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One of the most promising aspects of the Java system is the ability of Java-compatible browsers to import code from remote servers and execute it Clearly, a key design goal of the Java language and the Java-compatible browser is to ensure that this can be done in a secure manner The potential for bad code deliberately to disrupt local machines, to copy local files to remote hosts or to copy remote files to local hosts is such that the design of the Java security system has been considered at great depth The most documented security implementation is that of the Sun HotJava browser, which implements a layered approach to security, instead of favouring a monolithic system The layers are: the Java language and compiler, byte-code verification, class loader verification, and local system protection In this section we will look briefly at each layer and discuss the general security of the Java browser 1261 The Java language and compiler Some of the language design features of Java add additional security to the system as a whole For example, pointers can be used in C++ to manipulate client systems and casts from one type to another are not checked in C or C++ Because Java does not implement pointer arithmetic (it implements true arrays) it is not possible to use them at all, let alone illegally Equally, the use of the cast operator is checked to see if the casting of the first type to the second is legal If it is not, the code cannot be compiled The compiler goes to some lengths to ensure that the byte-codes which are produced are safe and legal, and will refuse to compile code which does not meet the strict Java language rules 1262 Byte-code verification While the Sun compilers are trustworthy, what is to stop someone writing a bad compiler which modifies the output of byte-codes for some hidden purpose This is especially likely considering that Sun are actively encouraging the development of development environments and Java systems To stop this happening the browser applies a range of tests to the code fragments before they are executed The range of tests includes checking for faked pointers, ensuring that the code does not try to access the local system illegally, and checking that objects are correctly used, that all objects are called legally, and that the general state of the Java system remains stable The checking of code fragments is one of Java s strongest security features The overall security of the system benefits from the layered approach to security since defective code must be sufficiently complex to pass all levels of testing before it is executed The code fragments are verified before they are passed to the class loader by the verifier This process ensures that the code doesn t cause overruns or underruns and that no illegal assignments or casts are performed
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1263 The class loader Once the code fragment has been passed by the verifier, it is passed to the class loader The class loader ensures that classes do not violate the rules by spoofing the system into using classes which are not part of the core Java system and ensures that each class is in a unique namespace which is not violated by other downloaded classes This checking is performed at run-time and ensures that the state of the Java system is not affected 1264 Local system protection The final layer of explicit security is the set of strict access controls that are placed on Java applets The Java browser examines calls from classes to the local system and checks to see if they are to the small section of the system which the Java system needs to access If an applet makes a call to a file which does not exist or to a file which is outside the area of the Java system, the user is presented with the information and the choice of either permitting or denying the request This policy aims to ensure that Java applets cannot write to or read from files which are not directly part of the Java system, including personal documents and system-wide files Some browsers will implement a system of Access Control Lists which define the areas of the file system that the Java code can read and write, and these can be overridden by the user specifying additional areas in environment variables The key idea to remember is that the Java applet cannot change the lists and therefore developers should not assume that the user has set up the access lists correctly prior to code execution 1265 The cost of security The cost of this kind of security is a restriction of the type of functionality which can be included in applets The Sun HotJava browser includes support for varying the level of security applied to downloaded classes, but commercial browsers are not required to follow the same model and, consequently, commercial and political pressures will ensure that a more restrictive security policy is implemented for the majority of browsers The scope and depth of the Java security system aims to protect the user from malicious damage while allowing the execution of code fragments from a range of unknown sources At the same time the tight security could be seen as limiting to the developer In the long term it is better that the Java system be oriented towards security instead of flexibility since the importance of security on the large networks is increasing and the publicity surrounding the potential for loss and damage is becoming greater Consequently, even if the strict security system does restrict the developers to certain models of access the interests of the Java community as a whole are best served by strong fences and thorough tests
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