Free of Memory not on the Heap

Incomplete Variant

Structure: Simple

Description

The product calls free() on a pointer to memory that was not allocated using associated heap allocation functions such as malloc(), calloc(), or realloc().

Extended Description

When free() is called on an invalid pointer, the program's memory management data structures may become corrupted. This corruption can cause the program to crash or, in some circumstances, an attacker may be able to cause free() to operate on controllable memory locations to modify critical program variables or execute code.

Common Consequences 1

Scope: IntegrityConfidentialityAvailability

Impact: Execute Unauthorized Code or CommandsModify Memory

There is the potential for arbitrary code execution with privileges of the vulnerable program via a "write, what where" primitive. If pointers to memory which hold user information are freed, a malicious user will be able to write 4 bytes anywhere in memory.

Detection Methods 2

FuzzingHigh

Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.

Automated Static AnalysisHigh

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Potential Mitigations 5

Phase: Implementation

Only free pointers that you have called malloc on previously. This is the recommended solution. Keep track of which pointers point at the beginning of valid chunks and free them only once.

Phase: Implementation

Before freeing a pointer, the programmer should make sure that the pointer was previously allocated on the heap and that the memory belongs to the programmer. Freeing an unallocated pointer will cause undefined behavior in the program.

Phase: Architecture and Design

Strategy: 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.

Phase: Architecture and Design

Use a language that provides abstractions for memory allocation and deallocation.

Phase: Testing

Use a tool that dynamically detects memory management problems, such as valgrind.

Demonstrative Examples 1

In this example, an array of record_t structs, bar, is allocated automatically on the stack as a local variable and the programmer attempts to call free() on the array. The consequences will vary based on the implementation of free(), but it will not succeed in deallocating the memory.

Code Example:

Bad

/* do something interesting with bar /

This example shows the array allocated globally, as part of the data segment of memory and the programmer attempts to call free() on the array.

Code Example:

Bad

/* do something interesting with bar / ... free(bar);}

Instead, if the programmer wanted to dynamically manage the memory, malloc() or calloc() should have been used.

Code Example:

Good

/* do something interesting with bar /

Additionally, global variables could be passed to free() when they are pointers to dynamically allocated memory.

Code Example:

Good

/* do something interesting with bar /

References 1

Valgrind

https://valgrind.org/(2025-07-24)

ID: REF-480

Modes of Introduction

Implementation

Functional Areas

Memory Management

Affected Resources

Memory

Related Weaknesses

ChildOf:

Mismatched Memory Management Routines (CWE-762)

CanPrecede:

Write-what-where Condition (CWE-123)

Taxonomy Mapping

CERT C Secure Coding
CERT C Secure Coding
Software Fault Patterns

Notes

OtherIn C++, if the new operator was used to allocate the memory, it may be allocated with the malloc(), calloc() or realloc() family of functions in the implementation. Someone aware of this behavior might choose to map this problem to Free of Memory not on the Heap or to its parent, Mismatched Memory Management Routines, depending on their perspective.