Referencing memory after it has been freed can cause a program
to crash, use unexpected values, or execute code.
Alternate Terms
Use-After-Free
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
C
C++
Common Consequences
Integrity
The use of previously freed memory may corrupt valid data, if the
memory area in question has been allocated and used properly
elsewhere.
Availability
If chunk consolidation occur after the use of previously freed data,
the process may crash when invalid data is used as chunk
information.
Access Control (instruction processing): If malicious data is entered
before chunk consolidation can take place, it may be possible to take
advantage of a write-what-where primitive to execute arbitrary
code.
Likelihood of Exploit
High
Demonstrative Examples
Example 1:
C Example:
#include <stdio.h>
#include <unistd.h>
#define BUFSIZER1 512
#define BUFSIZER2 ((BUFSIZER1/2) - 8)
int main(int argc, char **argv) {
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);
}
Example 2:
The following code illustrates a use after free error:
Choose a language that provides automatic memory management.
Implementation
Ensuring that all pointers are set to NULL once they memory they point
to has been freed can be an effective strategy. The utilization of
multiple or complex data structures may lower the usefulness of this
strategy.
Implementation
Use a static analysis tool to find instances of use after free.
Other Notes
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. Like double free errors and memory leaks, 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 chances 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.
Description
Architecture and Design
