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→Integer-overflow prevention: Attempt to rewrite section which in its totality didn't make sense before. Also made clear that the language is C++; other languages might limit the size of an "unsigned int" only to available system memory and/or throw an exception on overflow, in which case this problem doesn't exist in that language. Comparison to MAX also changed from < to <=, as the name MAX (="maximum") implies that the value MAX is a valid value. |
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[[Buffer overflow]]s, a common software security vulnerability, happen when a process tries to store data beyond a fixed-length buffer. For example, if there are 8 slots to store items in, there will be a problem if there is an attempt to store 9 items. In computer memory the overflowed data may overwrite data in the next ___location which can result in a security vulnerability (stack smashing) or program termination (segmentation fault).<ref name="bss2001"/>
An example of a [[C (programming language)|C]] program prone to a buffer overflow is<syntaxhighlight lang="c++">
int vulnerable_function(char * large_user_input) {
char dst[SMALL];
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[[Integer overflow]] occurs when an arithmetic operation results in an integer too large to be represented within the available space. A program which does not properly check for integer overflow introduces potential software bugs and exploits.
Below is a
<syntaxhighlight lang="c++">
bool
unsigned int sum = x + y;
return sum <= MAX;
}
</syntaxhighlight>
The problem with the code is it does not check for integer overflow on the addition operation. If the sum of x and y is greater than the maximum possible value of an <code>unsigned int</code>, the addition operation will overflow and perhaps<!-- Note that an overflow will not always result in the calculated sum being less than MAX; MAX might be relatively small and both x and y relatively big, so even an overflow might still be greater than MAX. Example: x=y=UINT_MAX, MAX=1000000. --> result in a value less than or equal to MAX, even though the sum of x and y is greater than MAX.
Below is a program which checks for overflow by confirming the sum is greater than or equal to both x and y. If the sum did overflow, the sum would be less than x or less than y.
<syntaxhighlight lang="c++">
bool
unsigned int sum = x + y;
return sum >= x && sum >= y && sum <= MAX;
}
</syntaxhighlight>
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