Difference between revisions of "Doubly freeing memory"

Revision as of 12:42, 2 December 2015

This is a Vulnerability. To view all vulnerabilities, please see the Vulnerability Category page.

Last revision (mm/dd/yy): 12/2/2015

Vulnerabilities Table of Contents

Description

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

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

Risk Factors

TBD

Examples

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

#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);
}

Related Attacks

• Attack 1
• Attack 2

Related Vulnerabilities

• Using freed memory
• Write-what-where condition

Related Controls

• 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.

Related Technical Impacts

• Technical Impact 1
• Technical Impact 2

References

TBD