Null pointer dereferencing occurs when a null variable is treated as if it were a valid object reference and used without checking its state. This condition results in a NullPointerException, which could result in denial of service. Programs must not dereference null pointers.
Noncompliant Code Example
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This noncompliant example shows a bug in Tomcat version 4.1.24, initially discovered by Reasoning \[[Reasoning 2003|AA. Bibliography#Reasoning 03]\]. The {{cardinality}} method was designed to return the number of occurrences of object {{obj}} in collection {{col}}. One valid use of the {{cardinality}} method is to determine how many objects in the collection are {{null}}. However, because membership in the collection is checked using the expression {{obj.equals(elt)}}, a null pointer dereference is guaranteed whenever {{obj}} is {{null}}. |
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public static int cardinality(Object obj, final Collection col) {
int count = 0;
Iterator it = col.iterator();
while (it.hasNext()) {
Object elt = it.next();
if ((null == obj && null == elt) || obj.equals(elt)) { // null pointer dereference
count++;
}
}
return count;
}
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Compliant Solution
This compliant solution eliminates the null pointer dereference.
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Note that explicit null checks as shown here are one acceptable approach to eliminating null pointer dereferences;
Risk Assessment
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Dereferencing a {{null}} pointer can lead to a denial of service. For example, Java Web Start applications and applets particular to JDK version 1.6, prior to update 4, were affected by a bug that had some noteworthy security consequences. In some isolated cases, the application or applet's attempt to establish an HTTPS connection with a server generated a {{NullPointerException}} \[[SDN 2008|AA. Bibliography#SDN 08]\]. The resulting failure to establish a secure HTTPS connection with the server caused a denial of service: clients were temporarily forced to use an insecure http channel for data exchange. In multithreaded programs, null pointer dereferences can violate cache coherency policies and can cause resource leaks. |
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
EXP08EXP01-J | low | likely | high | P3 | L3 |
Automated Detection
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Null pointer dereferences can happen in path-dependent ways. Limitations of automatic detection tools can require manual inspection of code \[[Hovemeyer 2007|AA. Bibliography#Hovemeyer 07]\] to detect instances of null pointer dereferences. Annotations for method parameters that must be non-null can reduce the need for manual inspection by assisting automated null pointer dereference detection; use of these annotations is strongly encouraged. |
The Coverity Prevent Version 5.0 FORWARD_NULL checker can detect the instance where reference is checked against null but then dereferenced anyway.
Related Guidelines
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="0ea10d808aeed200-663ed12c-432c4154-a54ab35d-fa350e7f6aaa61a3d623337e"><ac:plain-text-body><![CDATA[ | [ISO/IEC TR 24772:2010 | http://www.aitcnet.org/isai/] | "Null Pointer Dereference [XYH]" | ]]></ac:plain-text-body></ac:structured-macro> |
CWE ID 476, "NULL Pointer Dereference" |
Bibliography
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="11c145e2e04e6c3e-b9d87d9e-4dd84404-905d9d0f-8a5dba0466a31ca0a692b8a5"><ac:plain-text-body><![CDATA[ | [[API 2006 | AA. Bibliography#API 06]] | [method doPrivileged() | http://java.sun.com/javase/6/docs/api/java/security/AccessController.html#doPrivileged(java.security.PrivilegedAction)] | ]]></ac:plain-text-body></ac:structured-macro> |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="8a06a9a75c6d8892-f5a04a06-4be14627-83afae69-2681991a429cc6cd3191bf97"><ac:plain-text-body><![CDATA[ | [[Hovemeyer 2007 | AA. Bibliography#Hovemeyer 07]] |
| ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="bf23a0c2a8478e9c-8b52bf72-495743a0-a6b7a7dc-9df40b223b41daaccd73fe03"><ac:plain-text-body><![CDATA[ | [[Reasoning 2003 | AA. Bibliography#Reasoning 03]] | Defect ID 00-0001 | ]]></ac:plain-text-body></ac:structured-macro> | |
| Null Pointer Dereference | ||||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="e3e19af05d1bb531-9938ef2c-4fe04a2c-a6c49c40-8e339a999956eb6d8814c6d8"><ac:plain-text-body><![CDATA[ | [[SDN 2008 | AA. Bibliography#SDN 08]] | [Bug ID 6514454 | http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6514454] | ]]></ac:plain-text-body></ac:structured-macro> |
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