Use After Free

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following example demonstrates the weakness.

#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 Two

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);
}

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

See Also