According to C99, Section 6.2.5, "Types":
A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.
This behavior is more informally referred to as unsigned integer wrapping. Unsigned integer operations can wrap if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operators can result in wrapping:
Operator 
Wrap 

Operator 
Wrap 

Operator 
Wrap 

Operator 
Wrap 

yes 

yes 

yes 

< 
no 

yes 

yes 

>> 
no 

> 
no 

yes 

/= 
no 

& 
no 

>= 
no 

/ 
no 

%= 
no 

 
no 

<= 
no 
% 
no 

yes 

^ 
no 

== 
no 

++ 
yes 

>>= 
no 

~ 
no 

!= 
no 
 
yes 

&= 
no 

! 
no 

&& 
no 
= 
no 

= 
no 

un + 
no 

 
no 
yes 

^= 
no 

un  
no 

?: 
no 
The following sections examine specific operations that are susceptible to unsigned integer wrap. When operating on small types (smaller than int
), integer conversion rules apply. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand implicit conversion rules before trying to implement secure arithmetic operations (see INT02A. Understand integer conversion rules).
Integer values that originate from untrusted sources must not be allowed to wrap if they are used in any of the following ways:
 as an array index
 in any pointer arithmetic
 as a length or size of an object
 as the bound of an array (for example, a loop counter)
 as an argument to a memory allocation function
 in security critical code
Addition
Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type (see ARR37C. Do not add or subtract an integer to a pointer to a nonarray object and ARR38C. Do not add or subtract an integer to a pointer if the resulting value does not refer to an element within the array for rules about adding a pointer to an integer). Incrementing is equivalent to adding one.
NonCompliant Code Example
This code may result in an unsigned integer wrap during the addition of the unsigned operands ui1
and ui2
. If this behavior is unexpected, the resulting value may be used to allocate insufficient memory for a subsequent operation or in some other manner that could lead to an exploitable vulnerability.
unsigned int ui1, ui2, sum; sum = ui1 + ui2;
Compliant Solution
This compliant solution tests the suspect addition operation to guarantee there is no possibility of unsigned wrap.
unsigned int ui1, ui2, sum; if (UINT_MAX  ui1 < ui2) { /* handle error condition */ } sum = ui1 + ui2;
Subtraction
Subtraction is between two operands of arithmetic type, two pointers to qualified or unqualified versions of compatible object types, or between a pointer to an object type and an integer type. See ARR36C. Do not subtract or compare two pointers that do not refer to the same array, ARR37C. Do not add or subtract an integer to a pointer to a nonarray object, and ARR38C. Do not add or subtract an integer to a pointer if the resulting value does not refer to an element within the array for rules about pointer subtraction. Decrementing is equivalent to subtracting one.
NonCompliant Code Example
This code may result in an unsigned integer wrap during the subtraction of the unsigned operands ui1
and ui2
. If this behavior is unanticipated, it may lead to an exploitable vulnerability.
unsigned int ui1, ui2, result; result = ui1  ui2;
Compliant Solution
This compliant solution tests the suspect unsigned subtraction operation to guarantee there is no possibility of unsigned wrap.
unsigned int ui1, ui2, result; if (ui1 < ui2){ /* handle error condition */ } result = ui1  ui2;
Multiplication
Multiplication is between two operands of arithmetic type.
NonCompliant Code Example
The Mozilla Scalable Vector Graphics (SVG) viewer contains a heap buffer wrap vulnerability resulting from an unsigned integer wrap during the multiplication of the signed int
value pen>num_vertices
and the size_t
value sizeof(cairo_pen_vertex_t)
[[VU#551436]]. The signed int
operand is converted to unsigned int
prior to the multiplication operation (see [INT02A. Understand integer conversion rules]).
pen>num_vertices = _cairo_pen_vertices_needed(gstate>tolerance, radius, &gstate>ctm); pen>vertices = malloc(pen>num_vertices * sizeof(cairo_pen_vertex_t));
The unsigned integer wrap can result in allocating memory of insufficient size.
Compliant Solution
This compliant solution tests the suspect multiplication operation to guarantee that there is no unsigned integer wrap.
pen>num_vertices = _cairo_pen_vertices_needed(gstate>tolerance, radius, &gstate>ctm); if (pen>num_vertices > SIZE_MAX/sizeof(cairo_pen_vertex_t)) { /* handle error condition */ } pen>vertices = malloc(pen>num_vertices * sizeof(cairo_pen_vertex_t));
Left Shift Operator
The left shift operator is between two operands of integer type.
NonCompliant Code Example
This noncompliant code example can result in unsigned wrap left shifting the unsigned operand ui1
by ui2
bits.
unsigned int ui1, ui2, uresult; uresult = ui1 << ui2;
Compliant Solution
This compliant solution tests the suspect shift operation to guarantee there is no possibility of unsigned wrap. This solution must also be compliant with INT36C. Do not shift a negative number of bits or more bits than exist in the operand.
unsigned int ui1, ui2, uresult; if ( (ui2 >= sizeof(unsigned int)*CHAR_BIT)  (ui1 > (UINT_MAX >> ui2))) ) { /* handle error condition */ } else { uresult = ui1 << ui2; }
Exceptions
INT32EX1. Unsigned integers can exhibit modulo behavior (wrapping) only when this behavior is necessary for the proper execution of the program. It is recommended that the variable declaration be clearly commented as supporting modulo behavior and that each operation on that integer also be clearly commented as supporting modulo behavior.
Risk Assessment
Integer wrap can lead to buffer overflows and the execution of arbitrary code by an attacker.
Rule 
Severity 
Likelihood 
Remediation Cost 
Priority 
Level 

INT32C 
high 
likely 
high 
P9 
L2 
Automated Detection
Fortify SCA Version 5.0 with CERT C Rule Pack is able to detect violations of this rule.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
A Linux kernel vmsplice exploit, described at http://www.avertlabs.com/research/blog/index.php/2008/02/13/analyzingthelinuxkernelvmspliceexploit/,
documents a vulnerability and exploit arising directly out of unsigned integer wrapping.
References
[[Dowd 06]] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211223)
[[ISO/IEC 98991999]] Section 6.2.5, "Types," Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types <limits.h>"
[[ISO/IEC PDTR 24772]] "XYY Wraparound Error"
[[Seacord 05]] Chapter 5, "Integers"
[[Viega 05]] Section 5.2.7, "Integer overflow"
[[VU#551436]]
[[Warren 02]] Chapter 2, "Basics"
INT15A. Take care when converting from pointer to integer or integer to pointer 04. Integers (INT)