Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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CWE-476: NULL Pointer Dereference

Weakness ID: 476
Abstraction: Base
Status: Draft
Presentation Filter:
+ 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.

Extended Description

NULL pointer dereference issues can occur through a number of flaws, including race conditions, and simple programming omissions.

+ Time of Introduction
  • Implementation
+ Applicable Platforms






+ Common Consequences

Technical Impact: DoS: crash / exit / restart

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.


Technical Impact: Execute unauthorized code or commands

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

+ Likelihood of Exploit


+ Detection Methods

Automated Dynamic Analysis

This weakness can be detected using 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.

Effectiveness: Moderate

Manual Dynamic Analysis

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.

+ Demonstrative Examples

Example 1

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.

(Mitigation Code)
if (pointer1 != NULL) {
/* make use of pointer1 */
/* ... */

If you are working with a multithreaded 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();

Example 4

This application has registered to handle a URL when sent an intent:

(Bad Code)
Example Language: Java 
IntentFilter filter = new IntentFilter("com.example.URLHandler.openURL");
MyReceiver receiver = new MyReceiver();
registerReceiver(receiver, filter);
public class UrlHandlerReceiver extends BroadcastReceiver {
public void onReceive(Context context, Intent intent) {
if("com.example.URLHandler.openURL".equals(intent.getAction())) {
String URL = intent.getStringExtra("URLToOpen");
int length = URL.length();

The application assumes the URL will always be included in the intent. When the URL is not present, the call to getStringExtra() will return null, thus causing a null pointer exception when length() is called.

+ Observed Examples
race condition causes a table to be corrupted if a timer activates while it is being modified, leading to resultant NULL dereference; also involves locking.
large number of packets leads to NULL dereference
packet with invalid error status value triggers NULL dereference
chain: race condition for an argument value, possibly resulting in NULL dereference
chain: race condition might allow resource to be released before operating on it, leading to NULL dereference
chain: some unprivileged ioctls do not verify that a structure has been initialized before invocation, leading to NULL dereference
chain: IP and UDP layers each track the same value with different mechanisms that can get out of sync, possibly resulting in a NULL dereference
chain: uninitialized function pointers can be dereferenced allowing code execution
chain: improper initialization of memory can lead to NULL dereference
chain: game server can access player data structures before initialization has happened leading to NULL dereference
chain: unchecked return value can lead to NULL dereference
SSL software allows remote attackers to cause a denial of service (crash) via a crafted SSL/TLS handshake that triggers a null dereference.
Network monitor allows remote attackers to cause a denial of service (crash) via a malformed RADIUS packet that triggers a null dereference.
Network monitor allows remote attackers to cause a denial of service (crash) via a malformed Q.931, which triggers a null dereference.
Chat client allows remote attackers to cause a denial of service (crash) via a passive DCC request with an invalid ID number, which causes a null dereference.
Server allows remote attackers to cause a denial of service (crash) via malformed requests that trigger a null dereference.
OS allows remote attackers to cause a denial of service (crash from null dereference) or execute arbitrary code via a crafted request during authentication protocol selection.
Game allows remote attackers to cause a denial of service (server crash) via a missing argument, which triggers a null pointer dereference.
Network monitor allows remote attackers to cause a denial of service (crash) or execute arbitrary code via malformed packets that cause a NULL pointer 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.

+ 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
Weaknesses for Simplified Mapping of Published Vulnerabilities (primary)1003
ChildOfCategoryCategory465Pointer Issues
Development Concepts699
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
ChildOfCategoryCategory8672011 Top 25 - Weaknesses On the Cusp
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory871CERT C++ Secure Coding Section 03 - Expressions (EXP)
Weaknesses Addressed by the CERT C++ Secure Coding Standard868
ChildOfCategoryCategory876CERT C++ Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory971SFP Secondary Cluster: Faulty Pointer Use
Software Fault Pattern (SFP) Clusters (primary)888
ParentOfCompound Element: ChainCompound Element: Chain690Unchecked Return Value to NULL Pointer Dereference
Development Concepts (primary)699
MemberOfViewView630Weaknesses Examined by SAMATE
Weaknesses Examined by SAMATE (primary)630
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
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
CERT C++ Secure CodingEXP34-CPPEnsure a null pointer is not dereferenced
CERT C++ Secure CodingMEM32-CPPDetect and handle memory allocation errors
Software Fault PatternsSFP7Faulty Pointer Use
+ 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
2010-02-16CWE Content TeamMITREInternal
updated Potential_Mitigations, Relationships
2010-06-21CWE Content TeamMITREInternal
updated Demonstrative_Examples, Description, Detection_Factors, Potential_Mitigations
2010-09-27CWE Content TeamMITREInternal
updated Demonstrative_Examples, Observed_Examples, Relationships
2010-12-13CWE Content TeamMITREInternal
updated Relationships
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2011-06-27CWE Content TeamMITREInternal
updated Related_Attack_Patterns, Relationships
2011-09-13CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Observed_Examples, Related_Attack_Patterns, Relationships
2014-02-18CWE Content TeamMITREInternal
updated Demonstrative_Examples
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2015-12-07CWE Content TeamMITREInternal
updated Relationships
2017-01-19CWE Content TeamMITREInternal
updated Relationships

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Page Last Updated: May 05, 2017