Arch package security
Current state of affairs
A package can become malicious (we consider original sources safe) either intentionally or unintentionally. In the first case packager prepares the package (by modifying sources, adding scripts, modifying build scripts) with intention to make harm to user system or data. In the second case packager by mistake or omission creates the package that if installed puts the user's system in jeopardy.
Malicious package may enter the package universe when packager provides it directly or puts it into legitimate repository or mirror. Threat level depends here on what software contained in package can do and how popular package is. The former quality is possible to determine by careful inspection of package and its contents (also running software in testing environment). The latter can be estimated by basing on download statistics (assuming that package downloaded is package installed).
What to do to make package management more secure?
There's no way to stop packagers from intentionally forging packages that can do bad things to user system and providing them on their own repositories. There is however a way to make users to be more careful when downloading packages outside from official repositories. Malicious packager will have to convince users to download his package. He'll have to do it by advertising his package as the one containing very popular software that is missing from official repositories, software that is very rich in features, software that promises to fulfill user's subconscious needs (security, speed, ease of use). A chance to counter this efforts is to educate user to use packages outside of official repos only if absolutely necessary (at least until some way of preventing damage potentially caused by such software won't be devised systemwise).
Dangerous packages can also be injected on mirror level. Making this level more secure is in the hands of mirror administrators mostly but there also is a thing or two that Arch can do to help them. Firstly mirroring could be done through secure connection. This takes care of proving that packages are downloaded exactly from intended server, what in effect excludes possibility of man-in-the-middle attack. Secondly downloaded packages can be verified the same way as they would be on user machine. This would demand a bit of processing power from server's side but hopefully little enough to do it.
Finally packages can be signed and their signatures automatically verified what in connection with provided database of trusted packagers will lower the possibility of installing dangerous package from untrusted source. Package signing would be a subject of separate policy which will be described lower.
Arch Linux has relatively small and educated user base which makes it unlikely target of malicious packages based attack but it doesn't mean that obvious gaps in security shouldn't be taken care of. One of this gaps is package management security.