Null pointer dereferencing occurs when a null variable is treated as if it were a valid object or field and is used without checking its state. This condition results in a NullPointerException, which could result in denial of service. For additional information, see the related rule "void ERR15-J. Do not catch NullPointerException."
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 with 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;
}
|
Compliant Solution
This compliant solution eliminates the null pointer dereference.
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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) ||
(null != obj && obj.equals(elt))) {
count++;
}
}
return count;
}
|
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. |
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. A {{NullPointerException}} was generated in some isolated cases when the application or applet attempted to establish an HTTPS connection with a server \[[SDN 2008|AA. Bibliography#SDN 08]\]. The 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 |
|---|---|---|---|---|---|
EXP11-J | low | likely | high | P3 | L3 |
Automated Detection
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
Bibliography
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="31fc37623a0e6ae5-d092e359-4785442f-97c18f39-1755e8509a00f83863469fa1"><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="5712837b45d475e8-9c38e56d-45824421-97328d07-be9152449f9d0b3c09db38c2"><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="d8ea48f19871d9b0-57a0dcff-49394ddc-b9d4b4ca-c6ac6430a97780a989545b42"><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="68bac48741a97921-a9c2bd21-483744a8-9e1f950d-428fb7355731e2300c4e432b"><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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