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Last modified 04/29/97

Frequently Asked Questions - Applet Security

The goal for the JDK is to enable browsers to run untrusted applets in a trusted environment. Our approach is to be conservative at first, and to add functionality when it can be added securely. The intent is to prevent applets from inspecting or changing files on the client file system. Also, the intent is to prevent applets from using network connections to circumvent file protections or people's expectations of privacy.

JDK 1.1 provides the basic technology for loading and authenticating signed classes. This enables browsers to run trusted applets in a trusted environment. This does not make obselete the need to run untrusted applets in a secure way. In the release following JDK 1.1, we will provide tools for finer-grained control of flexible security policies.


Status of all known security-related bugs

JDK 1.1.1 Signing Flaw, (April 29, 1997)
The Secure Internet Programming team at Princeton University notified JavaSoft of a flaw in the way the Java runtime manages identities of signers. JavaSoft is testing the fix and plans to ship it to licensees within 48 hours. Full details here.

Privacy hole, related to IP addresses, fixed in May 1996 JDK 1.0.2 (March 17, 1997)
Recently we've been getting lots of inquires about an alleged privacy attack on the Java sandbox. The privacy attack is that an applet calls getLocalHost() to find out the IP address of the computer that it's running on. This is not a big deal on the open internet, since IP addresses are clearly well-known and publicized in the internet routing tables. But if the computer running the applet resides inside a firewall, the IP address should remain hidden from the visiting applet. This is exactly what was implemented at JavaSoft about a year go, in time for the May 1996 JDK 1.0.2 release. An applet that calls getLocalHost() will get the loopback host ("localhost/127.0.0.1") as an answer. This is a generic handle to the local computer, which does not reveal any private information that should remain private.

Fix Delivered for Virtual Machine bug (March 5, 1997)
As part of our ongoing internal security audit at JavaSoft, our engineering team came across a bug in the implementation of the verifier in the Java Virtual Machine.

Within 24 hours, we investigated the bug and evaluated a fix. We tested the fix and created a patch for our licensees. As we always do, we notified our licensees and shipped the patch immediately. We encourage each of our licensees to include the patch in their products as soon as possible.

We are aware of no actual attacks involving this bug. It is important to note that a program written in the Java language and compiled by a standard compiler cannot access or exploit this bug. This depends on someone hand-crafting "evil byte code" and trying to slip it by the verifier. The theoretical attack is complex and extremely difficult to exploit.

Update on Java Security and ActiveX (February 25, 1997)
We've received lots of inquiries on the recently publicized Chaos Computer Club's theoretical hack into Microsoft's ActiveX. This well-established hacker organization, headquartered in Hamburg, demonstrated on German television how its members could use an ActiveX control to theoretically electronically transfer funds from an individual's bank account without using a personal identification number or a transaction number.

Executable code on the Internet is a complex security issue. That's why Java was designed from the ground up for complex networked environments, and our basic architecture takes into account using executable code from unknown sources. Check out the rest of this document, Frequently Asked Questions--Java Security, for a full description of the Java applet security model.

In brief, all Java applets run in a protected space that prohibits access to local disk. Encryption and certification can be used in conjunction with applets to add extra levels of security in trusted or controlled environments--like corporate intranets. JavaSoft has included digital signature capability in the Java Development Kit, JDK 1.1, which shipped February 18, 1997.

ActiveX uses a different model. Because it allows arbitrary binary code to be executed, a malicious ActiveX component can be written to remove or alter files on the user's local disk or to make connections to other computers without the knowledge or approval of the user. There is also the risk that a well-behaved ActiveX component could have a virus attached to it. Unfortunately, viruses can be encrypted just as easily as ordinary code.

In some cases, digital signing is adequate. In cases where stronger protection is required, Java allows the alternative of running untrusted code securely, which is extremely important for the Internet.

The most complete security solution for complex networks like the Internet requires not a single security solution, but many. Unlike ActiveX, Java was designed from the ground up for secure network computing. Security is fundamental to Java, not bolted on.

Web Spoofing (December, 1996)
The Secure Internet Programming team at Princeton University published a technical report describing an Internet security attack they name web spoofing. In this scenario, an attacker

Web spoofing requires that the attacker be able to interpose his machine between the server and client, in a man-in-the-middle attack.

Web spoofing does not require and does not exploit Java technologies.

If consumers are using a browser that does not have JavaScript (LiveScript) enabled, they will be able to tell that they are being spoofed when they notice either of these visual cues in the browser status line:

If consumers use a browser with JavaScript (LiveScript) enabled, then even these rudimentary and subtle alerts can be hidden by a malicious script. Recall that such scripts are not written in Java, and are not subject to the Java security model. In that case, consumers would have no way of noticing the spoofing. For this reason, people concerned about web spoofing attacks might consider disabling scripting languages.

Note that in any case, some novice internet consumers will not be sensitive to visual cues, even if they aren't obliterated by scripting languages. However, it is often the case on the internet that web spoofing attacks are noticed quickly and given wide publicity. Given the nature of the web and how quickly email bounces around the net, there is strong likelihood that a spoofed web site will be found out quickly and publicized widely.

JDK 1.1 beta (December 13, 1996)
JDK 1.1 Beta 2 fixes the implementation inconsistency in the javakey security tool that caused jar files to be signed with invalid signatures. In JDK 1.1 Beta 1, signature checking always failed, and so all applets ran as untrusted, with minimal permissions enabled. This made the code signing feature unusable, but was not a security hole.

Please report anything else you find to http://java.sun.com/products/JDK/1.1/bugreport.html.

Illegal type cast attack (June 2, 1996)
David Hopwood, a Java researcher in the UK, has uncovered a way to manipulate the way objects are assigned and the way they collaborate, in order to undermine the Java type system. Hopwood contacted JavaSoft directly regarding the bug, and we have had a team working on a fix from the time that we were notified. We are also applying Hopwood's model to conduct a security review, to determine if there are other bugs that may apply. Fixed in JDK 1.1; fixes propagated to all Java licensees by mid-June 1996

New twist on previous classloader attack (May 18, 1996)
The May 18 edition of the New York Times reported that Ed Felten, Drew Dean and Dan Wallach of Princeton University's Safe Internet have found a new way to get past system restrictions on creating a classloader. This attack builds on work done by Tom Cargill. The applet sandbox security model states that applets are not allowed to create and use classloaders to define classes. Once an applet has its own classloader, it can use it to define and execute classes that would otherwise be barred from execution. Fixed in JDK 1.0.2, and in Netscape Navigator 3.0b4.

Hostile Applets (April, 1996)
A number of hostile web pages, that host malicious or rude resource-consuming applets, are popping up on the web. These sites demonstrate problems related to denial of service attacks. We are investigating ways to better monitor and control resource consumption of applets. Here are more details.

URL name resolution attack (April, 1996)
For a specific firewall-protected network configuration, an applet downloaded from a client inside the firewall would be able to connect to a single specific host behind the firewall. (It requires an unusual network configuration, but here are the details.) Fixed in JDK 1.0.2 and in Netscape Navigator 2.02.

Verifier implementation bug (Mar, 1996)
Researchers at Princeton found an implementation bug in the Java bytecode verifier. The verifier is a part of Java's runtime system which checks that applets downloaded over the Internet adhere to Java's language safery rules. Here's our response. Fixed in JDK 1.0.2 and in Netscape Navigator 2.02.

Class loader implementation bug (Mar, 1996)
David Hopwood at Oxford University found a bug in the class loader that could be exploited to load illegal byte code, which could then be used to load a class referenced by an absolute path name. Fixed in Netscape Navigator 2.01 and in JDK 1.0.1.

DNS attack (Feb, 1996)
Drew Dean, Ed Felten and Dan Wallach from Princeton described an attack on the applet security model, based on exploiting how the applet security manager uses DNS for name/IP address resolution. Sun's response to this security bug was posted to comp.lang.java on Feb 21. Fixed in Netscape Navigator 2.01 and in JDK 1.0.1.

Steve Gibbons also independently suggested this attack scenario: see http://www.aztech.net/~steve/java/

JavaScript (Feb, 1996)
JavaScript is a scripting language used with Netscape Navigator 2.0. There have been reports of privacy problems with JavaScript, and Netscape is committed to addressing those concerns. JavaScript cannot be used to invoke Java applets. The privacy problems reported with JavaScript are not present in Java applets.

Applet Security

Applets
  1. What are applets prevented from doing?
  2. Can applets read or write files?
  3. How do I let an applet read a file?
  4. How do I let an applet write a file?
  5. What system properties can be read by applets, and how?
  6. How do I hide system properties that applets are allowed to read by default?
  7. How can I allow applets to read system properties that they aren't allowed to read by default?
  8. How can an applet open a network connection to a computer on the internet?
  9. How can an applet open a network connection to its originating host?
  10. How can an applet maintain persistent state?
  11. Can an applet start another program running on the client?
  12. What features of the Java language help people build secure applets?
  13. What is the difference between applets loaded over the net, and applets loaded via the file system?
  14. What's the applet class loader, and what does it buy me?
  15. What's the applet security manager, and what does it buy me?
  16. Is there a summary of applet capabilities?
  17. If other languages are compiled to Java bytecodes, how does that affect the applet security model?
Examples
Tiny applet examples that demonstrate the security features of your web browser.
Glossary
Terms used in this FAQ.
See Also
Other references on Java security

Applets

  1. What are applets prevented from doing?

    In general, applets loaded over the net are prevented from reading and writing files on the client file system, and from making network connections except to the originating host.

    In addition, applets loaded over the net are prevented from starting other programs on the client. Applets loaded over the net are also not allowed to load libraries, or to define native method calls. If an applet could define native method calls, that would give the applet direct access to the underlying computer.

    There are other specific capabilities denied to applets loaded over the net, but most of the applet security policy is described by those two paragraphs above. Read on for the gory details.

  2. Can applets read or write files?

    In Java-enabled browsers, applets cannot read or write files at all.

    Sun's appletviewer allows applets to read files that reside in directories on the access control lists.

    If the file is not on the client's access control list, then applets cannot access the file in any way. Specifically, applets cannot

  3. How do I let an applet read a file?

    Applets loaded into a Java-enabled browser can't read files.

    Sun's appletviewer allows applets to read files that are named on the access control list for reading. The access control list for reading is null by default, in the JDK. You can allow applets to read directories or files by naming them in the acl.read property in your ~/.hotjava/properties file.

    Note: The "~" (tilde) symbol is used on UNIX systems to refer to your home directory. If you install a web browser on your F:\ drive on your PC, and create a top-level directory named .hotjava, then your properties file is found in F:\.hotjava\properties.

    For example, to allow any files in the directory home/me to be read by applets loaded into the appletviewer, add this line to your ~/.hotjava/properties file.

    	acl.read=/home/me
    
    You can specify one file to be read:
    	acl.read=/home/me/somedir/somefile
    
    Use ":" to separate entries:
    	acl.read=/home/foo:/home/me/somedir/somefile
    
    Allowing an applet to read a directory means that it can read all the files in that directory, including any files in any subdirectories that might be hanging off that directory.

  4. How do I let an applet write a file?

    Applets loaded into a Java-enabled browser can't write files.

    Sun's appletviewer allows applets to write files that are named on the access control list for writing. The access control list for writing is empty by default.

    You can allow applets to write to your /tmp directory by setting the acl.write property in your ~/.hotjava/properties file:

    	acl.write=/tmp
    
    You can allow applets to write to a particular file by naming it explicitly:
    	acl.write=/home/me/somedir/somefile
    
    Use : to separate entries:
    	acl.write=/tmp:/home/me/somedir/somefile
    

    Bear in mind that if you open up your file system for writing by applets, there is no way to limit the amount of disk space an applet might use.

  5. What system properties can be read by applets, and how?

    In both Java-enabled browsers and the appletviewer, applets can read these system properties by invoking System.getProperty(String key):

      key			meaning
      ____________		______________________________
      java.version		Java version number
      java.vendor		Java vendor-specific string
      java.vendor.url	Java vendor URL
      java.class.version	Java class version number
      os.name		Operating system name
      os.arch		Operating system architecture
      os.version	        Operating system version
      file.separator	File separator (eg, "/")
      path.separator	Path separator (eg, ":")
      line.separator	Line separator 
    
    Applets are prevented from reading these system properties:
      key			meaning
      ____________		_____________________________
      java.home		Java installation directory
      java.class.path	Java classpath
      user.name		User account name
      user.home		User home directory
      user.dir		User's current working directory
    
    To read a system property from within an applet, simply invoke System.getProperty(key) on the property you are interested in.

    For example,

      String s = System.getProperty("os.name");
    
  6. How do I hide system properties that applets are allowed to read by default?

    There's no way to hide the above ten system properties from applets loaded into a Java-enabled browser. The reason is that the browsers don't consult any external files as part their Java configuration, as a security precaution, including the ~/.hotjava/properties file.

    From the appletviewer, you can prevent applets from finding out anything about your system by redefining the property in your ~/.hotjava/properties file. For example, to hide the name of the operating system that you are using, add this line to your ~/.hotjava/properties file:

    	os.name=null
    
  7. How can I allow applets to read system properties that they aren't allowed to read by default?

    There's no way to allow an applet loaded into a Java-enabled browser to read system properties that they aren't allowed to read by default.

    To allow applets loaded into the appletviewer to read the property named by key, add the property key.applet=true to your ~/.hotjava/property file. For example, to allow applets to record your user name, add this line to your ~/.hotjava/properties file:

    	user.name.applet=true
    
  8. How can an applet open a network connection to a computer on the internet?

    Applets are not allowed to open network connections to any computer, except for the host that provided the .class files. This is either the host where the html page came from, or the host specified in the codebase parameter in the applet tag, with codebase taking precendence.

    For example, if you try to do this from an applet that did not originate from the machine foo.com, it will fail with a security exception:

    	Socket s = new Socket("foo.com", 25, true);
    
  9. How can an applet open a network connection to its originating host?

    Be sure to name the originating host exactly as it was specified when the applet was loaded into the browser.

    That is, if you load an HTML page using the URL

    	http://foo.state.edu/~me/appletPage.html
    
    then your applet will be able to connect to its host only by using the name foo.state.edu. Using the IP address for foo.state.edu won't work, and using a "shorthand" form of the host name, like foo.state instead of foo.state.edu, won't work.

  10. How can an applet maintain persistent state?

    There is no explicit support in the JDK applet API for persistent state on the client side. However, an applet can maintain its own persistent state on the server side. That is, it can create files on the server side and read files from the server side.

    Interesting examples are

  11. Can an applet start another program on the client?

    No, applets loaded over the net are not allowed to start programs on the client. That is, an applet that you visit can't start some rogue process on your PC. In UNIX terminology, applets are not allowed to exec or fork processes. In particular, this means that applets can't invoke some program to list the contents of your file system, and it means that applets can't invoke System.exit() in an attempt to kill your web browser. Applets are also not allowed to manipulate threads outside the applet's own thread group.

  12. What features of the Java language help people build secure applets?

  13. What is the difference between applets loaded over the net and applets loaded via the file system?

    There are two different ways that applets are loaded by a Java system. The way an applet enters the system affects what it is allowed to do.

    If an applet is loaded over the net, then it is loaded by the applet class loader, and is subject to the restrictions enforced by the applet security manager.

    If an applet resides on the client's local disk, and in a directory that is on the client's CLASSPATH, then it is loaded by the file system loader. The most important differences are

    Java-enabled browsers use the applet class loader to load applets specified with file: URLs. So, the restrictions and protections that accrue from the class loader and its associated security manager are now in effect for applets loaded via file: URLs.

    This means that if you specify the URL like so:

    	Location:  file:/home/me/public_html/something.html
    
    and the file something.html contains an applet, the browser loads it using its applet class loader.

  14. What's the applet class loader, and what does it buy me?

    Applets loaded over the net are loaded by the applet class loader. For example, the appletviewer's applet class loader is implemented by the class sun.applet.AppletClassLoader.

    The class loader enforces the Java name space hierarchy. The class loader guarantees that a unique namespace exists for classes that come from the local file system, and that a unique namespace exists for each network source. When a browser loads an applet over the net, that applet's classes are placed in a private namespace associated with the applet's origin. Thus, applets loaded from different network sources are partitioned from each other.

    Also, classes loaded by the class loader are passed through the verifier. The verifier checks that the class file conforms to the Java language specification - it doesn't assume that the class file was produced by a "friendly" or "trusted" compiler. On the contrary, it checks the class file for purposeful violations of the language type rules and name space restrictions. The verifier ensures that

    The verifier accomplishes that by doing a data-flow analysis of the bytecode instruction stream, along with checking the class file format, object signatures, and special analysis of finally clauses that are used for Java exception handling.

    Details on the verifier's design and implementation were presented in a paper by Frank Yellin at the December 1995 WWW conference in Boston.

    A web browser uses only one class loader, which is established at start-up. Thereafter, the system class loader cannot be extended, overloaded, overridden or replaced. Applets cannot create or reference their own class loader.

  15. What's the applet security manager, and what does it buy me?

    The applet security manager is the Java mechanism for enforcing the applet restrictions described above. The appletviewer's applet security manager is implemented by sun.applet.AppletSecurity.

    A browser may only have one security manager. The security manager is established at startup, and it cannot thereafter be replaced, overloaded, overridden, or extended. Applets cannot create or reference their own security manager.

  16. Is there a summary of applet capabilities?

    The following table is not an exhaustive list of applet capabilities. It's meant to answer the questions we hear most often about what applets can and cannot do.

    Key:

    		 Stricter ------------------------> Less strict
    
    			NN	NL	AN	AL	JS
    
    read file in /home/me,	no	no	no      yes     yes
    acl.read=null
    
    read file in /home/me,	no	no	yes	yes	yes
    acl.read=/home/me
    
    write file in /tmp,	no	no	no	yes	yes
    acl.write=null
    
    write file in /tmp,	no	no	yes	yes 	yes
    acl.write=/tmp
    
    get file info,		no	no	no	yes	yes
    acl.read=null
    acl.write=null
    
    get file info,		no	no	yes	yes	yes
    acl.read=/home/me
    acl.write=/tmp
    
    delete file,		no	no	no	no	yes
    using File.delete()
    
    delete file, 		no	no	no	yes 	yes
    using exec /usr/bin/rm
    
    read the user.name	no	yes	no	yes	yes
    property
    
    connect to port		no	yes	no	yes	yes
    on client
    
    connect to port		no	yes	no	yes	yes
    on 3rd host
    
    load library		no	yes	no	yes	yes
    
    exit(-1)		no	no	no 	yes	yes
    
    create a popup		no	yes	no	yes	yes
    window without 
    a warning
    
    

  17. If other languages are compiled to Java bytecodes, how does that affect the applet security model?

    The verifier is independent of Sun's reference implementation of the Java compiler and the high-level specification of the Java language. It verifies bytecodes generated by other Java compilers. It also verifies bytecodes generated by compiling other languages into the bytecode format. Bytecodes imported over the net that pass the verifier can be trusted to run on the Java virtual machine. In order to pass the verifier, bytecodes have to conform to the strict typing, the object signatures, the class file format, and the predictability of the runtime stack that are all defined by the Java language implementation.


Examples

None of these examples are malicious - the one line descriptions can be taken at face value. You can look at the source code for each applet, before visiting the page that has that applet inside. (The first link in each example takes you to the source code, and the second link takes you to an html page that includes the executable content for the example.)

Files:

System Properties: Sockets: Processes: Libraries and name spaces: Windows:

Glossary of terms used in this FAQ

Applet
A Java program that is run from inside a web browser. The html page loaded into the web browser contains an <applet> tag, which tells the browser where to find the Java .class files. For example,

	appletviewer http://foo.com/~jo/coolApplet.html

Standalone Java application
A Java program that is run by invoking the java interpreter. For example,

	java coolApplication

Server
The computer that hosts the web page that contains an applet. The .class files that make up the applet, and the .html files that reference the applet, reside on the server. When someone on the internet connects to a web page that contains an applet, the server delivers the .class files over the internet to the client that made the request.

The server is also known as the originating host.

Client
The computer that displays the web page that contains an applet.

The terms server and client are sometimes used to refer to computers, and are sometimes used to refer to computer programs. For example, www.sun.com is a server, and the httpd process running on www.sun.com is its server process. My computer at home is a client, and the web browser running on my computer at home acts as the client process.


See Also

Low-level Security in Java, Frank Yellin, WWW4 Conference, December, 1995
This paper presents the details of the lowest levels of the java security mechanism. Before any downloaded code is executed, it is scanned and verified to ensure that it conforms to the specifications of the virtual machine.

HotJava: The Security Story
This paper from May 1995 (the 1.0alpha3 release of Java and HotJava) provides a high-level overview of the security mechanisms, which were elaborated on for the JDK 1.0 release.

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