CWE-416: Use After Free

Description

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Submission Date :

July 19, 2006, midnight

Modification Date :

2023-10-26 00:00:00+00:00

Organization :

MITRE

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system's reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

Error conditions and other exceptional circumstances.
Confusion over which part of the program is responsible for freeing the memory.
In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process.
If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Example Vulnerable Codes

Example - 1

The following example demonstrates the weakness.



char *buf1R1;char *buf2R1;char *buf2R2;char *buf3R2;buf1R1 = (char *) malloc(BUFSIZER1);buf2R1 = (char *) malloc(BUFSIZER1);free(buf2R1);buf2R2 = (char *) malloc(BUFSIZER2);buf3R2 = (char *) malloc(BUFSIZER2);strncpy(buf2R1, argv[1], BUFSIZER1-1);free(buf1R1);free(buf2R2);free(buf3R2);#include <stdio.h>#include <unistd.h>#define BUFSIZER1 512#define BUFSIZER2 ((BUFSIZER1/2) - 8)int main(int argc, char **argv) {}

Example - 2

The following code illustrates a use after free error:



abrt = 1;free(ptr);
logError("operation aborted before commit", ptr);char* ptr = (char*)malloc (SIZE);if (err) {}...if (abrt) {}

When an error occurs, the pointer is immediately freed. However, this pointer is later incorrectly used in the logError function.

Related Weaknesses

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined to give an overview of the different insight to similar items that may exist at higher and lower levels of abstraction.

CWE-1265: Unintended Reentrant Invocation of Non-reentrant Code Via Nested Calls

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