Callers can trivially access and modify public nonfinal non-final static fields. Neither accesses nor modifications can be checked by a SecurityManager, and newly set values cannot be validated. Furthermore, multiple threads can modify nonfinal non-final public static data in ways that are not consistent.

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Replacing the function table gives the attacker access to the XPathContext used to evaluate XPath expressions. The XPathContext is used to set the reference node for evaluating XPath expressions. Manipulating this can cause unexpected behavior , and XML fields can be modified in inconsistent ways. Also, static variables are global across a Java runtime environmentthe Java Runtime Environment (JRE). They can be used as a communication channel between different application domains (e.g., by through code loaded into by different class loaders).

There are a few ways this problem can be avoided.

Compliant Solution

Treat public static fields as constants and declare them as final. Consider the use of enum types in the following example.

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The FuncLoader static instance in the noncompliant example could have been declared as final as shown below.

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public static final FuncLoader m_functions;
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Additionally, for mutable static state one can define assessor accessor methods and add appropriate security checks. Note that this is a different example and in this case , wherein the scope of the static variable has been changed to private.

public class MyClass {

    private static byte[] data;
  
  public static byte[] getData() {
        return data.clone();
    }

    public static void setData(byte[] b) {
        securityCheck();
       data = b.clone();
    }
}

As a cautionary note however, simply changing the modifier to final may not prevent attackers from indirectly retrieving an incorrect value from the static final variable before its initialization. See Eliminate Class Initialization Cycles MSC00-J. Eliminate class initialization cycles and OBJ03-J. Be careful about final reference for more on this problem.

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