable Creating Temporary Files Securely Where it is available, mkstemp() is the best choice for creating temporary files among the functions offered by the standard library But because of the file permissions problem with mkstemp() on older systems, you should require that all newly created files be accessible only to the current user by calling umask(077) before creating any temporary files; this forces all newly created files to be accessible only to the user who creates them Because
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umask settings are inherited from one process to another, do not rely on the default umask set by the shell or whatever process ran your program An attacker can explicitly invoke your program with a relaxed umask and violate your assumptions This solution does not address the risk of a denial-of-service attack mounted by an attacker who can predict the filename values that will be generated If this type of attack is a concern in your environment, you have two choices for creating temporary files safely Consider storing temporary files under a directory that is not publicly accessible, thereby eliminating all contention with attackers (refer to Example 1211 for an example of code to create such a directory) If you are writing your own code for creating temporary files, generate temporary filenames that will be difficult to guess by using a cryptographically secure pseudo-random number generator (PRNG) to create a random element in every temporary filename 11, Privacy and Secrets, includes a section dedicated to generating strong random numbers Command Injection Command injection vulnerabilities exist when a program executes a command that an attacker can influence These vulnerabilities take two primary forms: An attacker can change the command that the program executes: The attacker explicitly controls what the command is An attacker can change the environment in which the command executes: The attacker implicitly controls what the command means The first form of command injection receives more attention because it has the potential to affect any program that accepts input from its users and executes commands Both forms of the vulnerability fall under the general umbrella of input validation problems, which we address in 5, Handling Input But the second form of command injection is specifically pertinent to privileged programs because it occurs when an attacker can change the meaning of the command by altering an environment variable or by inserting a malicious executable in the program s search path This variety of command injection exploit follows the following sequence of events: 1 An attacker modifies a program s environment 2 The program executes a command using the malicious environment without specifying an absolute path or otherwise verifying the program being invoked
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