From Ars Technica:
For the time being, people using Macs should avoid using public networks, a step that can thwart many criminal eavesdroppers but will do little to prevent surveillance by the National Security Agency and other state-sponsored spies. Because the Google Chrome and Mozilla Firefox browsers appear to be unaffected by the flaw, people should also consider using those browsers when possible, although they shouldn't be considered a panacea.
The flaw, according to researchers, causes most iOS and Mac applications to skip a crucial verification check that's supposed to happen when many transport layer security (TLS) and secure sockets layer (SSL) connections are being negotiated. Specifically, affected apps fail to check that the ephemeral public key presented by servers offering Diffie Hellman-supported encryption is actually signed by the site's private key. Attackers with the ability to monitor the connection between the end-user and the server can exploit this failure to completely decrypt and manipulate the traffic by presenting the app with a counterfeit key.
An attacker "can basically set up a connection and pretend to be Google.com," Matt Green, a Johns Hopkins University professor specializing in encryption, told Ars. The attacker "can basically say: 'Hey I'm Google, here's my signature. And since nobody is actually going to check the signature, [the attacker] just puts nonsense in there."
Keep an eye out for updates to OS X, and patch your iOS devices as soon as possible.