The product attempts to close or release a resource or handle more than once, without any successful open between the close operations.
Code typically requires "opening" handles or references to resources such as memory, files, devices, socket connections, services, etc. When the code is finished with using the resource, it is typically expected to "close" or "release" the resource, which indicates to the environment (such as the OS) that the resource can be re-assigned or reused by unrelated processes or actors - or in some cases, within the same process. API functions or other abstractions are often used to perform this release, such as free() or delete() within C/C++, or file-handle close() operations that are used in many languages. Unfortunately, the implementation or design of such APIs might expect the developer to be responsible for ensuring that such APIs are only called once per release of the resource. If the developer attempts to release the same resource/handle more than once, then the API's expectations are not met, resulting in undefined and/or insecure behavior. This could lead to consequences such as memory corruption, data corruption, execution path corruption, or other consequences. Note that while the implementation for most (if not all) resource reservation allocations involve a unique identifier/pointer/symbolic reference, then if this identifier is reused, checking the identifier for resource closure may result in a false state of openness and closing of the wrong resource. For this reason, reuse of identifiers is discouraged.
Threat Mapped score: 1.8
Industry: Finiancial
Threat priority: P4 - Informational (Low)
CVE: CVE-2019-13351
file descriptor double close can cause the wrong file to be associated with a file descriptor.
CVE: CVE-2006-5051
Chain: Signal handler contains too much functionality (CWE-828), introducing a race condition that leads to a double free (CWE-415).
CVE: CVE-2004-0772
Double free resultant from certain error conditions.
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| Phase | Note |
|---|---|
| Implementation | N/A |
Intro: This example attempts to close a file twice. In some cases, the C library fclose() function will catch the error and return an error code. In other implementations, a double-free (CWE-415) occurs, causing the program to fault. Note that the examples presented here are simplistic, and double fclose() calls will frequently be spread around a program, making them more difficult to find during code reviews.
Body: There are multiple possible fixes. This fix only has one call to fclose(), which is typically the preferred handling of this problem - but this simplistic method is not always possible.
char b[2000]; FILE *f = fopen("dbl_cls.c", "r"); if (f) { b[0] = 0; fread(b, 1, sizeof(b) - 1, f); printf("%s\n'", b); int r1 = fclose(f); printf("\n-----------------\n1 close done '%d'\n", r1); int r2 = fclose(f); // Double close printf("2 close done '%d'\n", r2); }Intro: The following code shows a simple example of a double free vulnerability.
Body: Double free vulnerabilities have two common (and sometimes overlapping) causes:
char* ptr = (char*)malloc (SIZE); ... if (abrt) { free(ptr); } ... free(ptr);