The value 0x10000 is too large for a 16 bit binary register, so the addition results in an arithmetic overflow. If we add a and b and store the result in c, the addition would lead to an arithmetic overflow: c = a + b For a, the maximum 16 bit representable value 0xffff (hexadecimal value of 65535) is assigned, and for b the value of 0x1 (hexadecimal value of 1). Let's assume we have three 16 bit unsigned integer values a, b and c. The following example helps to clarify what exactly leads to an arithmetic overflow. Typical process register widths are shown in the following table. Process registers represent an amount of storage available in digital processors and its width defines the range of values that can be represented. To clarify the problem, I'll introduce the term process register. Integer overflows occur when the result of an arithmetic operation is a value, that is too large to fit in the available storage space. In their 2011 report MITRE places integer overflows in the “Top 25 Most Dangerous Software Errors”. These errors are also a source of serious vulnerabilities, such as integer overflow errors in OpenSSH and Firefox, both of which allow attackers to execute arbitrary code. These errors can lead to serious software failures, e.g., a truncation error on a cast of a floating point value to a 16-bit integer played a crucial role in the destruction of Ariane 5 flight 501 in 1996. A short paragraph in Understanding Integer Overflow in C/C++ (Will Dietz, Peng Li, John Regehr, and Vikram Adve) highlights the scope of such errors: overflows are known bugs in C which can lead to exploitable vulnerabilities. Verify the size of input variables to avoid integer overflow vulnerabilities. The fix implemented for this integer overflow in bufferobject.c is if (*size > count – offset) Offset + Size then becomes a negative value and skips the conditional check above. The conditional statement adds offset and size parameters which causes the overflow when large offset and size are given. The vulnerable code snippet in buffer function is the conditional statement: if (offset + *size > count ) In above exploit code, the maximum values in offset and size parameters cause the integer overflow and allows to dump heap. The Buffer function is similar to SubStr function which reads a substring of Size bits from Offset position in a String Object.īelow exploit code reads unlimited heap dumpīuffer function allocates memory on heap. The vulnerability is in buffer function in bufferobject.c fileīuffer(Object, Offset, Size) // buffer’s contents will refer to the base object’s buffer interface, starting as position offset and extending for size bytes Example: Integer overflow in bufferobject.c in Python (CVE-2014-7185) For instance, withdrawing 1 dollar from an account with a balance of 0 could cause an integer underflow and yield a new balance of 4,294,967,295. Integer overflow or underflow could cause significant damage when sensitive data is stored in integers. A buffer overflow can result when the data is copied. However, this value exceeds the maximum for this integer type, so the interpreted value will “wrap around” and becomes -128.Īn integer overflow during a buffer length calculation can result in allocating a buffer that is too small to hold the data to be copied into it. If a programmer stores the value 127 in such a variable and adds 1 to it, the result should be 128. When an integer overflow occurs, the interpreted value will appear to have “wrapped around” the maximum value and gets the minimum value, similar to a clock that represents 13:00 by pointing at 1:00.įor example, an 8-bit signed integer on most common computer architectures has a maximum value of 127 and a minimum value of -128. An Integer Overflow is the condition that occurs when the result of an arithmetic operation, such as multiplication or addition, exceeds the maximum size of the integer type used to store it.
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