CWE Rule 787

Out-of-bounds Write

Since R2023a

Description

Rule Description

The software writes data past the end, or before the beginning, of the intended buffer.

Polyspace Implementation

The rule checker checks for these issues:

• Destination buffer overflow in string manipulation

• Destination buffer underflow in string manipulation

Examples

Destination buffer overflow in string manipulation

Issue

This issue occurs when certain string manipulation functions write to their destination buffer argument at an offset greater than the buffer size.

For instance, when calling the function sprintf(char* buffer, const char* format), you use a constant string format of greater size than buffer.

Risk

Buffer overflow can cause unexpected behavior such as memory corruption or stopping your system. Buffer overflow also introduces the risk of code injection.

Fix

One possible solution is to use alternative functions to constrain the number of characters written. For instance:

• If you use sprintf to write formatted data to a string, use snprintf, _snprintf or sprintf_s instead to enforce length control. Alternatively, use asprintf to automatically allocate the memory required for the destination buffer.

• If you use vsprintf to write formatted data from a variable argument list to a string, use vsnprintf or vsprintf_s instead to enforce length control.

• If you use wcscpy to copy a wide string, use wcsncpy, wcslcpy, or wcscpy_s instead to enforce length control.

Another possible solution is to increase the buffer size.

Example — Buffer Overflow in sprintf Use

#include <stdio.h>

void func(void) {
    char buffer[20];
    char *fmt_string = "This is a very long string, it does not fit in the buffer";

    sprintf(buffer, fmt_string);  //Noncompliant
}

In this example, buffer can contain 20 char elements but fmt_string has a greater size.

Correction — Use snprintf Instead of sprintf

One possible correction is to use the snprintf function to enforce length control.

#include <stdio.h>

void func(void) {
    char buffer[20];
    char *fmt_string = "This is a very long string, it does not fit in the buffer";

    snprintf(buffer, 20, fmt_string);
}

Destination buffer underflow in string manipulation

Issue

This issue occurs when certain string manipulation functions write to their destination buffer argument at a negative offset from the beginning of the buffer.

For instance, for the function sprintf(char* buffer, const char* format), you obtain the buffer from an operation buffer = (char*)arr; ... buffer += offset;. arr is an array and offset is a negative value.

Risk

Buffer underflow can cause unexpected behavior such as memory corruption or stopping your system. Buffer underflow also introduces the risk of code injection.

Fix

If the destination buffer argument results from pointer arithmetic, see if you are decrementing a pointer. Fix the pointer decrement by modifying either the original value before decrement or the decrement value.

Example — Buffer Underflow in sprintf Use

#include <stdio.h>
#define offset -2

void func(void) {
    char buffer[20];
    char *fmt_string ="Text";

    sprintf(&buffer[offset], fmt_string);  //Noncompliant
}

In this example, &buffer[offset] is at a negative offset from the memory allocated to buffer.

Correction — Change Pointer Decrementer

One possible correction is to change the value of offset.

#include <stdio.h>
#define offset 2

void func(void) {
    char buffer[20];
    char *fmt_string ="Text";

    sprintf(&buffer[offset], fmt_string);     
}

Check Information

Category: Memory Buffer Errors

Version History

Introduced in R2023a