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• 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)
• in security-critical code

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_. Most operations on unsigned integer operations can result in overflow wrap if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operators can result in overflowwrapping:

The following sections examine specific operations that are susceptible to unsigned integer overflow. The specific tests that are required to guarantee that the operation does not result in an integer overflow depend on the signedness of the integer types. 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 INT02-A. Understand integer conversion rules).

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Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type. Incrementing is equivalent to adding one.

Non-Compliant Code Example

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This code may result in an unsigned integer overflow 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.

Code Block
bgColor #FFcccc
unsigned int ui1, ui2, sum; sum = ui1 + ui2;

Compliant Solution

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This compliant solution tests the suspect addition operation to guarantee there is no possibility of unsigned overflow.

Code Block
bgColor #ccccff
unsigned int ui1, ui2, sum; if (UINT_MAX - ui1 < ui2) { /* handle error condition */ } sum = ui1 + ui2;

Non-Compliant Code Example (Signed)

This code may result in a signed integer overflow during the addition of the signed operands si1 and si2. If this behavior is unanticipated, it could lead to an exploitable vulnerability.

int si1, si2, sum;

sum = si1 + si2;

Compliant Solution (Two's Complement Signed)

This compliant solution tests the addition operation to ensure no overflow occurs, assuming two's complement representation.

signed int si1, si2, sum;

if ( ((si1^si2) | (((si1^(~(si1^si2) & (1 << (sizeof(int)*CHAR_BIT-1))))+si2)^si2)) >= 0) {
   /* handle error condition */
}

sum = si1 + si2;

Compliant Solution (General Signed)

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signed int si1, si2, sum;

if (((si1>0) && (si2>0) && (si1 > (INT_MAX-si2))) ||
    ((si1<0) && (si2<0) && (si1 < (INT_MIN-si2)))) {
   /* handle error condition */
}

sum = si1 + si2;

This solution is more readable but contains branches and consequently may be less efficient than the solution that is specific to two's complement representation.

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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. Decrementing is equivalent to subtracting one.

Non-Compliant Code Example

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This code may result in an unsigned integer overflow during the subtraction of the unsigned operands ui1 and ui2. If this behavior is unanticipated, it may lead to an exploitable vulnerability.

Code Block
bgColor #FFcccc
unsigned int ui1, ui2, result; result = ui1 - ui2;

Compliant Solution

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This compliant solution tests the suspect unsigned subtraction operation to guarantee there is no possibility of unsigned overflow.

Code Block
bgColor #ccccff
unsigned int ui1, ui2, result; if (ui1 < ui2){ /* handle error condition */ } result = ui1 - ui2;

Non-Compliant Code Example (Signed)

This code can result in a signed integer overflow during the subtraction of the signed operands si1 and si2. If this behavior is unanticipated, 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.

signed int si1, si2, result;

result = si1 - si2;

Compliant Solution (Two's Complement Signed)

This compliant solution tests the suspect subtraction operation to guarantee there is no possibility of signed overflow, presuming two's complement representation.

signed int si1, si2, result;

if (((si1^si2) & (((si1 ^ ((si1^si2) & (1 << (sizeof(int)*CHAR_BIT-1))))-si2)^si2)) < 0) {
  /* handle error condition */
}
result = si1 - si2;

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Multiplication is between two operands of arithmetic type.

Non-Compliant Code Example

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This non-compliant code example can result in a signed integer overflow during the multiplication of the signed operands si1 and si2. If this behavior is unanticipated, 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.

Code Block
bgColor #FFcccc
signed int si1, si2, result; result = si1 * si2;

Compliant Solution (Signed)

This compliant solution guarantees there is no possibility of signed overflow.

signed int si1, si2, result;

signed long long tmp = (signed long long)si1 * (signed long long)si2;

/*
 * If the product cannot be represented as a 32-bit integer, handle as an error condition
 */
if ( (tmp > INT_MAX) || (tmp < INT_MIN) ) {
  /* handle error condition */
}
result = (int)tmp;

The preceding code is compliant only on systems where long long is at least twice the size of int. On systems where this relationship does not exist, the following compliant solution may be used to ensure signed overflow does not occur.

signed int si1, si2, result;

if (si1 > 0){  /* si1 is positive */
  if (si2 > 0) {  /* si1 and si2 are positive */
    if (si1 > (INT_MAX / si2)) {
      /* handle error condition */
    }
  } /* end if si1 and si2 are positive */
  else { /* si1 positive, si2 non-positive */
    if (si2 < (INT_MIN / si1)) {
        /* handle error condition */
    }
  } /* si1 positive, si2 non-positive */
} /* end if si1 is positive */
else { /* si1 is non-positive */
  if (si2 > 0) { /* si1 is non-positive, si2 is positive */
    if (si1 < (INT_MIN / si2)) {
      /* handle error condition */
    }
  } /* end if si1 is non-positive, si2 is positive */
  else { /* si1 and si2 are non-positive */
    if ( (si1 != 0) && (si2 < (INT_MAX / si1))) {
      /* handle error condition */
    }
  } /* end if si1 and si2 are non-positive */
} /* end if si1 is non-positive */

result = si1 * si2;

Non-Compliant Code Example (Unsigned)

The Mozilla Scalable Vector Graphics (SVG) viewer contains a heap buffer overflow vulnerability resulting from an unsigned integer overflow during the multiplication of the {{signed int}} value {{pen->num_vertices}} and the {{size_t}} value {{sizeof(cairo_pen_vertex_t)}} \[[VU#551436|AA. C References#VU551436]\].  The {{signed int}} operand is converted to {{unsigned int}} prior to the multiplication operation (see [INT02-A. Understand integer conversion rules]).

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The unsigned integer overflow can result in allocating memory of insufficient size.

Compliant Solution

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This compliant solution tests the suspect multiplication operation to guarantee that there is no unsigned integer overflow.

Code Block
bgColor #ccccff
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));

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Division

Division is between two operands of arithmetic type. Overflow can occur during twos-complement signed integer division when the dividend is equal to the minimum (negative) value for the signed integer type and the divisor is equal to -1. Both signed and unsigned division operations are also susceptible to divide-by-zero errors (see INT33-C. Ensure that division and modulo operations do not result in divide-by-zero errors).

Non-Compliant Code Example (Signed)

This code can result in a signed integer overflow during the division of the signed operands sl1 and sl2 or in a divide-by-zero error. The IA-32 architecture, for example, requires that both conditions result in a fault, which could easily result in a denial-of-service attack.

signed long sl1, sl2, result;

result = sl1 / sl2;

Compliant Solution (Signed)

This compliant solution guarantees there is no possibility of signed overflow or divide-by-zero errors.

signed long sl1, sl2, result;

if ( (sl2 == 0) || ( (sl1 == LONG_MIN) && (sl2 == -1) ) ) {
  /* handle error condition */
}
result = sl1 / sl2;

Modulo

The modulo operator provides the remainder when two operands of integer type are divided.

Non-Compliant Code Example (Signed)

This code can result in a divide-by-zero or an overflow error during the modulo operation on the signed operands sl1 and sl2. Overflow can occur during a modulo operation when the dividend is equal to the minimum (negative) value for the signed integer type and the divisor is equal to -1.

signed long sl1, sl2, result;

result = sl1 % sl2;

Compliant Solution (Signed)

This compliant solution tests the suspect modulo operation to guarantee there is no possibility of a divide-by-zero error or an overflow error.

signed long sl1, sl2, result;

if ( (sl2 == 0 ) || ( (sl1 == LONG_MIN) && (sl2 == -1) ) ) {
  /* handle error condition */
}
result = sl1 % sl2;

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Unary Negation

The unary negation operator takes an operand of arithmetic type. Overflow can occur during twos-complement unary negation when the operand is equal to the minimum (negative) value for the signed integer type.

Non-Compliant Code Example

This non-compliant code example can result in a signed integer overflow during the unary negation of the signed operand si1. If this behavior is unanticipated, 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.

signed int si1, result;

result = -si1;

Compliant Solution

This compliant solution tests the suspect negation operation to guarantee there is no possibility of signed overflow.

signed int si1, result;

if (si1 == INT_MIN) {
  /* handle error condition */
}
result = -si1;

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The left shift operator is between two operands of integer type.

Non-Compliant Code Example

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This code can result in an unsigned overflow during the shift operation of the unsigned operands ui1 and ui2. If this behavior is unanticipated, 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.

Code Block
bgColor #FFcccc
unsigned int ui1, ui2, uresult; uresult = ui1 << ui2;

Compliant Solution

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This compliant solution tests the suspect shift operation to guarantee there is no possibility of unsigned overflow. This solution must also be compliant with INT36-C. Do not shift a negative number of bits or more bits than exist in the operand.

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Wiki Markup

\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211-223) \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.5, "Types," Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types <limits.h>" \[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "XYY Wrap-around Error" \[[Seacord 05|AA. C References#Seacord 05]\] Chapter 5, "Integers" \[[Viega 05|AA. C References#Viega 05]\] Section 5.2.7, "Integer overflow" \[[VU#551436|AA. C References#VU551436]\] \[[Warren 02|AA. C References#Warren 02]\] Chapter 2, "Basics"

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