CWE-192

Integer Coercion Error. [Improper-Control-Of-A-Resource-Through-Its-Lifetime]

Required inputs: IR, StaticSemanticAnalysis

Integer coercion refers to a set of flaws pertaining to the type casting, extension, or truncation of primitive data types. Several flaws fall under the category of integer coercion errors. For the most part, these errors in and of themselves result only in availability and data integrity issues. However, in some circumstances, they may result in other, more complicated security related flaws, such as buffer overflow conditions.
Demonstrative Examples
Example 1

The following code is intended to read an incoming packet from a socket and extract one or more headers.

Example Language:C
    DataPacket *packet;
    int numHeaders;
    PacketHeader *headers;

    sock=AcceptSocketConnection();
    ReadPacket(packet, sock);
    numHeaders =packet->headers;

    if (numHeaders > 100) {
        ExitError("too many headers!");
    }
    headers = malloc(numHeaders * sizeof(PacketHeader);
    ParsePacketHeaders(packet, headers);

The code performs a check to make sure that the packet does not contain too many headers. However, numHeaders is defined as a signed int, so it could be negative. If the incoming packet specifies a value such as -3, then the malloc calculation will generate a negative number (say, -300 if each header can be a maximum of 100 bytes). When this result is provided to malloc(), it is first converted to a size_t type. This conversion then produces a large value such as 4294966996, which may cause malloc() to fail or to allocate an extremely large amount of memory (CWE-195). With the appropriate negative numbers, an attacker could trick malloc() into using a very small positive number, which then allocates a buffer that is much smaller than expected, potentially leading to a buffer overflow.

Example 2

The following code reads a maximum size and performs validation on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data.

Example Language:C
    int GetUntrustedInt () {
        return(0x0000FFFF);
    }

    void main (int argc, char **argv) {
        char path[256];
        char *input;
        int i;
        short s;
        unsigned int sz;

        i = GetUntrustedInt();
        s = i;
        /* s is -1 so it passes the safety check -    CWE-697    */
        if (s > 256) {
            DiePainfully("go away!\n");
        }

        /* s is sign-extended and saved in sz */
        sz = s;

        /* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */
        printf("i=%d, s=%d, sz=%u\n", i, s, sz);

        input = GetUserInput("Enter pathname:");

        /* strncpy interprets s as unsigned int, so it's treated as MAX_INT
        (    CWE-195    ), enabling buffer overflow (    CWE-119    ) */
        strncpy(path, input, s);
        path[255] = '\0'; /* don't want    CWE-170    */
        printf("Path is: %s\n", path);
    }

This code first exhibits an example of CWE-839, allowing "s" to be a negative number. When the negative short "s" is converted to an unsigned integer, it becomes an extremely large positive integer. When this converted integer is used by strncpy() it will lead to a buffer overflow (CWE-119).

Excerpts from CWE [https://cwe.mitre.org], Copyright (C) 2006-2026, the MITRE Corporation. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

cafe_message

{}

None

False

cast_overflow

Cast on result of arithmetic computation may cause overflow

None

False

cast_truncate

Cast may truncate value

None

False

cast_underflow

Cast on result of arithmetic computation may cause underflow

None

False

certain_shift_amount_negative

Shift by a negative bit count (undefined behavior)

None

False

certain_shift_amount_too_large

Shift by the integer width or more (undefined behavior)

None

False

certain_shift_right_negative

Right shift with negative left-hand-side

None

False

shift_amount_negative

Possible shift by a negative bit count (undefined behavior)

None

False

shift_amount_too_large

Possible shift by the integer width or more (undefined behavior)

None

False

shift_right_negative

Possible right shift with negative left-hand-side

None

False

static_cast_overflow

Cast on result of arithmetic computation may cause overflow

None

False

static_cast_underflow

Cast on result of arithmetic computation may cause underflow

None

False

static_cast_underflow_minus_1

Casting -1 to an unsigned type causes underflow

None

False

static_overflow

Arithmetic computation may cause overflow

None

False

static_underflow

Arithmetic computation may cause underflow

None

False

unsigned_cast_overflow

Cast on result of arithmetic computation may cause wrap-around

None

False

unsigned_cast_underflow

Cast on result of arithmetic computation may cause wrap-around (underflow)

None

False

Options

abstract_interpretation_maximal_tracked_array_index

abstract_interpretation_maximal_tracked_array_index : int = 10

The number of explicit indices in array expressions per routine tracked by the "symbolic expression analysis". For example, consider the following program. 

extern signed char a[6];
int main()
{
    if (a[2] < 0)
    {
        a[2]++;
    }
    if (a[3] < 0)
    {
        a[3]++;
    }
    if (a[4] < 0)
    {
        a[4]++;
    }
    return 0;
}

If the value of this option is set to 2, the first two array index expressions encountered in the routine are tracked. Hence, the analysis can use the facts a[2] < 0 and a[3] < 0 to infer that a[2]++ and a[3]++ do not overflow, but it will not track the third array access in this routine.

A higher value of the option can cause more consumption of memory and time for the analysis.

 

abstract_interpretation_overflow

abstract_interpretation_overflow : bool = False

Use abstract-interpretation-based "symbolic expression analysis" as additional postprocessing step.
 

abstract_interpretation_overflow_unrolling_level

abstract_interpretation_overflow_unrolling_level : int = 0

How many levels of conditions are traversed to compute additional constraints for the "symbolic expression analysis".
 

check_signed

check_signed : bool = True

Whether issues for signed integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.
 

check_unsigned

check_unsigned : bool = True

Whether wrap-around for unsigned integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.
 

message_predicate

message_predicate : typing.Callable[[Cafe_Message], bool] | None = None

If provided, a custom predicate to filter relevant messages. Receives the message node and should return True for messages to report.
 

relevant_expressions

relevant_expressions : RelevantExpressions = 'const_expressions_only'

Which (const / constant) expressions should be considered.

Note: this is only relevant for the purely static parts of the analysis. The StaticSemanticAnalysis-based checks for runtime errors will be performed independently.

 

reported_messages

reported_messages : set[int] | None = {514}

If provided, only messages of these types are reported.
 

reported_severities

reported_severities : set[str] = {'error', 'remark', 'warning'}

List of severities to display.
 

suppress_well_defined_findings

suppress_well_defined_findings : SuppressionMode = 'NONE'

Some overflows have well-defined semantics in all C/C++ standard versions. The typical example is UINT_MAX+1 which is well-defined as 0 via wraparound. This differs from INT_MAX+1 which is either undefined or implementation-defined depending on the considered standard version. Most CPUs will compute INT_MIN but this wraparound is not guaranteed by any C/C++ standard.

Both cases are overflows and are reported by this rule. However, one might want to suppress messages for the well-defined cases. To suppress these activate this option.

Different C and C++ standard versions differ in what is well-defined, implementation-defined, or undefined. Luckily, if we only consider well-defined and do not discern between implementation-defined and undefined, we end up with only two groups: pre-C++20 and since-C++20.

 

use_error_number

use_error_number : bool = False

Whether the error number from the frontend should be used.
 

use_rule_severity

use_rule_severity : bool = True

Whether the rule's severity or the compiler's severity should be used.
 

Option Types

These types are used by options listed above:

RelevantExpressions

An enumeration.
 

none

No (additional) checks for overflows in const-expressions or compile time constant expressions.

const_expressions_only

Whether the analysis should statically check const-expressions (i.e., const variables and literals) that might have been reduced to a literal during compilation.

const_and_compile_time_constant

Whether the analysis should statically check const-expressions (i.e., const variables and literals) as well as compile-time constant expressions (i.e., preprocessor defines, constexprs or literals) that might have been reduced to a literal during compilation.

SuppressionMode

An enumeration.
 

NONE

Suppress nothing.

PRE_CPP2020

Suppress findings that are well-defined before C++20. These are:

  • Over- and underflows of unsigned integers during addition, subtraction, and multiplication
  • Conversions from unsigned to unsigned integers
  • Wrap-around caused by left-shifting of unsigned integer

CPP2020

Suppress findings that are well-defined since C++20. These are:

  • Over- and underflows of unsigned integers during addition, subtraction, and multiplication
  • Conversions between signed and unsigned integers
  • Wrap-around caused by left-shifting
  • Shifting negative integers

Surprising mechanics of C++20 signed narrow integers

Since C++20, casts between signed and unsigned are defined as two-complement wrap-around. Overflows of signed integers are still undefined behavior and are reported by this rule. But, due to integer promotion rules, certain expressions are computed using wider integer types, which can lead to the false impression that this is no longer the case, because no overflow findings are reported there.

Suppose, that the code is compiled on a platform where short is smaller than or equal to half the size of an int. Very commonly the sizes are 2 and 4. This assumption is thus true for many platforms.

In this case, narrow signed integer types such as short or signed char are first implicitly promoted to int before the arithmetic operation is executed. Because of this promotion, the actual operation does not overflow and is thus well-defined. After the operation, an implicit cast is performed to the narrower type. This cast is well-defined in C++20 as wrapping around.

Consider the following snippet: 

    static_assert(sizeof(short) == 2);
    static_assert(sizeof(int) == 4);
    short a = 0x1000;
    short b = 0x1001;
    short c = a*b;
C++20 defines c as 0x1000. The reason is that a*b is implicitly promoted to static_cast<int> (a)*static_cast<int>(b). After the promotion, the multiplication does not overflow and yields a well-defined 0x1001000. This number is then implicitly cast to 0x1000 which is also a well-defined operation.

An analogous effect can be observed for signed short addition and multiplication. Another effect is that it is well-defined to shift by up to as many bits as int has even if the shifted integer has fewer bits.

DERIVE_FROM_IR

Derive the language version from the IR compilation flags and suppress findings accordingly.