![]() It should be noted that some compilers can help against these kinds of errors. Since you filled it with, basically, gargabe, the address the CPU jumped to did probably contain byte sequences that, interpreted as program text, cause an invalid memory access (or the address itself was already bad). The run-time error you experience may well be due to an overwritten return address. If you were writing an application that ran under a privileged user account, you'd just have provided an attacker with a first-grade entry to your costumer's system, an ACE hole, if you will. He might use something like the NOP sled technique to have your application start a shell for him. Thereby the attacker mounted a buffer overflow attack. (NB: Better use snprintf if it's available to you). string contains an address of the attacker's choosing at just the point where the uncontrolled sprintf function will overwrite the return address on the stack. ![]() |-|īy finding out what exactly the offset between buf and the return address on the stack is, a malicious user may manipulate input to your application in a way that the XXX. Now consider a stack frame of your function. the address of the instruction that is to be executed after the function returns, is pushed to the stack (among other things, typically). (3) When you call a function, the return address, i.e. (2) Strings expect the characters belonging to that string to be stored so that character n 1 is stored after character n. the smaller the addresses, the more the stack is filled. (1) Your stack typically "grows" backwards, i.e. comes from uncontrolled sources, you are very close to generating a buffer overflow vulnerability. And what physically happens next, may well be a major security hole. However, the world does not stop simply because someone did not define what exactly should happen next. As other posters have noted, it invokes UB. It makes a lot of sense to consider what happens in your and, more importantly, similar, cases. This entry was posted in Lesson by dgookin. Next week, I explore another interesting variation on the printf() function, asprintf(). Not only must I emulate its behavior, but I must also reverse engineer the printf() function as a standard I/O function, I couldn’t include its definition in stdio.h, which truly complicates matters. The logic here is that the return value can be compared with the function’s second argument to determine whether overflow would have occurred.Ĭoding my own snprintf() function would be truly complex. The return value, 16, is the same as for the sprintf() function. The output is capped at 12 bytes 11 for the string and one for the null character. R = snprintf(buffer,12,"The total is %d\n",5 8) Here is the snprintf() version of the sample code: ![]() This size value caps the string at n-1 characters (one for the null character), preventing overflow. A new second argument sets the buffer’s size. ) Īs with sprintf(), the first argument is a buffer into which the formatted output is stored. Int snprintf(char * restrict str, size_t size, const char * restrict format. The snprintf() function prevents this overflow from occurring. The output on my Macintosh is far more telling:Įven when the program doesn’t crash, you can see that 16 characters were stored in a 12 character buffer. As with printf(), this value is equal to the characters output, or negative for an error. This code saves the sprintf() function’s return value. R = sprintf(buffer,"The total is %d\n",5 8) No bounds-checking is performed, so the buffer can overflow, as the following code demonstrates: The function’s first argument is a buffer into which the formatted output is placed. Int sprintf(char * restrict str, const char * restrict format. The sprintf() function is part of the family of printf() functions, each of which formats output. Like the strlcpy() and strlcat() functions shown in previous Lessons, snprintf() is safer than its standard Library counterpart, sprintf(). Another non-standard library function is snprintf().
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