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 integer types (smaller than int
), integer promotions are applied. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand integer 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:
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 a valid array element for rules about adding a pointer to an integer). Incrementing is equivalent to adding one.
This noncompliant code example 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; 
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 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 a valid array element for rules about pointer subtraction. Decrementing is equivalent to subtracting one.
This noncompliant code example 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; 
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 is between two operands of arithmetic type.
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)}} \[[VU#551436AA. C References#VU551436]\]. 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.
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) ); 
The left shift operator is between two operands of integer type.
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; 
This compliant solution tests the suspect shift operation to guarantee there is no possibility of unsigned wrap. This solution must also be compliant with INT34C. 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; } 
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.
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 
Fortify SCA Version 5.0 with the CERT C Rule Pack is able to detect violations of this rule.
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
A Linux kernel vmsplice [exploitBB. Definitions#exploit], described at \[[Wojtczuk 08AA. C References#Wojtczuk 08]\], documents a vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping). 
\[[Dowd 06AA. C References#Dowd 06]\] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211223) \[[ISO/IEC 9899:1999AA. C References#ISO/IEC 98991999]\] Section 6.2.5, "Types," Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types {{<limits.h>}}" \[[ISO/IEC PDTR 24772AA. C References#ISO/IEC PDTR 24772]\] "XYY Wraparound Error" \[[Seacord 05AA. C References#Seacord 05]\] Chapter 5, "Integers" \[[Viega 05AA. C References#Viega 05]\] Section 5.2.7, "Integer overflow" \[[VU#551436AA. C References#VU551436]\] \[[Warren 02AA. C References#Warren 02]\] Chapter 2, "Basics" \[[Wojtczuk 08AA. C References#Wojtczuk 08]\] 
INT15A. Use intmax_t or uintmax_t for for formatted IO on programmerdefined integer types 04. Integers (INT)