Use After Free
Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.
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.
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
Weaknesses in this category are related to memory safety.
Weaknesses in this category are related to the rules and recommendations in the Memory Management (MEM) section of the SEI CERT C Coding Standard.
This category identifies Software Fault Patterns (SFPs) within the Faulty Resource Use cluster (SFP15).
This view (slice) covers all the elements in CWE.
CWE entries in this view are listed in the 2023 CWE Top 25 Most Dangerous Software Weaknesses.
CWE entries in this view are listed in the 2022 CWE Top 25 Most Dangerous Software Weaknesses.
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