Friday, April 04, 2008

The truth about Access 0-days

Security flaws in popular Office file formats (namely DOC, XLS and PPT) have been very common in the past few years, accounting for a large amount of Microsoft Security Bulletins (cf. slide #4 on this presentation). They have been also involved in high-profile targeted attacks.

However, flaws in lesser used Office file formats (namely PUB and MDB) were largely disregarded by Microsoft, for at least 2 reasons:
  • Access (.MDB) and Publisher (.PUB) applications are not part of Office Standard suite - they are available in higher grade SKUs only.
  • Access file format is considered "insecure by design" since automatic code execution on file opening cannot be blocked. Therefore MDB files are included in Microsoft blocked list. This list is enforced by Outlook application on attachments, among others.
A large amount of "buffer overflow"-like bugs involving MDB files have been floating around since year 2005 at least.

Some malware authors recently found a way to bypass Microsoft filters by sending 2 attachments in the same email (or the same ZIP file): the first one is an approved Office file format (let's say DOC), the other has an unknown extension.

However, when the Word document tries to open the second one as an ODBC Datasource using Jet Engine (where the flaw lies), it will disregard the extension.

McAfee Avert Labs blogged about that, but they missed something that is regularly re-discovered: OLE documents will be opened by the right Office application regardless of their extension.

How to reproduce:
  1. Create a new Word document named "test.doc".
  2. Rename "test.doc" into "" (the extension shall not be already registered).
  3. Double-click on "". Enjoy!
Therefore, if you want to be protected against Office-based attacks, you shall block any unknown extension (or rather, use a white-list of "known safe" extensions) at your mail gateway.

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