CWE-124: Buffer Underwrite ('Buffer Underflow')

What is Buffer Underwrite ('Buffer Underflow')?

• Overview: Buffer Underwrite ('Buffer Underflow') occurs when a program writes data to a memory location before the start of a buffer, often due to incorrect pointer arithmetic or using negative indices.

• Exploitation Methods:

• Attackers can exploit this vulnerability by manipulating input data to cause the program to write before the buffer, potentially corrupting memory.
• Common attack patterns include crafting inputs that decrement pointers or indices, allowing attackers to overwrite critical memory structures.

• Security Impact:

• Direct consequences of successful exploitation include memory corruption, which can lead to crashes or arbitrary code execution.
• Potential cascading effects include compromising data integrity or allowing privilege escalation.
• Business impact may involve data breaches, service outages, or loss of customer trust.

• Prevention Guidelines:

• Specific code-level fixes include validating buffer indices and ensuring pointers are within bounds before writing.
• Security best practices involve using safe libraries and functions that automatically handle buffer boundaries, such as using safer alternatives to functions like strcpy in C.
• Recommended tools and frameworks include static analysis tools to identify potential buffer underwrite vulnerabilities and memory management libraries that provide safer abstractions.

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Technical Details

Likelihood of Exploit: Medium

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

C Example: CWE-124 (Buffer Underwrite ('Buffer Underflow'))

// This code demonstrates a buffer underwrite vulnerability.
// The pointer 'ptr' is decremented before the start of the buffer,
// leading to a write operation that corrupts adjacent memory.

#include <stdio.h>
#include <string.h>

void vulnerableFunction() {
    char buffer[10];
    char *ptr = buffer;

    // Incorrectly decrementing the pointer before the start of the buffer
    ptr -= 1;  
    strcpy(ptr, "Hello");  // This writes outside the allocated buffer memory
    printf("Buffer content: %s\n", buffer);
}

int main() {
    vulnerableFunction();
    return 0;
}

Explanation:

In this vulnerable code, the pointer ptr is decremented before the start of the buffer, causing strcpy to write data outside the allocated buffer memory. This can lead to corruption of adjacent memory and unpredictable behavior, which is a classic example of buffer underwrite.

How to fix Buffer Underwrite ('Buffer Underflow')?

To fix a buffer underwrite vulnerability, ensure that you do not decrement pointers beyond the start of the allocated buffer. Always validate pointer arithmetic to ensure it remains within the bounds of the allocated memory. Use safe functions that check bounds or ensure that you initialize and work with pointers properly to avoid accessing out-of-bounds memory.

Best Practices:

1. Pointer Safety: Never decrement pointers to access memory before the buffer starts.
2. Use Safe Functions: Consider using functions like strncpy which allow you to specify the maximum number of characters to copy, preventing overflow/underflow.
3. Boundary Checks: Always perform boundary checks when manipulating pointers.

Fixed Code Example

// This code fixes the buffer underwrite by ensuring the pointer
// does not move outside the allocated buffer's boundaries.

#include <stdio.h>
#include <string.h>

void fixedFunction() {
    char buffer[10];
    char *ptr = buffer;

    // Ensure the pointer stays within valid buffer bounds
    // Use strncpy to prevent writing beyond the buffer size
    strncpy(ptr, "Hello", sizeof(buffer) - 1);
    ptr[sizeof(buffer) - 1] = '\0';  // Ensure null termination
    printf("Buffer content: %s\n", buffer);
}

int main() {
    fixedFunction();
    return 0;
}

Explanation:

In this fixed example, strncpy is used to ensure that no more than sizeof(buffer) - 1 characters are copied into the buffer. This prevents writing beyond the buffer's boundaries. Additionally, we explicitly null-terminate the string to ensure safe string handling. This approach ensures the pointer remains within the valid range of the buffer, thus preventing buffer underwrite.