Difference between revisions of "Overflow Binary Resource File"

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(Examples)
(Examples)
Line 37: Line 37:
 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 
</pre>
 
</pre>
Attempt to run one more time application will end with:
+
After an attempt to run one more time, the application will end with:
 
<pre>
 
<pre>
 
rezos@dojo-labs ~/owasp/binary $ ./read_binary_file
 
rezos@dojo-labs ~/owasp/binary $ ./read_binary_file
 
Segmentation fault
 
Segmentation fault
 
</pre>
 
</pre>
failure. Was it buffer overflow?
+
Failure. Was it buffer overflow?
 
<pre>
 
<pre>
 
rezos@dojo-labs ~/owasp/binary $ gdb -q ./read_binary_file
 
rezos@dojo-labs ~/owasp/binary $ gdb -q ./read_binary_file
Line 52: Line 52:
 
0xb7e4b9e3 in strcpy () from /lib/libc.so.6
 
0xb7e4b9e3 in strcpy () from /lib/libc.so.6
 
</pre>
 
</pre>
Yes, definietly that was a buffer overflow in a strcpy() function.
+
Yes, that was a buffer overflow in a strcpy() function.
  
 
Why?
 
Why?
  
fread(b, sizeof(b), 1, f); - it reads characters from the stream f, sizeof(b) once, to the buffer b. It looks OK.
+
fread(b, sizeof(b), 1, f); - reads characters from the stream f, sizeof(b) once, to the buffer b. It looks OK.
 
However there is no room for a '\0', which terminates the string.
 
However there is no room for a '\0', which terminates the string.
  
During executing strcpy(p, b); where both buffers are equal overflow takes place. What causes it is the absence of the
+
Whiile executing strcpy(p, b); where both buffers are equal, overflow takes place. What causes it is the absence of the
 
null byte/terminating character in a buffer b[]. The strcpy() function will copy into the buffer p[] everything starting
 
null byte/terminating character in a buffer b[]. The strcpy() function will copy into the buffer p[] everything starting
 
in b[0] and ending on the null byte. The attacker has successfully conducted the buffer overflow attack by crafting
 
in b[0] and ending on the null byte. The attacker has successfully conducted the buffer overflow attack by crafting

Revision as of 08:30, 4 August 2008

This is an Attack. To view all attacks, please see the Attack Category page.


Description

The source of the buffer overflows may be input data. When it comes from the Overflow Binary Resource File, the attacker has to modify/prepare binary file in such a way that the application, after reading this file, has become prone to a classic buffer overflow attack. The only difference between this attack and the classic one is the source of the input data. Common examples are specially crafted MP3, JPEG or ANI files, which cause buffer overflows.

Examples

The application reads the first 8 characters from the binary file.

rezos@dojo-labs ~/owasp/binary $ cat read_binary_file.c
#include <stdio.h>
#include <string.h>

int main(void)
{
       FILE *f;
       char p[8];
       char b[8];

       f = fopen("file.bin", "r");
       fread(b, sizeof(b), 1, f);
       fclose(f);

       strcpy(p, b);

       printf("%s\n", p);

       return 0;
}

The crafted file contaions more than 8 characters.

rezos@dojo-labs ~/owasp/binary $ cat file.bin
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

After an attempt to run one more time, the application will end with:

rezos@dojo-labs ~/owasp/binary $ ./read_binary_file
Segmentation fault

Failure. Was it buffer overflow?

rezos@dojo-labs ~/owasp/binary $ gdb -q ./read_binary_file
Using host libthread_db library "/lib/libthread_db.so.1".
(gdb) r
Starting program: /home/rezos/owasp/binary/read_binary_file

Program received signal SIGSEGV, Segmentation fault.
0xb7e4b9e3 in strcpy () from /lib/libc.so.6

Yes, that was a buffer overflow in a strcpy() function.

Why?

fread(b, sizeof(b), 1, f); - reads characters from the stream f, sizeof(b) once, to the buffer b. It looks OK. However there is no room for a '\0', which terminates the string.

Whiile executing strcpy(p, b); where both buffers are equal, overflow takes place. What causes it is the absence of the null byte/terminating character in a buffer b[]. The strcpy() function will copy into the buffer p[] everything starting in b[0] and ending on the null byte. The attacker has successfully conducted the buffer overflow attack by crafting special file.

References:

Related Threats

Related Attacks

Related Vulnerabilities

Related Countermeasures

1) Use safe equivalent functions, which check the buffers length, whenever it's possible.

Namely:

  • gets() -> fgets()
  • strcpy() -> strncpy()
  • strcat() -> strncat()
  • sprintf() -> snprintf()

2) These functions which doesn't have their safe equivalents should be rewritten with safe checks implemented. Time spent on that will benefit in the future. Remember that you have to do it only once.

3) Use compilers, which are able to identify unsafe functions, logic errors and check if the memory is overwritten when and where it shouldn't be.

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