Format string
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This document discusses format string vulnerabilities in C programs. It explains that if a program passes user input directly to a printf statement, an attacker can craft the input as a format string to control how the printf function operates. This allows viewing memory, crashing programs, and writing arbitrary values to memory by abusing how printf interprets format specifiers like %s, %x and %n. Countermeasures include compiler checks on format string usage and address space randomization.
![Lecture Notes (Syracuse University) Format String Vulnerability: 3
– For each %s, printf() will fetch a number from the stack, treat this number as an address,
and print out the memory contents pointed by this address as a string, until a NULL character
(i.e., number 0, not character 0) is encountered.
– Since the number fetched by printf() might not be an address, the memory pointed by this
number might not exist (i.e. no physical memory has been assigned to such an address), and the
program will crash.
– It is also possible that the number happens to be a good address, but the address space is protected
(e.g. it is reserved for kernel memory). In this case, the program will also crash.
• Viewing the stack
printf ("%08x %08x %08x %08x %08xn");
– This instructs the printf-function to retrieve five parameters from the stack and display them as
8-digit padded hexadecimal numbers. So a possible output may look like:
40012980 080628c4 bffff7a4 00000005 08059c04
• Viewing memory at any location
– We have to supply an address of the memory. However, we cannot change the code; we can only
supply the format string.
– If we use printf(%s) without specifying a memory address, the target address will be ob-
tained from the stack anyway by the printf() function. The function maintains an initial
stack pointer, so it knows the location of the parameters in the stack.
– Observation: the format string is usually located on the stack. If we can encode the target address
in the format string, the target address will be in the stack. In the following example, the format
string is stored in a buffer, which is located on the stack.
int main(int argc, char *argv[])
{
char user_input[100];
... ... /* other variable definitions and statements */
scanf("%s", user_input); /* getting a string from user */
printf(user_input); /* Vulnerable place */
return 0;
}
– If we can force the printf to obtain the address from the format string (also on the stack), we can
control the address.
printf ("x10x01x48x08 %x %x %x %x %s");
– x10x01x48x08 are the four bytes of the target address. In C language, x10 in a string
tells the compiler to put a hexadecimal value 0x10 in the current position. The value will take
up just one byte. Without using x, if we directly put "10" in a string, the ASCII values of the
characters ’1’ and ’0’ will be stored. Their ASCII values are 49 and 48, respectively.](https://image.slidesharecdn.com/formatstring-180304180708/85/Format-string-3-320.jpg)
![Lecture Notes (Syracuse University) Format String Vulnerability: 4
– %x causes the stack pointer to move towards the format string.
– Here is how the attack works if user input[] contains the following format string:
"x10x01x48x08 %x %x %x %x %s".
...
Addressofuser_input[]
0x10014808
user_input [ ]
%s
%x
%x
%x
%x
Print this
for the 1st %x
Print this
for the 4th %x
For %s: print out the contents pointed by this address
Print out the contents at the address 0x10014808 using format-string vlunerability
– Basically, we use four %x to move the printf()’s pointer towards the address that we stored
in the format string. Once we reach the destination, we will give %s to print(), causing it
to print out the contents in the memory address 0x10014808. The function printf() will
treat the contents as a string, and print out the string until reaching the end of the string (i.e. 0).
– The stack space between user input[] and the address passed to the printf() function
is not for printf(). However, because of the format-string vulnerability in the program,
printf() considers them as the arguments to match with the %x in the format string.
– The key challenge in this attack is to figure out the distance between the user input[] and
the address passed to the printf() function. This distance decides how many %x you need to
insert into the format string, before giving %s.
• Writing an integer to nearly any location in the process memory
– %n: The number of characters written so far is stored into the integer indicated by the corre-
sponding argument.
int i;
printf ("12345%n", &i);
– It causes printf() to write 5 into variable i.
– Using the same approach as that for viewing memory at any location, we can cause printf()
to write an integer into any location. Just replace the %s in the above example with %n, and the
contents at the address 0x10014808 will be overwritten.](https://image.slidesharecdn.com/formatstring-180304180708/85/Format-string-4-320.jpg)
Format string
- 1. Lecture Notes (Syracuse University) Format String Vulnerability: 1 Format String Vulnerability printf ( user input ); The above statement is quite common in C programs. In the lecture, we will find out what can go wrong if the program is running with privileges (e.g. Set-UID program). 1 Format String • What is a format string? printf ("The magic number is: %dn", 1911); The text to be printed is “The magic number is:”, followed by a format parameter ‘%d’, which is replaced with the parameter (1911) in the output. Therefore the output looks like: The magic number is: 1911. In addition to %d, there are several other format parameters, each having different meaning. The following table summarizes these format parameters: Parameter Meaning Passed as ------------------------------------------------------------------- %d decimal (int) value %u unsigned decimal (unsigned int) value %x hexadecimal (unsigned int) value %s string ((const) (unsigned) char *) reference %n number of bytes written so far, (* int) reference • The stack and its role at format strings The behavior of the format function is controlled by the format string. The function retrieves the parameters requested by the format string from the stack. printf ("a has value %d, b has value %d, c is at address: %08xn", a, b, &c);
- 2. Lecture Notes (Syracuse University) Format String Vulnerability: 2 Addressof FormatString Valueofa Valueofb Adressofc printf()’s internal pointer Moving in this direction Stack Stack grows in this direction • What if there is a miss-match between the format string and the actual arguments? printf ("a has value %d, b has value %d, c is at address: %08xn", a, b); – In the above example, the format string asks for 3 arguments, but the program actually provides only two (i.e. a and b). – Can this program pass the compiler? ∗ The function printf() is defined as function with variable length of arguments. There- fore, by looking at the number of arguments, everything looks fine. ∗ To find the miss-match, compilers needs to understand how printf() works and what the meaning of the format string is. However, compilers usually do not do this kind of analysis. ∗ Sometimes, the format string is not a constant string, it is generated during the execution of the program. Therefore, there is no way for the compiler to find the miss-match in this case. – Can printf() detect the miss-match? ∗ The function printf() fetches the arguments from the stack. If the format string needs 3 arguments, it will fetch 3 data items from the stack. Unless the stack is marked with a boundary, printf() does not know that it runs out of the arguments that are provided to it. ∗ Since there is no such a marking. printf() will continue fetching data from the stack. In a miss-match case, it will fetch some data that do not belong to this function call. – What trouble can be caused by printf() when it starts to fetch data that is meant for it? 2 Attacks on Format String Vulnerability • Crashing the program printf ("%s%s%s%s%s%s%s%s%s%s%s%s");
- 3. Lecture Notes (Syracuse University) Format String Vulnerability: 3 – For each %s, printf() will fetch a number from the stack, treat this number as an address, and print out the memory contents pointed by this address as a string, until a NULL character (i.e., number 0, not character 0) is encountered. – Since the number fetched by printf() might not be an address, the memory pointed by this number might not exist (i.e. no physical memory has been assigned to such an address), and the program will crash. – It is also possible that the number happens to be a good address, but the address space is protected (e.g. it is reserved for kernel memory). In this case, the program will also crash. • Viewing the stack printf ("%08x %08x %08x %08x %08xn"); – This instructs the printf-function to retrieve five parameters from the stack and display them as 8-digit padded hexadecimal numbers. So a possible output may look like: 40012980 080628c4 bffff7a4 00000005 08059c04 • Viewing memory at any location – We have to supply an address of the memory. However, we cannot change the code; we can only supply the format string. – If we use printf(%s) without specifying a memory address, the target address will be ob- tained from the stack anyway by the printf() function. The function maintains an initial stack pointer, so it knows the location of the parameters in the stack. – Observation: the format string is usually located on the stack. If we can encode the target address in the format string, the target address will be in the stack. In the following example, the format string is stored in a buffer, which is located on the stack. int main(int argc, char *argv[]) { char user_input[100]; ... ... /* other variable definitions and statements */ scanf("%s", user_input); /* getting a string from user */ printf(user_input); /* Vulnerable place */ return 0; } – If we can force the printf to obtain the address from the format string (also on the stack), we can control the address. printf ("x10x01x48x08 %x %x %x %x %s"); – x10x01x48x08 are the four bytes of the target address. In C language, x10 in a string tells the compiler to put a hexadecimal value 0x10 in the current position. The value will take up just one byte. Without using x, if we directly put "10" in a string, the ASCII values of the characters ’1’ and ’0’ will be stored. Their ASCII values are 49 and 48, respectively.
- 4. Lecture Notes (Syracuse University) Format String Vulnerability: 4 – %x causes the stack pointer to move towards the format string. – Here is how the attack works if user input[] contains the following format string: "x10x01x48x08 %x %x %x %x %s". ... Addressofuser_input[] 0x10014808 user_input [ ] %s %x %x %x %x Print this for the 1st %x Print this for the 4th %x For %s: print out the contents pointed by this address Print out the contents at the address 0x10014808 using format-string vlunerability – Basically, we use four %x to move the printf()’s pointer towards the address that we stored in the format string. Once we reach the destination, we will give %s to print(), causing it to print out the contents in the memory address 0x10014808. The function printf() will treat the contents as a string, and print out the string until reaching the end of the string (i.e. 0). – The stack space between user input[] and the address passed to the printf() function is not for printf(). However, because of the format-string vulnerability in the program, printf() considers them as the arguments to match with the %x in the format string. – The key challenge in this attack is to figure out the distance between the user input[] and the address passed to the printf() function. This distance decides how many %x you need to insert into the format string, before giving %s. • Writing an integer to nearly any location in the process memory – %n: The number of characters written so far is stored into the integer indicated by the corre- sponding argument. int i; printf ("12345%n", &i); – It causes printf() to write 5 into variable i. – Using the same approach as that for viewing memory at any location, we can cause printf() to write an integer into any location. Just replace the %s in the above example with %n, and the contents at the address 0x10014808 will be overwritten.
- 5. Lecture Notes (Syracuse University) Format String Vulnerability: 5 – Using this attack, attackers can do the following: ∗ Overwrite important program flags that control access privileges ∗ Overwrite return addresses on the stack, function pointers, etc. – However, the value written is determined by the number of characters printed before the %n is reached. Is it really possible to write arbitrary integer values? ∗ Use dummy output characters. To write a value of 1000, a simple padding of 1000 dummy characters would do. ∗ To avoid long format strings, we can use a width specification of the format indicators. • Countermeasures. – Address randomization: just like the countermeasures used to protect against buffer-overflow attacks, address randomization makes it difficult for the attackers to find out what address they want to read/write.