The default SecurityManager checks whether the caller of a particular method has sufficient permissions to proceed with an action. An action is defined in Java's security architecture as a level of access and requires certain permissions before it can be performed. For Example, the actions for java.io.FilePermission are read, write, execute, and delete [API 2013]. The "Permission Descriptions and Risks" guide [Oracle 2011d] enumerates the default permissions and the risks associated with granting these permissions to Java code.
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This guideline addresses the problem of excess privileges. See SEC50-J. Avoid granting excess privileges for another approach to solving this problem.
Noncompliant Code Example
This noncompliant code example contains a privileged block that is used to perform two sensitive operations: loading a library and setting the default exception handler.
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When used, the default security manager forbids the loading of the library unless the RuntimePermission loadLibrary.myLib is granted in the policy file. However, the security manager does not automatically guard a caller from performing the second sensitive operation of setting the default exception handler because the permission for this operation is nondefault and, consequently, unavailable. This security weakness can be exploited, for example, by programming and installing an exception handler that reveals information that a legitimate handler would filter out.
Compliant Solution
This compliant solution defines a custom permission ExceptionReporterPermission with target exc.reporter to prohibit illegitimate callers from setting the default exception handler. This can be achieved by subclassing BasicPermission, which allows binary-style permissions (either allow or disallow). The compliant solution then uses a security manager to check whether the caller has the requisite permission to set the handler. The code throws a SecurityException if the check fails. The custom permission class ExceptionReporterPermission is also defined with the two required constructors.
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By default, permissions cannot be defined to support actions using BasicPermission, but the actions can be freely implemented in the subclass ExceptionReporterPermission if required. BasicPermission is abstract even though it contains no abstract methods; it defines all the methods that it extends from the Permission class. The custom-defined subclass of the BasicPermission class must define two constructors to call the most appropriate (one- or two-argument) superclass constructor (because the superclass lacks a default constructor). The two-argument constructor also accepts an action even though a basic permission does not use it. This behavior is required for constructing permission objects from the policy file. Note that the custom-defined subclass of the BasicPermission class is declared to be final.
Applicability
Running Java code without defining custom permissions where default permissions are inapplicable can leave an application open to privilege escalation vulnerabilities.
Bibliography
| [API 2013] | Class FilePermission Class SecurityManager |
| [Oaks 2001] | Chapter 5, "The Access Controller," "Permissions" |
| [Oracle 2011d] | Permissions in the Java™ SE 6 Development Kit (JDK) |
| [Oracle 2013c] | Permissions in Java SE 7 Development Kit (JDK) |
| [Policy 2010] | "Permission Descriptions and Risks" |
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