Unsafe Mobile Code

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Vulnerabilities Table of Contents

Description
Mobile code, such as a Java Applet, is code that is transmitted across a network and executed on a remote machine. Because mobile code developers have little if any control of the environment in which their code will execute, special security concerns become relevant. One of the biggest environmental threats results from the risk that the mobile code will run side-by-side with other, potentially malicious, mobile code. Because all of the popular web browsers execute code from multiple sources together in the same JVM, many of the security guidelines for mobile code are focused on preventing manipulation of your objects' state and behavior by adversaries who have access to the same virtual machine where your program is running.

Access Violation
The program violates secure coding principles for mobile code by returning a private array variable from a public access method.

Returning a private array variable from a public access method allows the calling code to modify the contents of the array, effectively giving the array public access and contradicting the intentions of the programmer who made it private.

Example

The following Java Applet code mistakenly returns a private array variable from a public access method.

public final class urlTool extends Applet { private URL[] urls; public URL[] getURLs { return urls; }			...	}.

Dangerous Array Declaration
The program violates secure coding principles for mobile code by declaring an array public, final, and static.

In most cases an array declared public, final, and static is a bug. Because arrays are mutable objects, the final constraint requires that the array object itself be assigned only once, but makes no guarantees about the values of the array elements. Since the array is public, a malicious program can change the values stored in the array. In most situations the array should be made private.

Example

The following Java Applet code mistakenly declares an array public, final, and static.

public final class urlTool extends Applet { public final static URL[] urls; ...	}

Dangerous Public Field
The program violates secure coding principles for mobile code by declaring a member variable public but not final.

All public member variables in an Applet and in classes used by an Applet should be declared final to prevent an attacker from manipulating or gaining unauthorized access to the internal state of the Applet.

Example

The following Java Applet code mistakenly declares a member variable public but not final.

public final class urlTool extends Applet { public URL url; ...	}

Inner Class
The program violates secure coding principles for mobile code by making use of an inner class.

Inner classes quietly introduce several security concerns because of the way they are translated into Java bytecode. In Java source code, it appears that an inner class can be declared to be accessible only by the enclosing class, but Java bytecode has no concept of an inner class, so the compiler must transform an inner class declaration into a peer class with package level access to the original outer class. More insidiously, since an inner class can access private fields in their enclosing class, once an inner class becomes a peer class in bytecode, the compiler converts private fields accessed by the inner class into protected fields.

Example

The following Java Applet code mistakenly makes use of an inner class.

public final class urlTool extends Applet { private final class urlHelper { ...		}		...	}

Public finalize Method
The program violates secure coding principles for mobile code by declaring a finalizemethod public.

A program should never call finalize explicitly, except to call super.finalize inside an implementation of finialize. In mobile code situations, the otherwise error prone practice of manual garbage collection can become a security threat if an attacker can maliciously invoke one of your finalize methods because it is declared with public access. If you are using finalize as it was designed, there is no reason to declare finalize with anything other than protected access.

Example

The following Java Applet code mistakenly declares a public finalize method.

public final class urlTool extends Applet { public void finalize { ...		}		...	}

Risk Factors

 * Talk about the factors that make this vulnerability likely or unlikely to actually happen
 * Discuss the technical impact of a successful exploit of this vulnerability
 * Consider the likely [business impacts] of a successful attack

Related Attacks

 * Attack 1
 * Attack 2

Related Vulnerabilities

 * Vulnerability 1
 * Vulnerabiltiy 2

Related Controls

 * Use encapsulation

Related Technical Impacts

 * Technical Impact 1
 * Technical Impact 2