What is Free of Pointer not at Start of Buffer (CWE-761)?

TL;DR: Free of Pointer not at Start of Buffer (CWE-761) is a vulnerability where calling free() on an incorrect pointer can lead to undefined behavior. Fix it by tracking the original allocation address.

What is Free of Pointer not at Start of Buffer?

Free of Pointer not at Start of Buffer (CWE-761) is a type of memory management vulnerability that occurs when the product calls free() on a pointer to a memory resource allocated on the heap, but the pointer is not at the start of the buffer. As defined by the MITRE Corporation under CWE-761.

Quick Summary

Free of Pointer not at Start of Buffer can lead to undefined behavior and potential security issues such as modification of memory or execution of unauthorized code. It impacts system availability by causing crashes, exits, or restarts leading to denial-of-service conditions. Jump to: Overview · How it Works · Business Impact · Attack Scenario · Detection · Fixing · Framework-Specific Fixes · Asking AI · Best Practices Checklist · FAQ

Jump to: Quick Summary · Free of Pointer not at Start of Buffer Overview · How Free of Pointer not at Start of Buffer Works · Business Impact of Free of Pointer not at Start of Buffer · Free of Pointer not at Start of Buffer Attack Scenario · How to Detect Free of Pointer not at Start of Buffer · How to Fix Free of Pointer not at Start of Buffer · Framework-Specific Fixes for Free of Pointer not at Start of Buffer · How to Ask AI to Check Your Code for Free of Pointer not at Start of Buffer · Free of Pointer not at Start of Buffer Best Practices Checklist · Free of Pointer not at Start of Buffer FAQ · Vulnerabilities Related to Free of Pointer not at Start of Buffer · References · Scan Your Own Site

Free of Pointer not at Start of Buffer Overview

What: A memory management vulnerability where calling free() on an incorrect pointer can lead to undefined behavior. Why it matters: It can cause crashes, exits, or restarts leading to denial-of-service conditions and unauthorized code execution. Where it occurs: In C++ applications that improperly manage heap memory allocation and deallocation. Who is affected: Developers using languages like C++ without proper memory management practices. Who is NOT affected: Applications that use managed languages with automatic garbage collection.

How Free of Pointer not at Start of Buffer Works

Root Cause

The root cause lies in the misuse of pointer arithmetic to traverse a buffer, leading to incorrect deallocation of heap-allocated memory resources.

Attack Flow

1. An attacker manipulates a pointer within a buffer.
2. The manipulated pointer is passed to free().
3. This results in undefined behavior and potential security issues.

Prerequisites to Exploit

• The application must use pointer arithmetic incorrectly.
• The application must call free() on the manipulated pointer without proper validation.

Vulnerable Code

void exampleFunction(char* buffer) {
    char* ptr = buffer + 10;
    // Manipulate ptr within buffer...
    free(ptr); // Incorrect deallocation
}

This code is vulnerable because it deallocates memory from a pointer that may not be at the start of the allocated buffer.

Secure Code

void exampleFunction(char* buffer) {
    char* originalPtr = buffer;
    char* ptr = buffer + 10;
    // Manipulate ptr within buffer...
    free(originalPtr); // Correct deallocation
}

This code is secure because it tracks the original allocation address and uses that for proper deallocation.

Business Impact of Free of Pointer not at Start of Buffer

Availability: Crashes, exits, or restarts leading to denial-of-service conditions.

• Example: The application crashes when an incorrect pointer is passed to free().
• Consequences: Financial losses due to downtime, compliance penalties for service interruptions, and reputational damage from system unavailability.

Free of Pointer not at Start of Buffer Attack Scenario

1. An attacker manipulates a pointer within a buffer in the application.
2. The manipulated pointer is passed to free() without proper validation.
3. This results in undefined behavior leading to a crash or exit.
4. The application becomes unavailable, causing denial-of-service conditions.

How to Detect Free of Pointer not at Start of Buffer

Manual Testing

• Review code for improper use of pointer arithmetic and deallocation.
• Ensure pointers passed to free() are properly validated against the original allocation address.

Automated Scanners (SAST/DAST)

Static analysis can detect instances where a pointer is manipulated within a buffer before being passed to free(). Dynamic testing confirms the behavior in runtime scenarios.

PenScan Detection

PenScan’s scanner engines such as ZAP, Nuclei, and Wapiti can identify potential issues related to incorrect pointer deallocation.

False Positive Guidance

A false positive occurs when the code manipulates a pointer but ensures it is properly validated before calling free().

How to Fix Free of Pointer not at Start of Buffer

• Use a separate variable to track progress through memory and preserve the originally allocated address for later freeing.
• Consider using smart pointers provided by C++ libraries such as boost.
• Utilize vetted libraries or frameworks that provide protection against invalid pointer operations.
• Employ languages that offer abstractions for memory allocation and deallocation.

Framework-Specific Fixes for Free of Pointer not at Start of Buffer

void exampleFunction(char* buffer) {
    char* originalPtr = buffer;
    char* ptr = buffer + 10;
    // Manipulate ptr within buffer...
    free(originalPtr); // Correct deallocation
}

This code ensures proper memory management by tracking the original allocation address.

How to Ask AI to Check Your Code for Free of Pointer not at Start of Buffer

Free of Pointer not at Start of Buffer Best Practices Checklist

✅ Use a separate variable to track progress through memory. ✅ Preserve the originally allocated address for later freeing. ✅ Consider using smart pointers in C++. ✅ Utilize vetted libraries or frameworks that provide protection against invalid pointer operations. ✅ Employ languages with automatic garbage collection.

Free of Pointer not at Start of Buffer FAQ

How does the free of pointer not at start of buffer vulnerability work?

The product calls free() on a pointer to a memory resource that was allocated on the heap, but the pointer is not at the start of the buffer. This can lead to undefined behavior and potential security issues.

What are the common consequences of Free of Pointer not at Start of Buffer?

It can result in modification of memory, denial-of-service attacks, or execution of unauthorized code.

How can I detect free of pointer not at start of buffer vulnerabilities?

Use static analysis tools and manual testing to identify instances where the pointer is not at the start of the allocated buffer before calling free().

What are some best practices for preventing Free of Pointer not at Start of Buffer?

Track progress through memory separately from the original allocation address, use smart pointers in C++, or leverage libraries that provide protection against invalid pointer operations.

Can you show an example of vulnerable code for free of pointer not at start of buffer?

Vulnerable code would involve manipulating a pointer within a buffer and then calling free() on it without ensuring the pointer points to the beginning of the allocated memory block.

What is the impact of Free of Pointer not at Start of Buffer on system availability?

It can cause crashes, exits, or restarts leading to denial-of-service conditions.

How does using smart pointers help prevent free of pointer not at start of buffer issues in C++?

Smart pointers automatically manage memory and ensure proper deallocation without requiring manual pointer manipulation.

Vulnerabilities Related to Free of Pointer not at Start of Buffer

| CWE | Name | Relationship | |—|—|—| | CWE-763 | Release of Invalid Pointer or Reference | ChildOf | | CWE-404 | Improper Resource Shutdown or Release | ChildOf |

References

• MITRE
• NVD

Scan Your Own Site

Manual code review catches what you know to look for. An automated scan catches what you didn’t. Scan your own website using PenScan to find Free of Pointer not at Start of Buffer and other risks before an attacker does.