Warning: Some malicious client mods are stealing passwords
This message was posted by Notch on his blog.
We'd just like to point out you should always be cautious when choosing your modules/plugins.
Always make sure to read te bukkit forum post where you downloaded the plugin from and see if you notice any negative comments or complaints about the plugin before installing it.
When you install a mod, the mods gets full access to your computer, and can do ANYTHING to it.
I don’t know which mods are the culprits.
When the modding support is added, there will be support for sandboxing mods to give them less access to important stuff. You can chose to trust a mod if you want to, and if it needs to do fancy things, but the default will be to run the mod sandboxed.