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>On Mon, 2002-11-18 at 17:10, Andrew Griffiths wrote:
>
>> Another thing to use is consistency, for example,
>>
>> char dst[50];
>> strncpy(dst, user_supplied_data, sizeof(dst));
>> strncat(dst, sizeof(dst) - strlen(dst) -1, moreuserdata);
>>
>> This could be exploitable if user_supplied_data is 50 or more bytes long.
>>
>> In specific,
>>
>> 50 - 50 - 1 == -1
>
>If sizeof(dst) is 50, then a 0 terminated string is is 49 chars long
>(len(dst) is 49). That means we've got 50-49-1 = 0 which is correct as
>there is no room left in dst.

"strncpy" doesn't NUL terminate, so the strlen() in the above code can
return any value, including values *over* 50. Since the "n"
argument to strncat (which, incidentally, is the 3rd, not 2nd argument),
is unsigned, that would mean unlimited copying.

>Of course in your example you allow dst to overflow in the strncpy.
>Using
> strncpy(dst, user_supplied_data, sizeof(dst)-1);
>would have prevented that if my math is correct.

As long as you NUL terminated the string.

Casper

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