![]() Format string attack detection & defensesįollowing section describes about how we can detect and defend against format string attacks.X-Original-To: from ( ) Now this memory location can be where return pointer resides, thus overwriting it attackers can take control of the program. So we have seen as to how we can write a number to a memory location. It is being provided, but in this case there no such argument, so it will look at the stack and pick the next item, which is the buffer, and which is loaded with “xa0xfcxfbxaf” which in memory will be interpreted as 0xaffbfca0 (because it is interpreted as little endian) and thus the value 5 is written in this location: What is the memory location one would ask? snprintf will look out for next argument. So the stack will become like thisĪfter this, the next thing that comes in the input is ‘%n’ and as stated earlier this format string is used to store the number of characters, which in this case is 5, and write it in the memory of the next argument. ![]() Look at the above stack diagram and the next value in the stack after input is of integer ‘a’ which is set to 1, so now there are total of five characters in the buffer (4 ASCII+ 1 decimal). After this attacker enters %d, this means to print a decimal integer but what integer will it print. So 2 hexadecimal gets translated into 1 ASCII character so there are 4 ASCII characters in the input. Note that “” which is used to escape and x indicates a hexadecimal number. Let’s understand this input from the user. Snprintf(buffer, sizeof_buffer, “xa0xfcxfbxaf%d%n”) and the stack will be like this: “xa0xfcxfbxaf%d%n”, so that the sprint statement will become This can be collected from the attacker by looking at the source code or by printing the content of stack. Now let’s say the value to change is at address 0xaffbfca0. So the corresponding stack will look this this Now let’s try with a more complex example, Suppose there is some value that attacker wants to change in the stack and following is the program for that: main() Notice we have just written to the memory location of variable ‘test’ using a printf. This will load the number 6 into the memory location of the test. For example, consider the following printf command: printf("Testing%n",&test) The “%n” format is used to store the number of characters before encountering %n. Writing to the stackįor now we have been reading contents from the stack by passing format string as user input but we can also write to the stack. We will see the value if next three hexadecimal values from the buffer. Now this input from the attacker is interpreted as a format string and sprint will fetch the next three hexadecimal values from the stack and load it in the variable buffer. Suppose the attacker enters “%x%x%x” into the input above, then the above sprint statement will become sprintf (buffer,sizeof_buffer, "%x%x%x") ![]() In this sprint statement, there is no format string so it can come under a format string attack. Reading from stackĬonsider the following sprintf statement "sprintf (buffer,sizeof_buffer,input) " To understand this attack even better, we will showcase this attack with the help of an example and then trace it down in stack. Since there is none, printf will print a garbage value which should not be treated as a failure because we can successfully print a garbage value with a printf command. The printf command will dutifully interpret the output as a format for printing a decimal integer, and in turn it will go to the stack to grab an integer. Now you may argue “What is the difference between these two as both of them will compile without any errors?” Imagine if the output is set to “%d” in the first printf. ![]() He uses the following printf: printf(output) Suppose a programmer is writing a code and in that he is using printf statement to print something. These vulnerabilities are associated with the ‘printf’ statement. ![]()
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