The weakness affects DKIM, or , a security system used by major email senders. DKIM wraps a cryptographic signature around an email that verifies the domain name through which the message was sent, which helps more easily filter out spoofed messages from legitimate ones.

The problem lies with signing keys that are less than 1,024 bits, which can be factored due to increasing computer power. US-CERT said issued Wednesday that signing keys less than 1,024 bits are weak, and that keys up to RSA-768 bits have been factored.

The issue came to light after Florida-based mathematician Zachary Harris was sent an email from a Google recruiter that used only a 512-bit key, according to a Wednesday by Wired magazine.

Thinking it might be some clever test by Google, he factored the key, then used it to send a spoofed message from Sergey Brin to Larry Page, Google's founders.

It wasn't a test but in fact a serious problem, one in which emails that could be bogus would be trusted. According to the DKIM standard, email messages that have keys shorter that 1,024 bits are not necessarily rejected.