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Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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ID

CWE-825: Expired Pointer Dereference

Weakness ID: 825
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The program dereferences a pointer that contains a location for memory that was previously valid, but is no longer valid.
+ Extended Description
When a program releases memory, but it maintains a pointer to that memory, then the memory might be re-allocated at a later time. If the original pointer is accessed to read or write data, then this could cause the program to read or modify data that is in use by a different function or process. Depending on how the newly-allocated memory is used, this could lead to a denial of service, information exposure, or code execution.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
+ Relevant to the view "Development Concepts" (CWE-699)
+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Confidentiality

Technical Impact: Read Memory

If the expired pointer is used in a read operation, an attacker might be able to control data read in by the application.
Availability

Technical Impact: DoS: Crash, Exit, or Restart

If the expired pointer references a memory location that is not accessible to the program, or points to a location that is "malformed" (such as NULL) or larger than expected by a read or write operation, then a crash may occur.
Integrity
Confidentiality
Availability

Technical Impact: Execute Unauthorized Code or Commands

If the expired pointer is used in a function call, or points to unexpected data in a write operation, then code execution may be possible.
+ Alternate Terms
Dangling pointer
+ Demonstrative Examples

Example 1

The following code shows a simple example of a use after free error:

(bad)
Example Language:
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.

Example 2

The following code shows a simple example of a double free error:

(bad)
Example Language:
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.

+ Observed Examples
ReferenceDescription
access of expired memory address leads to arbitrary code execution
stale pointer issue leads to denial of service and possibly other consequences
read of value at an offset into a structure after the offset is no longer valid
+ Potential Mitigations

Phase: Architecture and Design

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.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory8672011 Top 25 - Weaknesses On the Cusp
MemberOfViewView884CWE Cross-section
+ Notes

Maintenance

There are close relationships between incorrect pointer dereferences and other weaknesses related to buffer operations. There may not be sufficient community agreement regarding these relationships. Further study is needed to determine when these relationships are chains, composites, perspective/layering, or other types of relationships. As of September 2010, most of the relationships are being captured as chains.

Research Gap

Under-studied and probably under-reported as of September 2010. This weakness has been reported in high-visibility software, but applied vulnerability researchers have only been investigating it since approximately 2008, and there are only a few public reports. Few reports identify weaknesses at such a low level, which makes it more difficult to find and study real-world code examples.

Terminology

Many weaknesses related to pointer dereferences fall under the general term of "memory corruption" or "memory safety." As of September 2010, there is no commonly-used terminology that covers the lower-level variants.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2010-09-22CWE Content TeamMITRE
Modifications
Modification DateModifierOrganizationSource
2011-06-27CWE Content TeamMITRE
updated Demonstrative_Examples, Potential_Mitigations, Relationships
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2013-02-21CWE Content TeamMITRE
updated Alternate_Terms

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Page Last Updated: November 14, 2017