Apple has warned that the Mac OS X operating system contains 13 security flaws, some of them highly serious. The company issued a cumulative patch for the bugs on Wednesday.

The impact of the flaws includes remote code execution, security bypass, spoofing, cross site scripting, denial of service and other effects, according to Apple. Some of the flaws can be exploited from the Internet.

The most serious of the flaws could allow an attacker to remotely execute malicious code, effectively taking over the system. These include flaws in CoreFoundation, curl, and two bugs in the Safari browser.

Safari is also vulnerable to less serious attacks, such as causing the browser to download files into a different location and a spoofing flaw involving JavaScript dialogue boxes.

Other flaws could allow the downgrading of SSL connections to an earlier, less secure SSL version - known as a protocol downgrade attack; privilege escalation by local users; a cross-site scripting flaw in Apache; and the ability to forge syslog entries.

Security experts say Apple's security practices have improved, but are still at pains to make it clear that Mac OS X isn't as secure as people might think. The SANS Institute last week highlighted Mac security flaws in its list of top 20 security issues, partly in order to give users a wake-up call, according to the organisation.

"Although Mac OS X has security features implemented out of the box such as built-in personal firewall, un-necessary services turned off by default and easy ways to increase the OS security, the user still faces many vulnerabilities," SANS said in the report.

SANS noted that vulnerabilities continue to crop up regularly in Safari. "in certain cases exploit code has also been posted publicly," the group said.

Apple's way of releasing updates cumulatively doesn't make things easier for system administrators, SANS said. "Apple frequently issues Mac OS X cumulative security updates that tend to include fixes for a large number of vulnerabilities with risk ratings ranging from critical to low. This complicates the tracking of vulnerabilities for this OS," SANS' report said.