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

Noncompliant code example

This is an example from the JDK 1.4.2 software.[[Function Table]]

package org.apache.xpath.compiler;

public class FunctionTable {
    public static FuncLoader m_functions;
}

An attacker can replace the function table as follows

FunctionTable.m_functions = <new_table>;

Replacing the function table gives the attacker access to the XPathContext used to evaluate XPath expression. Static variables are global across a Java runtime environment. They can be used as a communication channel between different application domains (e.g. by code loaded into 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.

public class MyClass {
    public static final int LEFT  = 1;
    public static final int RIGHT = 2;
}

...
public static final FuncLoader m_functions;
...

Compliant Solution

Additionally for mutable static state one can define assessor methods and add appropriate security checks.

public class MyClass {

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

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

However as a cautionary note, simply changing the modifier to final may not prevent attackers from indirectly indirectly retrieving an incorrect value from the static final variable before its initialization. See Eliminate Class Initialization Cycles for more on this problem.

Risk Assessment

Unauthorized modifications to public static variables can result in unexpected behavior and can bypass important security checks and/or invoke malicious code.

References

Avoiding Antipatterns Antipattern 5, Misusing Public Static Variables

Java Secure Coding Guidelines Section 3.1, Treat public static fields as constants

Function Table Field detail, public static FuncLoader m_functions