The security update for users of Apple's OS X computer operating software follows a fix issued for iPhones last week, meaning all Apple device users now have access to the patch.
The flaw allowed attackers with access to a mobile user's network, such as a shared unsecured wireless service offered by a cafe, to see or alter exchanges between the user and protected sites such as Google Inc's Gmail or Facebook.
Governments with access to telecom carrier data could do the same, experts said.
On Tuesday, Apple said in a statement that the Mac security update also improved features such as its FaceTime videoconferencing service and email.
The flaw appeared related to the way in which well-understood protocols were implemented, and how Apple's software recognizes digital certificates used by websites to establish encrypted connections.
Researchers have said the bug could have been present for months. Apple has not said when or how it learned about the flaw in the way iOS handles sessions, in what are known as secure sockets layer (SSL) or transport layer security. Nor has it said whether the flaw was being exploited.
A spokesman for the company declined to comment on Tuesday.