fprintf
This function is similar to printf
()
, but it writes the formatted output to a file rather than to the terminal.
Prototype
This function is similar to printf()
, but it writes the formatted output to a file rather than to the terminal. It takes a file pointer as the first argument and a format string as the second argument, and it can take additional arguments that provide the data to be formatted and written to the file.
Vulnerable example
the fprintf()
function is being called with a file pointer and a format string as arguments. The file pointer, output_file
, points to a file called "output.txt"
, and the format string is provided by the user as the first command-line argument.
Prevent
There is multiple ways to prevent format string exploitation :
Check the input for certain characters or patterns that may indicate an attempt to exploit the function, and replace or remove these characters as necessary. For example, check the input for the
%
character, which is used to introduce formatting commands in the format string, and replace it with a different character or remove it entirely.
Use a different function to print the user input. For example, use the
fputs()
function, which writes a string to a file but does not interpret formatting commands.
Use the
%s
formatting command to print the user input, rather than using the user input as the format string itself. This will prevent theprintf()
function from interpreting the user input as a format string.
It is important to choose the right approach based on your specific needs and the requirements of your program. It may also be necessary to combine multiple approaches to effectively prevent format string exploits.
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