Outlook (Express) S/MIME Certificate Chain Vulnerability

Summary
Outlook's S/MIME implementation is vulnerable to the certificate chain spoofing attack, despite Microsoft's claim that IE is the only affected application. The vulnerability allows anyone to forge the digital signature on an email that is to be viewed with Outlook. No warnings are given, no dialogs are shown.


Details
Affected clients:
* Mozilla is NOT vulnerable.
* Outlook Express 5 is vulnerable (Tested on fully patched Win2k SP3 system)

In a very similar way to the IE SSL vulnerability, an attacker generates a bad certificate chain:

[Issuer:VeriSign | Subject:VeriSign] > [Issuer:VeriSign | Subject:www.thoughtcrime.org] > [Issuer:www.thoughtcrime.org | Subject:Bill Gates/billgates@microsoft.com]

In addition, Outlook will fail to check the Basic Constraints on the intermediate certificate and accepts the leaf certificate as valid.

Severity:
As it stands, there is virtually no difference between signed and unsigned email in Outlook. Unless carefully inspected, signed email in Outlook is essentially meaningless. This also applies to any signed email received over the past 5+ years.

Prudent users who must continue using Outlook for signed email should manually inspect and verify received certificate chains.

Exploit:
erased - Forum Admin

Vendor Notification Status:
Microsoft knows about this, of course, but "isn't even sure whether to call this a 'vulnerability'" according to their official response.

-----

source: securiteam