methods/https.cc in apt before 0.8.11 accepts connections when the certificate host name fails validation and Verify-Host is enabled, which allows man-in-the-middle attackers to obtain repository credentials via unspecified vectors.
https://bugs.launchpad.net/ubuntu/+source/apt/+bug/868353
http://www.ubuntu.com/usn/USN-1283-1
http://people.canonical.com/~ubuntu-security/cve/2011/CVE-2011-3634.html