CERT Advisory - Weaknesses in Java Bytecode Verifier

[Dave Farber <farber () central cis upenn edu>]

CERT(sm) Advisory CA-96.07
March 29, 1996


Topic: Weaknesses in Java Bytecode Verifier


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The CERT Coordination Center has received reports of weaknesses in the
bytecode verifier portion of Sun Microsystems' Java Development Kit (JDK)
versions 1.0 and 1.0.1. The JDK is built into Netscape Navigator 2.0 and 2.01.
We have not received reports of the exploitation of this vulnerability.


When applets written with malicious intent are viewed, those applets can
perform any operation that the legitimate user can perform on the machine
running the browser. For example, a maliciously written applet could remove
files from the machine on which the browser is running--but only if the
legitimate user could also.


Problem applets have to be specifically written with malicious intent, and
users are at risk only when connecting to "untrusted" web pages. If you use
Java-enabled products on a closed network or browse the World Wide Web but
never connect to "untrusted" web pages, you are not affected.


The CERT staff recommends disabling Java in Netscape Navigator and not using
Sun's appletviewer to browse applets from untrusted sources until patches are
available from these vendors.


As we receive additional information relating to this advisory, we will
place it in


        ftp://info.cert.org/pub/cert_advisories/CA-96.07.README


We encourage you to check our README files regularly for updates on
advisories that relate to your site.


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I.   Description


     The Java Programming Language is designed to allow an executable
     computer program, called an applet, to be attached to a page viewable
     by a World Wide Web browser. When a user browsing the Web visits that
     page, the applet is automatically downloaded onto the user's machine
     and executed, but only if Java is enabled.


     It is possible for an applet to generate and execute raw machine code
     on the machine where the browser is running. This means that a
     maliciously written applet can perform any action that the legitimate
     user can perform; for example, an applet can read, delete, or change
     files that the user owns. Because applets are loaded and run
     automatically as a side-effect of visiting a Web page, someone could 
     "booby-trap" their Web page and compromise the machine of anyone visiting
     the page. This is the problem described in the Wall Street Journal on
     March 26, 1996 ("Researchers Find Big Security Flaw in Java Language," by
     Don Clark).  


     Note: The security enhancements announced by Sun Microsystems in
           JDK version 1.0.1 and by Netscape Communications in Netscape
           Navigator version 2.01 do *not* fix this flaw.


II.  Impact


     If Java is enabled and a Web page containing a maliciously written
     applet is viewed by any of the vulnerable browsers or Sun's appletviewer,
     that applet can perform any operation that the legitimate user can
     perform. For example, the applet could read, delete, or in other ways
     corrupt the user's files and any other files the user has access to, such
     as /etc/passwd. 




III. Solution


     We recommend obtaining vendor patches as soon as they become available.
     Until you can install the patches, we urge you to apply the workarounds
     described below.


     A. Java Development Kit users 


        Sun reports that source-level fixes will be supplied to source
        licensees in the next few days. The fixes will also be included in
        the next JDK version, v1.0.2, which will be released within the next
        several weeks. 


        The JDK itself is a development kit, and it can safely be used to
        develop applets and applications. If you choose to use the
        appletviewer as a rudimentary browser, do not use it to browse
        applets from untrusted sources until you have installed the v1.0.2
        browser. 




     B. Netscape users 


        If you use Netscape 2.0 or 2.01, disable Java using the
        "Security Preferences" dialog box. You do not need to disable
        JavaScript as part of this workaround.


        For the latest news about fixes for Netscape Navigator, consult
        the following for details:


                http://home.netscape.com/




IV.  Information for HotJava (alpha3) users


     Sun Microsystems has provided the following information for users of
     HotJava (alpha3). 


          Sun made available last year a demonstration version of a browser
          called "HotJava." That version (alpha3) is proof-of-concept 
          software only, not a product. HotJava (alpha3) uses an entirely
          different security architecture from JDK 1.0 or JDK 1.0.1. It will
          not be tested for any reported security vulnerabilities that it
          might be susceptible to, and Sun neither supports it nor recommends
          its use as a primary browser. When HotJava is released as a product,
          it will be based on an up-to-date version of the JDK and fully
          supported. 


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The CERT Coordination Center thanks Drew Dean, Ed Felten, and Dan Wallach of
Princeton University for providing information for this advisory. We thank
Netscape Communications Corporation and Sun Microsystems, Inc. for their
response to this problem.
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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident
Response and Security Teams (FIRST).


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The CERT Coordination Center can support a shared DES key and PGP. Contact the
CERT staff for more information. 


Location of CERT PGP key
         ftp://info.cert.org/pub/CERT_PGP.key


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------------------------
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Copyright 1996 Carnegie Mellon University
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