Destination buffer underflow in string manipulation

Function writes to buffer at a negative offset from beginning of buffer

Description

This defect 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.

Examples

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);     
}

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);     
}

Result Information

Group: Static memory

Language: C | C++

Default: On for handwritten code, off for generated code

Command-Line Syntax: STRLIB_BUFFER_UNDERFLOW

Impact: High

CWE ID: 124, 786, 787