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The default {{SecurityManager}} checks whether the caller of a particular method has sufficient permissions to proceed with an action. An action is a level of access; for instance, the actions for {{java.io.FilePermission}} are "read,", "write,", "execute,", and "delete" \[[Sun 2006|AA. References#Sun 06]\]. The "Permission Descriptions and Risks" guide \[[Permissions 2008|AA. References#Permissions 08]\] enumerates the default permissions and the risks associated with granting these permissions to Java code. |
Sometimes, stronger restrictions than those provided by the default security manager are necessary. Custom permissions prove to be more suitable for privilege separation in such cases. Failure to provide custom permissions when no corresponding default permissions exist can lead to privilege escalation vulnerabilities where that enable untrusted callers can to execute restricted operations or actions.
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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. When used, the default security manager does not permit the loading of the library unless the RuntimePermission loadLibrary.awt 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 . This is because the permission for this operation is not non-default nondefault and , consequently , is 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.
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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).
By default, permissions cannot be defined to support actions using BasicPermission, but the actions can be freely implemented in the subclass , 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 has to define two constructors to call the most appropriate (one- or two-argument) superclass constructor (the superclass lacks a default constructor). The two-argument constructor also accepts an action even though a basic permission does not use it. This 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 in accordance with guideline SEC53-J. Classes that derive from a sensitive class or implement a sensitive interface must be declared final.
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Assuming that the above sources reside in the c:\package directory on a Windows-based system, for example, the policy file needs to grant two permissions, ExceptionReporterPermission exc.reporter and RuntimePermission loadlibrary.awt.
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