c-String vulnerabilities

format string vulnerability
scanf format string vulnerability
formatting string can be done by which analyzer
format string attack prevention
format string vulnerability ctf
format string overwrite return address
format string vulnerability write to address
vsnprintf vulnerability

I was reading about the vulnerabilities in strings in C and then I came across this code. Could anyone give me an explanation why this is happening? Thanks in advance.

int main (int argc, char* argv[]) {
  char a[16];
  char b[16];
  char c[32];
  strncpy(a, "0123456789abcdef", sizeof(a));
  strncpy(b, "0123456789abcdef", sizeof(b));
  strncpy(c, a, sizeof(c));

  printf("a = %s\n", a);
  printf("b = %s\n", b);
  printf("c = %s\n", c);
}

output:

a = 0123456789abcdef0123456789abcdef
b = 0123456789abcdef
c = 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef

Format String Software Attack, Format string attack on the main website for The OWASP Foundation. •The Format Function is an ANSI C conversion function, like printf, fprintf, which the attacker could explore the vulnerability by inserting formatting characters in a form of� For more information, please see the Exploiting Format String Vulnerabilities article from 2001. Similar Functions to printf. The whole printf function family is vulnerable. Here is an example of snprintf:

Reading beyond the end of the string is undefined behavior (UB). With UB there are no guarantees that the code will behave in one way or the other. The behavior can differ on different systems, compilers, linkers, compilation/linking flags, depending on (seemingly) unrelated code, and the version of all of the above.

On many systems, variables sit consecutively on the stack, in reverse order. Replace your printf with:

printf("a (%p) = %s\n", a, a);
printf("b (%p) = %s\n", b, b);
printf("c (%p) = %s\n", c, c);

It prints the addresses of the arrays:

a (0x7fff559adad0) = 0123456789abcdef<F0>ښU<FF>
b (0x7fff559adac0) = 0123456789abcdef0123456789abcdef<F0>ښU<FF>
c (0x7fff559adaa0) = 0123456789abcdef<F0>ښU<FF>

As evident from the addresses, the printout of b starts at address 0x7fff559adac0 but continues well into the address of a (which starts 16 bytes after the start of b).

Also note that the strings have junk at the end. The just is because the '\0' terminator is missing in the string, and printf goes on to read the following junk (UB in its own sake).

This happens because:

strncpy(a, "0123456789abcdef", sizeof(a));

sets a[] to have all its bytes equal to "0123456789abcdef" without a null terminator. Having no '\0' printf does not know where to stop, and will result in UB.

strncpy(b, "0123456789abcdef", sizeof(b));

also sets b[] to have all its bytes equal to "0123456789abcdef" without a null terminator. Here also, any printf causes UB. But this time instead of random junk, it simply reads the next string.

To add insult to injury, the line

 strncpy(c, a, sizeof(c));

reads 32 bytes from a 16 byte array. This is also UB. On your (and my) system it readsa and a lot of junk after it. In theory, this can crash your program with access-violation or a segmentation fault.

Some viruses and worms use such overflows to read or write data they are not supposed to.

Format String Vulnerabilities, Never let the user input format string/s; Buffer Overflow. 1. Never let the user input format string/s. What you should not do. #FormatString.c #include <include.h>. c-String vulnerabilities. Ask Question Asked 9 months ago. Active 9 months ago. Viewed 107 times 1. I was reading about the vulnerabilities in strings in C and then I

strncpy is a dangerous function, because it only adds null termination if there is room left. This is what happens in your code, you copy exactly 16 bytes

The strncpy function was actually never intended to be used for C strings, but for an ancient Unix string format that didn't use null termination. It is a function that should be avoided for most purposes. In particular, it is not a "safe version of strcpy" - but a more dangerous function than strcpy, as we can see from the bugs here.

The solution is to check the size to copy in advance, before you copy. And then use strcpy. For example:

char a[16];
const char to_copy[] = "0123456789abcdef";
_Static_assert(sizeof(to_copy) <= sizeof(a), "to_copy is too big");
strcpy(a, to_copy);

To fix your current program, you need to allocate room for the null terminator, like this:

#include <string.h>
#include <stdio.h>

int main (void)
{
  char a[16+1];
  char b[16+1];
  char c[32+1];
  const char to_copy[] = "0123456789abcdef";
  _Static_assert(sizeof(to_copy) <= sizeof(a), "to_copy is too big");
  _Static_assert(sizeof(to_copy) <= sizeof(b), "to_copy is too big");

  strcpy(a, to_copy);
  strcpy(b, to_copy);
  strcpy(c, a);

  printf("a = %s\n", a);
  printf("b = %s\n", b);
  printf("c = %s\n", c);
}

Format String Vulnerability and Prevention with Example , A simple C program with format. // string vulnerability. #include<stdio.h>. int main( int argc, char ** argv). {. char buffer[100];. strncpy (buffer� Misuse of format strings creates vulnerabilities The definition of printf assumes that a developer will always use a defined format string with additional arguments to handle any untrusted user input.

c-String vulnerabilities, The n in strncpy does not mean the same as the n in strncat or snprintf ; strncpy was born to manipulate fixed size buffer strings such as� This article covers how to exploit format String vulnerabilities to fetch and write arbitrary values to and from the stack. Overview : In this article, we will learn what Format String Vulnerabilities are, how we exploit it to read specific values from the stack, further we will also have a look at how we can use different format specifiers to write arbitrary values to the stack.

2.3. String Vulnerabilities and Exploits, Buffer overflows are a primary source of software vulnerabilities. Type-unsafe languages, such as C and C++, are especially prone to such� I was reading about vulnerabilities in code and came across this Format-String Vulnerability.. Wikipedia says:. Format string bugs most commonly appear when a programmer wishes to print a string containing user supplied data.

String Vulnerabilities | Secure Coding in C and C++: Strings, 2.3 String Vulnerabilities. Figure 2–9 shows a simple example of a program that checks a user password and grants or denies access. 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

Comments
  • First of all, there are cases when strncpy will not add the null-terminator. And secondly C have no bounds-checking of any kind. It's your responsibility as a programmer to make sure strings are terminated and that your code doesn't go out of bounds.
  • If the second string length is longer than the specified length, the string is unterminated. If that isn't corrected, the behaviour of the print() statements is undefined. Set the last character of each array to zero AFTER the call to strncpy(). For example, a[sizeof(a)-1] = '\0'