Apple has patched versions of its iOS and OS X operating systems to fix yet another extremely critical cryptography vulnerability that leaves some users open to surreptitious eavesdropping. Readers are urged to install the updates immediately.

The flaw resides in the secure transport mechanism of iOS version 7.1 and earlier for iPhones and iPads and the Mountain Lion 10.8.5 and Mavericks 10.9.2 versions of Mac OS X, according to advisories here and here. The bug makes it possible to bypass HTTPS encryption protections that are designed to prevent eavesdropping and data tampering by attackers with the capability to monitor traffic sent by and received from vulnerable devices. Such "man-in-the-middle" attackers could exploit the bug by abusing the "triple handshake" carried out when secure connections are established by applications that use client certificates to authenticate end users.

"In a 'triple handshake' attack, it was possible for an attacker to establish two connections which had the same encryption keys and handshake, insert the attacker's data in one connection, and renegotiate so that the connections may be forwarded to each other," Apple's warning explained. "To prevent attacks based on this scenario, Secure Transport was changed so that, by default, a renegotiation must present the same server certificate as was presented in the original connection."

The patch comes three months after the disclosure of a separate serious HTTPS vulnerability dubbed "goto fail" that similarly threatened iOS and OS X Mavericks users. It wasn't fixed in OS X until four days after the bug became widely known, a delay that prompted criticism from security professionals because it potentially gave attackers a window to exploit Mavericks machines. By contrast, the triple handshake bug may be considered less severe because it affects a smaller class of applications. Still, it's a serious bug because those apps are typically used by businesses and government agencies, where security is especially sensitive. More information about triple-handshake weaknesses is available here

More recently, the Internet was severely threatened by another extremely critical vulnerability in HTTPS software—the so-called Heartbleed bug in the widely used OpenSSL cryptographic library. Apple has reportedly updated its Airport Base Stations to fix that critical flaw as well, according to Macworld.

The iOS and OS X updates Apple issued Tuesday, which Ars wrote about earlier here, fix a variety of other serious security vulnerabilities, some of which also affect Mac OS X Lion. Users should install them as soon as possible.

Post updated to add details about client authentication.