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According to the Java API Class SecurityManager documentation [API 20132014],

The security manager is a class that allows applications to implement a security policy. It allows an application to determine, before performing a possibly unsafe or sensitive operation, what the operation is and whether it is being attempted in a security context that allows the operation to be performed. The application can allow or disallow the operation.

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Regarding the implementation and use of custom security managers as opposed to default ones, the Java security architecture specification [SecuritySpec 2010] states:

We encourage the use of AccessController in application code, while customization of a security manager (via subclassing) should be the last resort and should be done with extreme care. Moreover, a customized security manager, such as one that always checks the time of the day before invoking standard security checks, could and should utilize the algorithm provided by AccessController whenever appropriate.

Many of the Java SE APIs perform security manager checks by default before performing sensitive operations. For example, the constructor of class java.io.FileInputStream throws a SecurityException if the caller does not have the permission to read a file. Because SecurityException is a subclass of RuntimeException, the declarations of some API methods (for example, those of the java.io.FileReader class) may lack a throws clause that lists the SecurityException. Avoid depending on the presence or absence of security manager checks that are not specified in the API method's documentation.

Noncompliant Code Example (Command-Line Installation)

This noncompliant code example fails to install any security manager from the command line. Consequently, the program runs with all permissions enabled; that is, there is no security manager to prevent any nefarious actions the program might perform.

java LocalJavaApp

Compliant Solution (Default Policy File)

Any Java program can attempt to install a SecurityManager programmatically, although the currently active security manager may forbid this operation. Applications designed to run locally can specify a default security manager by use of a flag on the command line at invocation.

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The default security policy file java.policy—found in the /path/to/java.home/lib/security directory on UNIX-like systems and its equivalent on Microsoft Windows systems—grants a few permissions (reading system properties, binding to unprivileged ports, and so forth). A user-specific policy file may be located in the user's home directory. The union of these policy files specifies the permissions granted to a program. The java.security file can specify which policy files are used. If either of the systemwide java.policy or java.security files is deleted, no permissions are granted to the executing Java program.

Compliant Solution (Custom Policy File)

Use double equals (==) instead of the single equals (=) when overriding the global Java security policy file with a custom policy file:

java -Djava.security.manager \
     -Djava.security.policy==policyURL \
     LocalJavaApp

Compliant Solution (Additional Policy Files)

The appletviewer automatically installs a security manager with the standard policy file. To specify additional policy files, use the -J flag.

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Note that the policy file specified in the argument is ignored when the policy.allowSystemProperty property in the security properties file (java.security) is set to false; the default value of this property is true. Default Policy Implementation and Policy File Syntax [Policy 2010] discusses in depth the issues of and syntax for writing policy files.

Noncompliant Code Example (Programmatic Installation)

A SecurityManager can also be activated using the static System.setSecurityManager() method. Only one SecurityManager may be active at a time. This method replaces the currently active SecurityManager with the SecurityManager provided in its argument or no SecurityManager if its argument is null.

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An active SecurityManager that enforces a sensible security policy will prevent the system from deactivating it, causing this code to throw a SecurityException.

Compliant Solution (Default Security Manager)

This compliant solution instantiates and sets the default security manager:

try {
  System.setSecurityManager(new SecurityManager());
} catch (SecurityException se) {
  // Cannot set security manager, log appropriately
}

Compliant Solution (Custom Security Manager)

This compliant solution demonstrates how to instantiate a custom SecurityManager class called CustomSecurityManager  by invoking its constructor with a password; this custom security manager is then installed as the active security manager.

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After this code executes, APIs that perform security checks use the custom security manager. As noted earlier, custom security managers should be installed only when the default security manager lacks the required functionality.

Applicability

Java security fundamentally depends on the existence of a security manager. In its absence, sensitive actions can execute without restriction.

Programmatic detection of the presence or absence of a SecurityManager at runtime is straightforward. Static analysis can address the presence or absence of code that would attempt to install a SecurityManager if the code were executed. Checking whether the SecurityManager is installed early enough, whether it specifies the desired properties, or whether it is guaranteed to be installed may be possible in some special cases but is generally undecidable.

Bibliography

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