Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.
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 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.
Alternate Terms
Dangling pointer
Use-After-Free
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
C
C++
Common Consequences
Scope
Effect
Integrity
Technical Impact: Modify memory
The use of previously freed memory may corrupt valid data, if the
memory area in question has been allocated and used properly
elsewhere.
Availability
Technical Impact: DoS: crash / exit /
restart
If chunk consolidation occurs after the use of previously freed data,
the process may crash when invalid data is used as chunk
information.
Integrity
Confidentiality
Availability
Technical Impact: Execute unauthorized code or
commands
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
(Bad Code)
Example
Language: C
#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:
(Bad Code)
Example
Language: C
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.
Chain: race condition (CWE-362) from improper handling of a page transition in web client while an applet is loading (CWE-368) leads to use after free (CWE-416)
Choose a language that provides automatic memory management.
Phase: Implementation
When freeing pointers, be sure to set them to NULL once they are
freed. However, the utilization of multiple or complex data structures
may lower the usefulness of this strategy.
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Page Last Updated:
May 05, 2017
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