Status: Draft Weakness ID: 415 (Weakness Variant)Summary The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. Extended Description When a program calls free() twice with the same argument, the program's memory management data structures become corrupted. This corruption can cause the program to crash or, in some circumstances, cause two later calls to malloc() to return the same pointer. If malloc() returns the same value twice and the program later gives the attacker control over the data that is written into this doubly-allocated memory, the program becomes vulnerable to a buffer overflow attack. Access Control Doubly freeing memory may result in a write-what-where condition, allowing an attacker to execute arbitrary code. Example 1: The following code shows a simple example of a double free vulnerability. C Example: char* ptr = (char*)malloc (SIZE); ... if (abrt) { free(ptr); } ... free(ptr); Double free vulnerabilities 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 Although some double free vulnerabilities are not much more complicated than the previous example, most are spread out across hundreds of lines of code or even different files. Programmers seem particularly susceptible to freeing global variables more than once. Example 2: While contrived, this code should be exploitable on Linux distributions which do not ship with heap-chunk check summing turned on. C Example: #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); }
Architecture and Design Choose a language that provides automatic memory management. 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. Implementation Use a static analysis tool to find double free instances.
This is usually resultant from another weakness, such as an unhandled error or race condition between threads. It could also be primary to weaknesses such as buffer overflows.
A weakness where code path has: 1. start statement that relinquishes a dynamically allocated memory resource 2. end statement that relinquishes the dynamically allocated memory resource It could be argued that Double Free would be most appropriately located as a child of "Use after Free", but "Use" and "Release" are considered to be distinct operations within vulnerability theory, therefore this is more accurately "Release of a Resource after Expiration or Release", which doesn't exist yet. Submissions PLOVER. (Externally Mined) Modifications Eric Dalci. Cigital. 2008-07-01. (External) updated Potential_Mitigations,
Time_of_Introduction KDM Analytics. 2008-08-01. (External) added/updated white box definitions CWE Content Team. MITRE. 2008-09-08. (Internal) updated Applicable_Platforms, Common_Consequences,
Description, Maintenance_Notes, Relationships, Other_Notes,
Relationship_Notes, Taxonomy_Mappings CWE Content Team. MITRE. 2008-11-24. (Internal) updated Relationships,
Taxonomy_Mappings CWE Content Team. MITRE. 2009-05-27. (Internal) updated Demonstrative_Examples |
|
Page Last Updated:
May 26, 2009
|
|
CWE is a Software Assurance strategic initiative sponsored by the National Cyber Security Division of the U.S. Department of Homeland Security. This Web site is hosted by The MITRE Corporation. Contact cwe@mitre.org for more information. |
|||
