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Unique Filenames Table 124 lists C Library functions that attempt to generate a unique filename for a new temporary file These functions suffer from an inherent underlying race condition on the filename chosen4 Although the functions guarantee that the filename is unique at the time it is selected, there is no mechanism to prevent an attacker from creating a file with the same name after it is selected but before the application attempts to open the file The likelihood of successful attacks against these functions is increased by the fact that they use very poor sources of randomness in the names they generate; this makes it more likely that an attacker will be able to create a malicious collision
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Table 124 Common functions that attempt to generate a unique temporary filename Function
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char* mktemp (char *template)
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Description The mktemp() function generates a unique filename by modifying template [] If successful, it returns the template as modified If mktemp() cannot find a unique filename, it makes template an empty string and returns that This function constructs and returns a valid filename that does not refer to any existing file If the result argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls and therefore makes this function non-reentrant Otherwise, the result argument should be a pointer to an array of at least L_tmpnam characters, and the result is written into that array This function generates a unique temporary filename If prefix is not a null pointer, up to five characters of this string are used as a prefix for the filename The return value is a string newly allocated with malloc(), so you should release its storage with free when it is no longer needed
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char* tmpnam (char *result)
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char* tempnam (const char *dir, const char *prefix)
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Source: [FSF, 2001]
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4 On Windows, the GetTempFileName() function suffers from the same vulnerability
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If an attacker does manage to create the file first, depending on how the file is opened, the existing contents or access permissions of the file might remain intact If the contents of the file are malicious in nature, an attacker might be able to inject dangerous data into the application when it reads data back from the temporary file If an attacker precreates the file with relaxed access permissions, an attacker might be able to later access, modify, or corrupt data that the application stores in the temporary file If the attacker precreates the file as a link to another important file, the application might truncate or write data to the file and unwittingly perform damaging operations for the attacker Finally, in the best case, the file can be opened with open() using the O_CREAT and O_EXCL flags, which will fail if the file already exists and, therefore, prevent these types of attacks However, if an attacker can accurately predict a sequence of temporary filenames, he or she might be able to prevent the program from opening necessary temporary storage, effectively causing a denial-of-service attack Again, this type of attack is trivial to mount, given the small amount of randomness used in the selection of the filenames these functions generate Unique Files Table 125 lists the C Library functions that attempt to not only generate a unique filename, but also open the file
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Table 125 Common functions that attempt to open a unique temporary file Open Temporary Files
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FILE* tmpfile (void)
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Description This function creates a temporary binary file for update mode, as if by calling fopen() with mode wb+ The file is deleted automatically when it is closed or when the program terminates The mkstemp() function generates a unique file name just as mktemp() does, but it also opens the file for you with open() [with the O_EXCL flag] If successful, it modifies the template in place and returns a file descriptor for that file open for reading and writing If mkstemp() cannot create a uniquely named file, it returns -1 The file is opened using mode 0600
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int mkstemp (char *template)
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Source: [FSF, 2001]
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