It is very common for Microsoft security bulletins to include “Kill-Bits” to disable individual ActiveX controls / COM objects. Here is the final part of our three-part Kill-Bit FAQ.
The Kill-Bit FAQ – Part 3 of 3
Are there issues that could complicate the implementation of a Kill-Bit based fix?
Yes. Here’s one interesting example: if the vulnerable code is in a separate binary from the one that implements the ActiveX control (the one referenced by the registered CLSID for the control) then the Kill-Bit may not have the intended effect.
Per the top portion of Figure 1 below, imagine Control AX.1 references some vulnerable code in DLL.1. The proposed fix plan is as follows:
Now imagine that the old DLL B.1 binary is dropped onto the system and registered. The system is now in a “downgraded” and vulnerable state, as depicted in Figure 1. The Kill-Bit does not automatically address this problem because even the new “fixed” AX.2 can still reference the old vulnerable DLL.1.
Consequently, in the event that you need to fix a vulnerable control and the vulnerable code is actually in a separate binary, make sure that the new control is not able to use the old / vulnerable binary even if that binary is reintroduced onto the system. You can achieve this by performing a handshake or version check between the new control and the new / fixed binary.
You should always carefully consider the applicability of the Kill-Bit before deciding to use it. For example, if an attack vector exists through a non-Kill-Bit-aware application then a Kill-Bit obviously will not be effective. See “If I Kill-Bit my vulnerable object / control, should I still release a fixed version?” in part 2.
Thanks to Matt Thomlinson for providing Figure 1 above!
Can I lock my ActiveX control down to a specific web site as an additional security measure?
Yes, use SiteLock. Try to avoid implementing this functionality from scratch – there are many ways to get this wrong.
SWI recommends using SiteLock only as “defense-in-depth” as it is not bulletproof. (For example, if a Cross-Site Scripting flaw exists anywhere on the domain it can potentially be abused to bypass this restriction.)
Where are some additional resources on ActiveX Controls?
Most relevant to this FAQ:
Other good stuff:
- Security Vulnerability Research & Defense Bloggers