Apple has patched a bug in the Bash system, which is present in Mac OS X.
The vulnerability, known as ShellShock, allows attackers to remotely take control of a user's computer by allowing them to execute shell commands. The problem, which is up to 25 years old, was discovered on 25 September.
Apple had previously said that while it was "working quickly" to resolve the situation, most people were "not at risk" from the bug, unless they were advanced users who had configured advanced Unix services.
The security update applies to desktop operating systems OS X 10.6.6 Snow Leopard through to OS X 10.9.6 Mavericks, as well as a variety of applications on these operating systems.
For Windows operating systems, there are updates to iTunes from version 10.2 on XP SP 2, Vista and 7 through to iTunes 11.2 for XP SP 3 and up, Vista, 7 and 8, as well as updates for Quicktime from 7.7 to 7.7.5 for Windows XP SP 2 and up, Vista, and 7.
Full details of the security updates, of which there are 144 in total, are available here.