OBJ10-J. Do not use public static non-final variables

Client code can trivially access public static fields. Neither reads nor writes to such variables are checked by a security manager. Furthermore, new values cannot be validated programmatically before they are stored in these fields.

In the presence of multiple threads, non-final public static fields can be modified in inconsistent ways. (See rule "TSM01-J. Do not let the (this) reference escape during object construction" for an example.)

Improper use of public static fields can also result in type safety issues. For example, untrusted code can supply an unexpected subtype when the variable is defined to be of a more general type, such as java.lang.Object [[Gong 2003]].

Noncompliant Code Example

This noncompliant code example is adopted from JDK v1.4.2 [[FT 2008]]. It declares a function table containing a public static field.

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 XPathContext, which is used to set the reference node for evaluating XPath expressions. Manipulating XPathContext can cause XML fields to be modified in inconsistent ways, resulting in unexpected behavior. Also, because static variables are global across the JRE, they can be used as a covert communication channel between different application domains (for example, through code loaded by different class loaders).

Note that this vulnerability was repaired in JDK v1.4.2_05.

Compliant Solution

This compliant solution declares the FuncLoader static field final and treats it like a constant.

public static final FuncLoader m_functions;
// Initialize m_functions in a constructor

Fields declared static final are also safe for multithreaded use. (See rule "TSM03-J. Do not publish partially initialized objects.") However, remember that simply changing the modifier to final might not prevent attackers from indirectly retrieving an incorrect value from the static final variable before its initialization. (See rule "DCL12-J. Prevent class initialization cycles" for more information.) Furthermore, individual members of the referenced object can also be changed if the object itself is mutable. (See rule "OBJ02-J. Never conflate immutability of a reference with that of the referenced object.")

It is also permissible to use a wrapper method to retrieve the value of m_functions. This has encapsulation advantages because it permits m_functions to be declared private. See rule "OBJ01-J. Declare data members as private and provide accessible wrapper methods" for more information.

Noncompliant Code Example (serialVersionUID)

This noncompliant code example uses a public static non-final serialVersionUID field in a class designed for serialization.

class DataSerializer implements Serializable {
  public static long serialVersionUID = 1973473122623778747L;
  // ...
}

Compliant Solution

This compliant solution declares the serialVersionUID field final and limits its accessibility to private.

class DataSerializer implements Serializable {
  private static final long serialVersionUID = 1973473122623778747L;
}

The serialization mechanism uses the serialVersionUID field internally, so accessible wrapper methods are unnecessary.

Risk Assessment

Unauthorized modifications of public static variables can result in unexpected behavior and violation of class invariants. Furthermore, because static variables can be visible to code loaded by different class loaders when those class loaders are in the same delegation chain, such variables can be used as a covert communication channel between different application domains in some cases.

Related Guidelines

Bibliography

OBJ09-J. Do not expose sensitive private members of an outer class from within a nested class      04. Object Orientation (OBJ)      OBJ11-J. Prevent access to partially initialized objects