CWE

Free of Pointer not at Start of Buffer

ID: 761		Date: (C)2012-05-14   (M)2022-10-10
Type: weakness		Status: INCOMPLETE
Abstraction Type: Variant

Description

The application 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.

Extended Description

This can cause the application to crash, or in some cases, modify critical program variables or execute code.

This weakness often occurs when the memory is allocated explicitly on the heap with one of the malloc() family functions and free() is called, but pointer arithmetic has caused the pointer to be in the interior or end of the buffer.

Applicable Platforms
None

Time Of Introduction

• Implementation

Common Consequences

Scope	Technical Impact	Notes
Integrity Availability Confidentiality	Modify memory DoS: crash / exit / restart Execute unauthorized code or commands

Detection Methods
None

Potential Mitigations

Phase	Strategy	Description	Effectiveness	Notes
Implementation		When utilizing pointer arithmetic to traverse a buffer, use a separate variable to track progress through memory and preserve the originally allocated address for later freeing.
Implementation		When programming in C++, consider using smart pointers provided by the boost library to help correctly and consistently manage memory.
Architecture and Design Implementation Operation	Libraries or Frameworks	Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, glibc in Linux provides protection against free of invalid pointers.
Architecture and Design		Use a language that provides abstractions for memory allocation and deallocation.
Testing		Use a tool that dynamically detects memory management problems, such as valgrind.

Relationships

Related CWE	Type	View	Chain
CWE-761 ChildOf CWE-891	Category	CWE-888

Demonstrative Examples   (Details)

1. Consider the following code in the context of a parsing application to extract commands out of user data. The intent is to parse each command and add it to a queue of commands to be executed, discarding each malformed entry. (Demonstrative Example Id DX-79)
2. In this example, the programmer dynamically allocates a buffer to hold a string and then searches for a specific character. After completing the search, the programmer attempts to release the allocated memory and return SUCCESS or FAILURE to the caller. Note: for simplification, this example uses a hard-coded "Search Me!" string and a constant string length of 20. (Demonstrative Example Id DX-77)
3. This code attempts to tokenize a string and place it into an array using the strsep function, which inserts a \0 byte in place of whitespace or a tab character. After finishing the loop, each string in the AP array points to a location within the input string. (Demonstrative Example Id DX-78)

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings
None

References:

1. .boost C++ Library Smart Pointers.
2. .Valgrind.