Late last week, the Internet Engineering Task Force (IETF) approved a fix to the protocol that guards most sensitive transactions and communications online. But experts expect it to take a year for the fix to be fully applied.

The patch repairs a flaw in the protocol that encrypts sensitive communications, including most banking and credit-card transactions. It repairs the Transport Layer Security (TLS) protocol, which has superseded the Secure Socket Layer (SSL) protocol. TLS is built into Web browsers and Web servers and protects high-value information.

The current flaw, discovered by Marsh Ray and Steve Dispensa of a Kansas-based authentication company called Phone Factor, gives an attacker the ability to hijack the first moment of the encrypted conversation between a Web browser and a Web server. This allows the attacker to add a command of his own, which could be as serious as an order to withdraw money from the victim's account. One security researcher demonstrated the attack on Twitter, showing that the flaw could be used to command the server to reveal a user's password.

"The reason it's striking is that it's actually a TLS error, or at least arguably so," says Eric Rescorla, a security consultant at a company called RTFM and one of the authors of the draft fix to the protocol. Rescorla says the flaw shows how difficult it actually is to design security protocols for the Internet.

To make use of the flaw, an attacker would first have to set up a "man in the middle attack" and intercept traffic between the client and the server. This might be done by hijacking a particular server on the Internet, for example.

The attacker could then exploit a feature of TLS called "renegotiation," which allows a Web server or client to change some of the parameters of an encrypted session while that session is happening. Dispensa explains that the protocol does not make sure that the parties communicating after renegotiation are the same ones as before.

Ray and Dispensa admit that exploiting the flaw would require considerable technical skill, but they say it is significant because it affects servers and clients even if they've implemented the protocol perfectly. "It's pretty clear that nobody understood this property of TLS," Rescorla says.