[Full-disclosure] Java Web Start argument injection vulnerability
jouko at iki.fi
Fri Mar 18 19:06:49 GMT 2005
Java Web Start is a technology for easy client-side deployment of Java
applications. "Using Java Web Start technology, standalone Java
software applications can be deployed with a single click over the
network" (from Sun Microsystems's website).
Java Web Start is installed with Java Runtime Environment (JRE). During
installation, file type associations are added to make web browsers
automatically (with a single click) open Java Web Start's .JNLP files
(the behavior may vary between different web browsers).
There is a vulnerability in the way Web Start handles Java system
properties defined in JNLP files. A malicious user can pass command
line arguments to the Java virtual machine. They can be used to disable
the Java "sandbox" and compromise the system. The attack can be carried
out when the victim user views a web page crafted by the attacker.
The <property> tag in a JNLP file can be used to define Java system
properties. System properties are key-value pairs which usually store
attributes of the current working environment, e.g. "java.home"
containing the Java installation path and "java.version" containing its
version. Due to the nature of some of the system properties, setting
their values in JNLP files is restricted.
A few system properties are considered "secure" and if defined in a
JNLP file, they are passed to the Java executable (javaw.exe) via the
-Dproperty=value command line argument. However, a malicious user can
use this feature to inject extra command line arguments to the Java
For instance, a JNLP file can contain this property tag:
<property name="sun.java2d.noddraw" value="true HELLO" />
The property "sun.java2d.noddraw" is considered secure by Web Start, so
it is accepted and the startup command for the application is something
javaw.exe -Dsun.java2d.noddraw=true HELLO (other args) your.application
This would produce a Web Start error message saying the main class
can't be found, as javaw.exe interprets "HELLO" as the main class name
instead of "your.application". The problem is that Web Start fails to
use quote symbols around the property argument.
To exploit the flaw, an attacker can pass command line arguments
affecting the Java security policies. Normally an unsigned, untrusted
Java applet operates inside a "sandbox" and can't e.g. access local
files. By exploiting this flaw, the default "sandbox" security policy
can be overridden with an arbitrary policy file hosted on the
attacker's web server. The new policy can grant full permissions to the
application, which could then e.g. read or write arbitrary files on the
victim system, or download and launch viruses, keyloggers or other
malware. The attacker may set up a JNLP file on a web server so that it
will be launched without further user interaction when the victim
visits the site, e.g. with the IFRAME tag.
As the application is made in Java, the same exploit can work on any
platform supporting Java Web Start. A proof-of-concept exploit was
produced which detects the operating system and starts an OS-dependant
binary executable when a web page is visited - the same exploit works
with Internet Explorer on Windows and Mozilla Firefox and Opera on
If Internet Explorer is used, the JNLP file is opened automatically
without further interaction. Other web browsers may, depending on
file type configuration, display a dialogue asking whether the file
should be opened or saved. Some versions of e.g. Opera require manual
configuration in order to open JNLP files.
In addition to the web browser attack vector, the attacker could
replace an existing JNLP file on a web site with a malicious one. Web
Start applications can be started from desktop shortcut icons, from the
Web Start menu, or from command line. All of these starting methods are
suspectible to the attack.
Java Web Start in J2SE 1.4.2 releases prior 1.4.2_07 are vulnerable.
J2SE 5.0 and later, and releases prior to 1.4.2 are NOT vulnerable.
Sun Microsystems was informed about the problem on September 25, 2004.
The issue was fixed in J2SE 1.4.2_07. Sun's advisory can be found at
The advisory also contains workarounds for the problem.
The vulnerability was found and researched by Jouko Pynnönen, Finland.
Jouko Pynnönen Web: http://iki.fi/jouko/
jouko at iki.fi
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