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Home : Advisories : Weaknesses in Java Bytecode Verifier

Title: Weaknesses in Java Bytecode Verifier
Released by: CERT
Date: 29th March 1996
Printable version: Click here

Hash: SHA1


CERT(*) Advisory CA-96.07

Original issue date: March 29, 1996

Last revised: September 24,1997

              Updated copyright statement

              A complete revision history is at the end of this file.

Topic: Weaknesses in Java Bytecode Verifier

- -----------------------------------------------------------------------------

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. We further recommend upgrading to Netscape 2.02

but still disabling Java and JavaScript if you don't need these programs.

We will update this advisory as we receive additional information.

Please check advisory files regularly for updates that relate to your site.

- -----------------------------------------------------------------------------

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


     B. Netscape users

        Upgrade to Netscape version 2.02, which addresses the Java Bytecode

        Verifier problems discussed in the advisory.

        Until you can do so, 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.

        After you update to version 2.02, you should still disable Java and

        JavaScript if these programs are not being used.  (This also applies

        to Netscape Version 3.0b4.)  Note that in order to display Netscape's

        home page, you must have JavaScript enabled.

        For the latest news about fixes for Netscape Navigator, consult

        the following for details:


        Netscape 2.02 and additional information about it are available from


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


V.  Information for Macintosh users

    Macintosh version 2.01 does not support Java, so there is nothing to

    disable as part of the solution to the problems described in this


- ---------------------------------------------------------------------------

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.

- ---------------------------------------------------------------------------

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).

We strongly urge you to encrypt any sensitive information you send by email.

The CERT Coordination Center can support a shared DES key and PGP. Contact the

CERT staff for more information.

Location of CERT PGP key


CERT Contact Information

- ------------------------

Email    cert@cert.org

Phone    +1 412-268-7090 (24-hour hotline)

                CERT personnel answer 8:30-5:00 p.m. EST

                (GMT-5)/EDT(GMT-4), and are on call for

                emergencies during other hours.

Fax      +1 412-268-6989

Postal address

        CERT Coordination Center

        Software Engineering Institute

        Carnegie Mellon University

        Pittsburgh PA 15213-3890


CERT publications, information about FIRST representatives, and other

security-related information are available for anonymous FTP from



CERT advisories and bulletins are also posted on the USENET newsgroup


To be added to our mailing list for CERT advisories and bulletins, send your

email address to


- ------------------------------------------------------------------------------

Copyright 1996 Carnegie Mellon University. Conditions for use, disclaimers,

and sponsorship information can be found in

http://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .

If you do not have FTP or web access, send mail to cert@cert.org with

"copyright" in the subject line.

CERT is registered in the U.S. Patent and Trademark Office.


Revision history

Sep. 24, 1997  Updated copyright statement

Aug. 30, 1996  Information previously in the README was inserted into the


June 26, 1996  Introduction - added a note about Netscape 2.02.

               Sec. III.B - added a pointer to Netscape 2.02 and a

               recommendation about disabling Java and JavaScript.

Apr. 1, 1996   Sec. III.B - added a note about viewing Netscape's home page.

               Sec. V - added this section for Macintosh users.


Version: PGP for Personal Privacy 5.0

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