Use After Free

Referencing memory after it has been freed can cause a program to crash.

Use after free errors occur when a program continues to use a pointer after it has been freed. 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

Use after free errors sometimes have no effect and other times cause a program to crash. While it is technically feasible for the freed memory to be re-allocated and for an attacker to use this reallocation to launch a buffer overflow attack, we are unaware of any exploits based on this type of attack.

Example: The following code illustrates a use after free error:

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