Macromedia Flash Activex Buffer overflow

Release Date:
05/02/2002

Severity:
High (Remote code execution)

Systems Affected:
Systems with Flash Activex Ocx Version 6, revision 23
(Possibly older versions)
This includes most installations of Windows.

Description:
All users of Internet Explorer are potentially affected because this is
a Macromedia signed ocx. We advise them to upgrade their flash version
immediately to version 6, revision 29 (see the Vendor Status section
below).

This is an unusual advisory in a number of ways. 

One, it was found while investigating an access error encountered during
normal web surfing, which was suspicious. Within a few hours we had
confirmed on multiple Operating Systems that this was an exploitable
condition that overwrote EIP. 

Two, while we tested on these systems with the latest install from the
vendor's site, when we contacted the vendor they informed us that they
had just released a new build this same day which already fixed the
problem. They asked us to confirm this. We tried the link they gave us
and it did indeed fix the problem and was a new build. Testing the link
later that night confirmed the link we used to install the ocx now had
the fixed, latest version.

In this, we congratulate Macromedia for: finding the bug, fixing it, and
releasing the build in a timely fashion. This truly shows that they are
dedicated to security just as they have stated they are.

However, because there is a signed flash ocx out there which has been
downloaded by an untold number of people, and potentially could still be
used in an exploit scenario against those without the latest ocx we felt
the need to release this advisory.

Furthermore, this issue was found in the wild, and it is not safe to
assume it could not be found by others with malicious intent. Nor do we
believe it is safe to assume this has not been found by users with
malicious intent.

A vulnerability in the parameter handling to the Flash OCX, which could
lead to the execution of attacker supplied code via email, web or any
other avenue in which Internet Explorer is used to display html that an
attacker can supply. This includes software which uses the web browser
activex.

Example:

<OBJECT classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000">
<PARAM NAME=movie
VALUE="http://www.notthere8979873.com/notthere.swf?AAA[...unstated, but
fixed number]XXXXXXXX">
</OBJECT>

Where X overwrites the EIP consistently across Windows platforms.

Technical Description:

Flash.ocx is an activex object installed with Internet Explorer, and is
used to display flash objects on the web. 

Proper bounds checking is not in place in the "movie" parameter which
overwrites EIP at an unsaid, but fixed number of bytes across Windows
platforms.

Because the ocx is signed by Macromedia: there is a chance the older
activex could be used against people without flash; people whom have an
older version of flash not affected may be forced to "upgrade" to the
affected version; and, of course, those with the affected versions need
to upgrade lest the exploit works out of the box on them.

There has been considerable debate about legacy activex objects which
have exploits within them. In general, if someone uses the codebase
parameter to point to an affected version of the activex, the system
will first try and grab the activex from Microsoft's activex store on
the web. Then, it will try the activex specified in the codebase tag by
the malicious user. 

We do not believe this method is fool-proof. 

We do not believe the method is full proof because of the potential of
the activex storehouse check failing and because of the potentiality for
the activex to be called by other methods. (At least a few potential
other methods are in the RFC for applets and objects).

However, the other option of setting the "kill bit" for the affected
activex and reassigning the fixed activex version with a new classid is
only a suggestion we will make in this case. We do not believe it is
necessarily mandatory.

Risk should be mitigated to a satisfactory level by users upgrading to
the new ocx.

Vendor Status:
Visit Macromedia's site to get the latest Flash ocx to eliminate these
issues.
http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash

Credit:
Drew Copley

Greetings:
Fat code: presented by Yahoo and Weight Watchers. KROQ, and corn dog
manufacturers world-wide.

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