The application 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.
Time of Introduction
Implementation
Common Consequences
Scope
Effect
Integrity
Confidentiality
Availability
Technical Impact: Execute unauthorized code or
commands; Modify 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.
Demonstrative Examples
Example 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.
(Bad Code)
Example
Language: C
void foo(){
record_t bar[MAX_SIZE];
/* do something interesting with bar */
...
free(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.
(Bad Code)
Example
Language: C
record_t bar[MAX_SIZE]; //Global var
void foo(){
/* do something interesting with bar */
...
free(bar);
}
Instead, if the programmer wanted to dynamically manage the memory,
malloc() or calloc() should have been used.
Additionally, you can pass global variables to free() when they are
pointers to dynamically allocated memory.
(Good Code)
record_t *bar; //Global var
void foo(){
bar = (record_t*)malloc(MAX_SIZE*sizeof(record_t));
/* do something interesting with bar */
...
free(bar);
}
Potential Mitigations
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.
Phases: Architecture and Design; Implementation; Operation
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.
In 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 CWE-590 or to its parent, CWE-762, depending on their perspective.
Description
