According to C99, Section The C Standard, 6.2.5, "Types"paragraph 9 [ISO/IEC 9899:2011], states
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 informally called 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 

No 

No 
Yes 

No 

Yes 

no
Although unsigned left shift <<
can result in wrapping, modulo behavior is permitted by this standard because of common usage, because this behavior is usually expected by the programmer, and because the behavior is welldefined.
No 
The following sections examine specific operations that are susceptible to unsigned integer wrap. When operating on small integer types (smaller with less precision 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 Programmers should understand integer conversion rules before trying to implement secure arithmetic operations. (See recommendation INT02C. Understand integer conversion rules.)
Integer values must not be allowed to wrap, especially if they are used in any of the following ways:
 as an array index
 in Integer operands of any pointer arithmetic
 as a length or size of an object
 as the bound of an array (for example, a loop counter)
 as , including array indexing
 The assignment expression for the declaration of a variable length array
 The postfix expression preceding square brackets
[]
or the expression in square brackets[]
of a subscripted designation of an element of an array object  Function arguments of type
size_t
orrsize_t
(for example, an argument to a memory allocation function)  in In securitycritical code
The C Standard defines arithmetic on atomic integer types as readmodifywrite operations with the same representation as regular integer types. As a result, wrapping of atomic unsigned integers is identical to regular unsigned integers and should also be prevented or detected.
Anchor  


Addition
Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type. This rule applies only to addition between two operands of arithmetic type. (See rules ARR37C. Do not add or subtract an integer to a pointer to a nonarray object and ARR38ARR30C. Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array element for information about adding a pointer to an integer.) form or use outofbounds pointers or array subscripts.)
Incrementing is equivalent to adding one1.
Noncompliant Code Example
This noncompliant code example may can result in an unsigned integer wrap during the addition of the unsigned operands ui1
and ui2
ui_a
and ui_b
. 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 can lead to an exploitable vulnerability.
Code Block  

 
void func(unsigned int ui1ui_a, ui2,unsigned usum; /* Initialize ui1 and ui2 */ int ui_b) { unsigned int usum = ui1ui_a + ui2ui_b; /* ... */ } 
Compliant Solution (
...
Precondition Test)
This compliant solution performs a precondition precondition test of the operands of the addition to guarantee there is no possibility of unsigned wrap.:
Code Block  

 
#include <limits.h> void func(unsigned int ui1ui_a, ui2,unsigned usum; /* Initialize ui1 and ui2 */ int ui_b) { unsigned int usum; if (UINT_MAX  ui1ui_a < ui2ui_b) { /* handleHandle error condition */ } else { usum = ui1ui_a + ui2ui_b; } /* ... */ } 
Compliant Solution (
...
Postcondition Test)
This compliant solution performs a postcondition postcondition test to ensure that the result of the unsigned addition operation usum
is not less than either of the operands.the first operand:
Code Block  

 
void func(unsigned int ui1ui_a, ui2,unsigned usum; /* Initialize ui1 and ui2 */ int ui_b) { unsigned int usum = ui1ui_a + ui2ui_b; if (usum < ui1  usum < ui2) { ui_a) { /* handleHandle error condition **/ } /* ... */ } 
Anchor  


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 rules This rule applies only to subtraction between two operands of arithmetic 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 ARR38and ARR30C. Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array elementform or use outofbounds pointers or array subscripts for information about pointer subtraction.)
Decrementing is equivalent to subtracting one1.
Noncompliant Code Example
This noncompliant code example may can result in an unsigned integer wrap during the subtraction of the unsigned operands ui1
and ui2
ui_a
and ui_b
. If this behavior is unanticipated, it may lead to an exploitable vulnerability.
Code Block  

 
void func(unsigned int ui1ui_a, ui2,unsigned udiff; /* Initialize ui1 and ui2 */ int ui_b) { unsigned int udiff = ui1ui_a  ui2ui_b; /* ... */ } 
Compliant Solution (
...
Precondition Test)
This compliant solution performs a precondition precondition test of the unsigned operands of the subtraction operation to guarantee there is no possibility of unsigned wrap.:
Code Block  

 
void func(unsigned int ui1ui_a, ui2,unsigned udiff; /* Initialize ui1 and ui2 */ if (ui1 < ui2int ui_b) { unsigned int udiff; if (ui_a < ui_b){ /* handleHandle error condition */ } else { udiff = ui1ui_a  ui2ui_b; } /* ... */ } 
Compliant Solution (
...
Postcondition Test)
This compliant solution performs a postcondition postcondition test that the result of the unsigned subtraction operation udiff
is not greater than the minuend.:
Code Block  

 
void func(unsigned int ui1ui_a, ui2,unsigned udiff ; /* Initialize ui1 and ui2 */ int ui_b) { unsigned int udiff = ui1ui_a  ui2ui_b; if (udiff > ui1ui_a) { /* handleHandle error condition **/ } /* ... */ } 
Anchor  


Multiplication
Multiplication is between two operands of arithmetic type.
Noncompliant Code Example
Wiki MarkupThe Mozilla Scalable Vector Graphics (SVG) viewer contains a heap buffer overflow vulnerability resulting from an unsigned integer wrap during the multiplication of the {{signed int}} value {{Mozilla Foundation Security Advisory 200701 describes a heap buffer overflow vulnerability in the Mozilla Scalable Vector Graphics (SVG) viewer 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. Bibliography#VU551436]\]. The {{signed
int
}} operand is converted to {{size_t
}} prior to the multiplication operation so that the multiplication takes place between two {{size_t
}} integers, which are unsigned. (See recommendation [INT02C. Understand integer conversion rules].)
Code Block  


 
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 operands of the multiplication to guarantee that there is no unsigned integer wrap.:
Code Block  


 
pen>num_vertices = _cairo_pen_vertices_needed( gstate>tolerance, radius, &gstate>ctm ); if (pen>num_vertices > SIZE_MAX / sizeof(cairo_pen_vertex_t)) { /* handleHandle error condition */ } pen>vertices = malloc( pen>num_vertices * sizeof(cairo_pen_vertex_t) ); 
Exceptions
INT30CEX1.: 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.
INT32INT30CEX2.: Checks for wraparound can be omitted when it can be determined at compile  time that wraparound will not occur. As such, the following operations on unsigned integers require no validation:
 Operations on two compiletime constants
 Operations on a variable and 0 (except division or remainder by 0, of course)
 Subtracting any variable from its type's maximum. For instance; for example, any
unsigned int
may safely be subtracted fromUINT_MAX
.  Multiplying any variable by 1
 Division or remainder, as long as the divisor is nonzero.
 Rightshifting any type maximum by any number smaller no larger than the type size. For instanceprecision; for example,
UINT_MAX >> x
is valid as long asx < sizeof(unsigned int)
.Leftshifting 1 by any number smaller than the type size0 <= x < 32
(assuming that the precision ofunsigned int
is 32 bits)
Anchor  


INT30CEX3. The leftshift operator takes two operands of integer type. Unsigned left shift <<
can exhibit modulo behavior (wrapping). This exception is provided because of common usage, because this behavior is usually expected by the programmer, and because the behavior is well defined. For examples of usage of the leftshift operator, see INT34C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand.
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 

INT30C 
High 
Likely 
High  P9  L2 
Automated Detection
Tool  Version  Checker  Description 

Astrée 
Fortify SCA
Section 

V. 5.0 
Section 

can detect violations of this rule with the CERT C Rule Pack 
Section 

Compass/ROSE 
Sectioncan
 integeroverflow  Fully checked  
CodeSonar 
 ALLOC.SIZE.ADDOFLOW  Addition overflow of allocation size  
Compass/ROSE  Can detect violations of this rule by ensuring that operations are checked for overflow before being performed 
(Be mindful of exception INT30EX2 because it excuses many operations from requiring validation, including all the operations that would validate a potentially dangerous operation. For instance, adding two 
s together requires validation involving subtracting one of the numbers from 
because it cannot wrap.) 
Related Vulnerabilities
Wiki Markup 

[CVE20091385http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE20091385] results from a violation of this rule. The value performs an unchecked subtraction on the {{length}} of a buffer, and then adds that many bytes of data to another buffer \[[xorl 2009http://xorl.wordpress.com/2009/06/10/cve20091385linuxkernele1000integerunderflow/]\]. This can cause a buffer overflow, which allows an attacker to execute arbitrary code. 
Wiki Markup 

A Linux kernel vmsplice [exploitBB. Definitions#exploit], described at \[[Wojtczuk 2008AA. Bibliography#Wojtczuk 08]\], documents a vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping). 
...
Coverity 
 INTEGER_OVERFLOW  Implemented  
Klocwork 
 NUM.OVERFLOW CWARN.NOEFFECT.OUTOFRANGE  
LDRA tool suite 
 493 S, 494 S  Partially implemented  
Parasoft C/C++test 
 CERT_CINT30a  Avoid integer overflows  
Polyspace Bug Finder 
 CERT C: Rule INT30C  Checks for:
Rule fully covered.  
PRQA QAC 
 2910 [C], 2911 [D], 2912 [A], 2913 [S], 3383, 3384, 3385, 3386  Partially implemented  
PRQA QAC++ 
 2910, 2911, 2912, 2913  
PVSStudio 
 V658  
TrustInSoft Analyzer 
 unsigned overflow  Exhaustively verified. 
Related Vulnerabilities
CVE20091385 results from a violation of this rule. The value performs an unchecked subtraction on the length
of a buffer and then adds those many bytes of data to another buffer [xorl 2009]. This can cause a buffer overflow, which allows an attacker to execute arbitrary code.
A Linux Kernel vmsplice exploit, described by Rafal Wojtczuk [Wojtczuk 2008], documents a vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping).
Don Bailey [Bailey 2014] describes an unsigned integer wrap vulnerability in the LZO compression algorithm, which can be exploited in some implementations.
CVE20144377 describes a vulnerability in iOS 7.1 resulting from a multiplication operation that wraps, producing an insufficiently small value to pass to a memory allocation routine, which is subsequently overflowed.
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
CERT C++ Secure Coding Standard: INT30CPP. Ensure that unsigned integer operations do not wrap
ISO/IEC 9899:1999 Section 6.2.5, "Types," Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types <limits.h>
"
ISO/IEC TR 24772 "XYY Wraparound Error"
MITRE CWE: CWE190, "Integer Overflow (Wrap or Wraparound)"
Bibliography
Wiki Markup 

\[[Dowd 2006AA. Bibliography#Dowd 06]\] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211223)
\[[Seacord 2005AA. Bibliography#Seacord 05]\] Chapter 5, "Integers"
\[[Viega 2005AA. Bibliography#Viega 05]\] Section 5.2.7, "Integer overflow"
\[[VU#551436AA. Bibliography#VU551436]\]
\[[Warren 2002AA. Bibliography#Warren 02]\] Chapter 2, "Basics"
\[[Wojtczuk 2008AA. Bibliography#Wojtczuk 08]\]
\[[xorl 2009AA. Bibliography#xorl 2009]\] ["CVE20091385: Linux kernel E1000 Integer Underflow"http://xorl.wordpress.com/2009/06/10/cve20091385linuxkernele1000integerunderflow/] 
04. Integers (INT) Key here (explains table format and definitions)
Taxonomy  Taxonomy item  Relationship 

CERT C  INT02C. Understand integer conversion rules  Prior to 20180112: CERT: Unspecified Relationship 
CERT C  ARR30C. Do not form or use outofbounds pointers or array subscripts  Prior to 20180112: CERT: Unspecified Relationship 
CERT C  ARR36C. Do not subtract or compare two pointers that do not refer to the same array  Prior to 20180112: CERT: Unspecified Relationship 
CERT C  ARR37C. Do not add or subtract an integer to a pointer to a nonarray object  Prior to 20180112: CERT: Unspecified Relationship 
CERT C  CON08C. Do not assume that a group of calls to independently atomic methods is atomic  Prior to 20180112: CERT: Unspecified Relationship 
ISO/IEC TR 24772:2013  Arithmetic WrapAround Error [FIF]  Prior to 20180112: CERT: Unspecified Relationship 
CWE 2.11  CWE190, Integer Overflow or Wraparound  20161202: CERT: Rule subset of CWE 
CWE 2.11  CWE131  20170516: CERT: Partial overlap 
CWE 2.11  CWE191  20170518: CERT: Partial overlap 
CWE 2.11  CWE680  20170518: CERT: Partial overlap 
CERTCWE Mapping Notes
Key here for mapping notes
CWE131 and INT30C
 Intersection( INT30C, MEM35C) = Ø
 Intersection( CWE131, INT30C) =
 Calculating a buffer size such that the calculation wraps. This can happen, for example, when using malloc() or operator new[] to allocate an array, multiplying the array item size with the array dimension. An untrusted dimension could cause wrapping, resulting in a toosmall buffer being allocated, and subsequently overflowed when the array is initialized.
 CWE131 – INT30C =
 Incorrect calculation of a buffer size that does not involve wrapping. This includes offbyone errors, for example.
INT30C – CWE131 =
 Integer wrapping where the result is not used to allocate memory.
CWE680 and INT30C
Intersection( CWE680, INT30C) =
 Unsigned integer overflows that lead to buffer overflows
CWE680  INT30C =
 Signed integer overflows that lead to buffer overflows
INT30C – CWE680 =
 Unsigned integer overflows that do not lead to buffer overflows
CWE191 and INT30C
Union( CWE190, CWE191) = Union( INT30C, INT32C) Intersection( INT30C, INT32C) == Ø
Intersection(CWE191, INT30C) =
 Underflow of unsigned integer operation
CWE191 – INT30C =
 Underflow of signed integer operation
INT30C – CWE191 =
 Overflow of unsigned integer operation
Bibliography
[Bailey 2014]  Raising Lazarus  The 20 Year Old Bug that Went to Mars 
[Dowd 2006]  Chapter 6, "C Language Issues" ("Arithmetic Boundary Conditions," pp. 211–223) 
[ISO/IEC 9899:2011]  Subclause 6.2.5, "Types" 
[Seacord 2013b]  Chapter 5, "Integer Security" 
[Viega 2005]  Section 5.2.7, "Integer Overflow" 
[VU#551436]  
[Warren 2002]  Chapter 2, "Basics" 
[Wojtczuk 2008]  
[xorl 2009]  "CVE20091385: Linux Kernel E1000 Integer Underflow" 
...