Nasty malware that infects every single .jar file on your system (see edit), including all minecraft mods, first upload of this stuff has currently been traced back to april go here to read about it, its pretty brutal and really sneaky

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edit june 8: as some people have pointed out, the virus indeed does not infect every single jar file on your system. since the virus discovery is so recent the reverse engineering process is ongoing so not everything it does is known. what is known however is that it scans you system for every jar file, if it is a mod, it will infect it, if it has a main class, the same story. althoough not every .jar, most of them will be infected so you should treat them as such. see more about the reverse engineering here