Reviewing Code for Buffer Overruns and Overflows

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OWASP Code Review Guide Table of Contents

Contents


The buffer

A Buffer is an amount of contiguous memory set aside for storing information. Example: A program has to remember certain things, like what your shopping cart contains or what data was inputted prior to the current operation this information is stored in memory in a buffer.

How to locate the potentially vulnerable code

In locating potentially vulnerable code from a buffer overflow standpoint one should look for particular signatures such as:

Arrays:

 int x[20];
 int y[20][5];
 int x[20][5][3];

Format Strings:

 printf() ,fprintf(), sprintf(), snprintf().
 %x,  %s, %n, %d, %u, %c, %f

Over flows:

 strcpy (), strcat (), sprintf (), vsprintf ()


Vulnerable Patterns for buffer overflows

‘Vanilla’ buffer overflow:

Example: A program might want to keep track of the days of the week (7). The programmer tells the computer to store a space for 7 numbers. This is an example of a buffer. But what happens if an attempt to add 8 numbers is performed? Languages such as C and C++ do not perform bounds checking and therefore if the program is written in such a language the 8th piece of data would overwrite the program space of the next program in memory would result in data corruption. This can cause the program to crash at a minimum or a carefully crafted overflow can cause malicious code to be executed, as the overflow payload is actual code.


 void copyData(char *userId) {  
    char  smallBuffer[10]; // size of 10  
    strcpy(smallBuffer, userId);
 }  
 int main(int argc, char *argv[]) {  
 char *userId = "01234567890"; // Payload of 11
 copyData (userId); // this shall cause a buffer overload
 }


Buffer overflows are the result of stuffing more code into a buffer than it is meant to hold.


The Format String:

A format function is a function within the ANSI C specification. It can be used to tailor primitive C data types to human readable form. They are used in nearly all C programs to output information, print error messages, or process strings.

Some format parameters:

 %x        hexadecimal (unsigned int)
 %s        string ((const) (unsigned) char *)
 %n        number of bytes written so far, (* int)
 %d        decimal (int)
 %u        unsigned decimal (unsigned int)


Example:


 printf ("Hello: %s\n", a273150);


The %s in this case ensures that the parameter (a273150) is printed as a string.

Through supplying the format string to the format function we are able to control the behaviour of it. So supplying input as a format string makes our application do things its not meant to! What exactly are we able to make the application do?

Crashing an application:

 printf (User_Input);

If we supply %x (hex unsigned int) as the input, the printf function shall expect to find an integer relating to that format string, but no argument exists. This can not be detected at compile time. At runtime this issue shall surface.

Walking the stack:

For every % in the argument the printf function finds it assumes that there is an associated value on the stack. In this way the function walks the stack downwards reading the corresponding values from the stack and printing them to user

Using format strings we can execute some invalid pointer access by using a format string such as:


 printf ("%s%s%s%s%s%s%s%s%s%s%s%s");


Worse again is using the %n directive in printf(). This directive takes an int* and writes the number of bytes so far to that location.

Where to look for this potential vulnerability. This issue is prevalent with the printf() family of functions, printf(),fprintf(), sprintf(), snprintf(). Also syslog() (writes system log information) and setproctitle(const char *fmt, ...); (which sets the string used to display process identifier information).


Integer overflows:

  1. include <stdio.h>
   int main(void){
           int val;
           val = 0x7fffffff; 	/* 2147483647*/
           printf("val = %d (0x%x)\n", val, val);
           printf("val + 1 = %d (0x%x)\n", val + 1 , val + 1); /*Overflow the int*/
           return 0;
   }


The binary representation of 0x7fffffff is 1111111111111111111111111111111; this integer is initialised with the highest positive value a signed long integer can hold.

Here when we add 1 to the hex value of 0x7fffffff the value of the integer overflows and goes to a negative number (0x7fffffff + 1 = 80000000) In decimal this is (-2147483648). Think of the problems this may cause!! Compilers will not detect this and the application will not notice this issue.

We get these issues when we use signed integers in comparisons or in arithmetic and also comparing signed integers with unsigned integers

Example:

int myArray[100];

   int fillArray(int v1, int v2){
       if(v2 > sizeof(myArray) / sizeof(int)){
           return -1; /* Too Big !! */
       }
       myArray [v2] = v1;
       return 0;
   }

Here if v2 is a massive negative number so the if condition shall pass. This condition checks to see if v2 is bigger than the array size. The line myArray[v2] = v1 assigns the value v1 to a location out of the bounds of the array causing unexpected results.

Good Patterns & procedures to prevent buffer overflows:

Example:

void copyData(char *userId) {
   char  smallBuffer[10]; // size of 10
   strncpy(smallBuffer, userId, 10); // only copy first 10 elements
   smallBuffer[9] = 0; // Make sure it is terminated.
}

int main(int argc, char *argv[]) {
   char *userId = "01234567890"; // Payload of 11
   copyData (userId); // this shall cause a buffer overload
}

The code above is not vulnerable to buffer overflow as the copy functionality uses a specified length, 10.

C library functions such as strcpy (), strcat (), sprintf () and vsprintf () operate on null terminated strings and perform no bounds checking. gets () is another function that reads input (into a buffer) from stdin until a terminating newline or EOF (End of File) is found. The scanf () family of functions also may result in buffer overflows.

Using strncpy(), strncat(), snprintf(), and fgets() all mitigate this problem by specifying the maximum string length. The details are slightly different and thus understanding their implications is required.

Always check the bounds of an array before writing it to a buffer.

The Microsoft C runtime also provides additional versions of many functions with an _s suffix (strcpy_s, strcat_s, sprintf_s). These functions perform additional checks for error conditions and call an error handler on failure. (See Security Enhancements in the CRT)

.NET & Java

C# or C++ code in the .NET framework can be immune to buffer overflows if the code is managed. Managed code is code executed by a .NET virtual machine, such as Microsoft's. Before the code is run, the Intermediate Language is compiled into native code. The managed execution environments own runtime-aware complier performs the compilation; therefore the managed execution environment can guarantee what the code is going to do. The Java development language also does not suffer from buffer overflows; as long as native methods or system calls are not invoked, buffer overflows are not an issue. Finally ASP pages are also inmune to buffer overflows due to Integer Overflow checks performed by the VBScript intepreter while executing the code.