NULL Pointer Dereference
Weakness ID: 476 (Weakness Base)Status: Draft
+ Description

Description Summary

A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.
+ Time of Introduction
  • Implementation
+ Applicable Platforms






+ Common Consequences

NULL pointer dereferences usually result in the failure of the process unless exception handling (on some platforms) is available and implemented. Even when exception handling is being used, it can still be very difficult to return the software to a safe state of operation.


In very rare circumstances and environments, code execution is possible.

+ Likelihood of Exploit


+ Demonstrative Examples

Example 1

NULL pointer dereference issue can occur through a number of flaws, including race conditions, and simple programming omissions. While there are no complete fixes aside from conscientious programming, the following steps will go a long way to ensure that NULL pointer dereferences do not occur. Before using a pointer, ensure that it is not equal to NULL:

(Good Code)
Example Language:
if (pointer1 != NULL) {
/* make use of pointer1 */
/* ... */

When freeing pointers, ensure they are not set to NULL, and be sure to set them to NULL once they are freed:

(Good Code)
Example Language:
if (pointer1 != NULL) {
pointer1 = NULL;

If you are working with a multi-threaded or otherwise asynchronous environment, ensure that proper locking APIs are used to lock before the if statement; and unlock when it has finished.

Example 2

This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.

(Bad Code)
Example Language:
void host_lookup(char *user_supplied_addr){
struct hostent *hp;
in_addr_t *addr;
char hostname[64];
in_addr_t inet_addr(const char *cp);

/*routine that ensures user_supplied_addr is in the right format for conversion */
addr = inet_addr(user_supplied_addr);
hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);
strcpy(&hostname, hp->h_name);

If an attacker provides an address that appears to be well-formed, but the address does not resolve to a hostname, then the call to gethostbyaddr() will return NULL. Since the code does not check the return value from gethostbyaddr (CWE-252), a NULL pointer dereference would then occur in the call to strcpy().

Note that this example is also vulnerable to a buffer overflow (see CWE-119).

Example 3

In the following code, the programmer assumes that the system always has a property named "cmd" defined. If an attacker can control the program's environment so that "cmd" is not defined, the program throws a NULL pointer exception when it attempts to call the trim() method.

(Bad Code)
Example Language: Java 
String cmd = System.getProperty("cmd");
cmd = cmd.trim();
+ Observed Examples
CVE-2005-3274race condition causes a table to be corrupted if a timer activates while it is being modified, leading to resultant NULL dereference; also involves locking.
CVE-2002-1912large number of packets leads to NULL dereference
CVE-2005-0772packet with invalid error status value triggers NULL dereference
+ Potential Mitigations

Phase: Implementation

If all pointers that could have been modified are sanity-checked previous to use, nearly all NULL pointer dereferences can be prevented.

Phase: Requirements

The choice could be made to use a language that is not susceptible to these issues.

Phase: Implementation

Check the results of all functions that return a value and verify that the value is non-null before acting upon it.

Effectiveness: Moderate

Checking the return value of the function will typically be sufficient, however beware of race conditions (CWE-362) in a concurrent environment.

This solution does not handle the use of improperly initialized variables (CWE-665).

Phase: Architecture and Design

Identify all variables and data stores that receive information from external sources, and apply input validation to make sure that they are only initialized to expected values.

Phase: Implementation

Explicitly initialize all your variables and other data stores, either during declaration or just before the first usage.

Phase: Testing

Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible.

Phase: Testing

Use dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Phase: Testing

Identify error conditions that are not likely to occur during normal usage and trigger them. For example, run the program under low memory conditions, run with insufficient privileges or permissions, interrupt a transaction before it is completed, or disable connectivity to basic network services such as DNS. Monitor the software for any unexpected behavior. If you trigger an unhandled exception or similar error that was discovered and handled by the application's environment, it may still indicate unexpected conditions that were not handled by the application itself.

+ Weakness Ordinalities

NULL pointer dereferences are frequently resultant from rarely encountered error conditions, since these are most likely to escape detection during the testing phases.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)Named Chain(s) this relationship pertains toChain(s)
ChildOfWeakness ClassWeakness Class398Indicator of Poor Code Quality
Development Concepts (primary)699
Seven Pernicious Kingdoms (primary)700
Research Concepts (primary)1000
ChildOfCategoryCategory730OWASP Top Ten 2004 Category A9 - Denial of Service
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory737CERT C Secure Coding Section 03 - Expressions (EXP)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfCategoryCategory742CERT C Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C Secure Coding Standard734
ChildOfCategoryCategory8082010 Top 25 - Weaknesses On the Cusp
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
PeerOfWeakness BaseWeakness Base373State Synchronization Error
Research Concepts1000
MemberOfViewView630Weaknesses Examined by SAMATE
Weaknesses Examined by SAMATE (primary)630
CanFollowWeakness BaseWeakness Base252Unchecked Return Value
Research Concepts1000
Unchecked Return Value to NULL Pointer Dereference690
CanFollowWeakness VariantWeakness Variant789Uncontrolled Memory Allocation
Research Concepts1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
7 Pernicious KingdomsNull Dereference
CLASPNull-pointer dereference
PLOVERNull Dereference (Null Pointer Dereference)
OWASP Top Ten 2004A9CWE More SpecificDenial of Service
CERT C Secure CodingEXP34-CEnsure a null pointer is not dereferenced
CERT C Secure CodingMEM32-CDetect and handle memory allocation errors
+ White Box Definitions

A weakness where the code path has:

1. start statement that assigns a null value to the pointer

2. end statement that dereferences a pointer

3. the code path does not contain any other statement that assigns value to the pointer

+ Content History
Submission DateSubmitterOrganizationSource
7 Pernicious KingdomsExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-08-01KDM AnalyticsExternal
added/updated white box definitions
2008-09-08CWE Content TeamMITREInternal
updated Applicable Platforms, Common Consequences, Relationships, Other Notes, Taxonomy Mappings, Weakness Ordinalities
2008-11-24CWE Content TeamMITREInternal
updated Relationships, Taxonomy Mappings
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative Examples
2009-10-29CWE Content TeamMITREInternal
updated Relationships
2009-12-28CWE Content TeamMITREInternal
updated Common Consequences, Demonstrative Examples, Other Notes, Potential Mitigations, Weakness Ordinalities