Anybody know how to find better descriptions of these bugs, or the patches that fixed them?
(edited to emphasize that the above isn't some kind of formal pronouncement of The Truth, but it is the logic I used when deciding how to prioritize this situation, which ended up with a conclusion of "not very important, not at all". Your situation not being identical to mine will almost certainly result in a somewhat different conclusion, much as my conclusion would be different if I were in your situation instead of mine.)
http://www.ubuntu.com/usn/usn-2348-1/
It was discovered that APT did not re-verify downloaded files when the If-Modified-Since wasn't met. (CVE-2014-0487)
It was discovered that APT did not invalidate repository data when it switched from an unauthenticated to an authenticated state. (CVE-2014-0488)
It was discovered that the APT Acquire::GzipIndexes option caused APT to skip checksum validation. This issue only applied to Ubuntu 12.04 LTS and Ubuntu 14.04 LTS, and was not enabled by default. (CVE-2014-0489)
It was discovered that APT did not correctly validate signatures when downloading source packages using the download command. This issue only applied to Ubuntu 12.04 LTS and Ubuntu 14.04 LTS. (CVE-2014-0490)
Edit:
Regarding #1(If-Modified-Since), the vulnerability is that if a hash in the Release file changes, but the file being referred to by the Release file gets served with a 304 response, apt will ignore the updated file and continue to use the old version of the file. even though the old version of the file doesn't match the new hash. An attacker could exploit this to prevent a system from receiving updates, though thankfully it doesn't seem to be possible to exploit this to cause apt to trust an arbitrary package.
Original incorrect speculation below:
Regarding #1 (If-Modified-Since), I'm wondering if perhaps their HTTP client incorrectly accepts a response body with a 304 response (contrary to the HTTP spec)? In that case a malicious server could deliver a file that's blindly trusted as long as it has a status of 304. (This is pure speculation, but I can't think of any other reason this would be a big deal.)
