View source for Doubly freeing memory

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==Overview==

Freeing or deleting the same memory chunk twice may - when combined with other flaws - result in a write-what-where condition.

==Consequences ==

*	Access control: Doubly freeing memory may result in a write-what-where condition, allowing an attacker to execute arbitrary code.

==Exposure period ==

*	Requirements specification: A language which handles memory allocation and garbage collection automatically might be chosen.

*	Implementation: Double frees are caused most often by lower-level logical errors.

==Platform ==

*	Language: C, C++, Assembly

*	Operating system: All

==Required resources ==

Any

==Severity ==

High

==Likelihood of exploit ==

Low to Medium

==Avoidance and mitigation ==

*	Implementation: Ensure that each allocation is freed only once. After freeing a chunk, set the pointer to NULL to ensure the pointer cannot be freed again. In complicated error conditions, be sure that clean-up routines respect the state of allocation properly. If the language is object oriented, ensure that object destructors delete each chunk of memory only once.

==Discussion ==

Doubly freeing memory can result in roughly the same write-what-where condition that the use of previously freed memory will. 

==Examples ==

While contrived, this code should be exploitable on Linux distributions which do not ship with heap-chunk check summing turned on.

<pre>
#include <stdio.h>
#include <unistd.h>

#define BUFSIZE1    512
#define BUFSIZE2    ((BUFSIZE1/2) - 8)

int main(int argc, char **argv) { 
  char *buf1R1;    
  char *buf2R1;    
  char *buf1R2;    

  buf1R1 = (char *) malloc(BUFSIZE2);    
  buf2R1 = (char *) malloc(BUFSIZE2);    
  
  free(buf1R1);    
  free(buf2R1);    

  buf1R2 = (char *) malloc(BUFSIZE1);    
  strncpy(buf1R2, argv[1], BUFSIZE1-1);    
  
  free(buf2R1);    
  free(buf1R2);
}
</pre>

==Related problems ==

*	[[Using freed memory]]

*	[[Write-what-where condition]]


[[Category:Vulnerability]]
[[Category:Range and Type Error Vulnerability]]
[[Category:OWASP_CLASP_Project]]
[[Category:Code Snippet]]
[[Category:C]]
[[Category:Implementation]]