Apple today patched 27 vulnerabilities in Safari for Mac OS X and Windows, 85% of them critical bugs that could be exploited to hijack Macs or PCs.
Of the 27 flaws fixed in Safari 5.0.3 for Mac and Windows, four were patched by Apple two months ago in its iOS mobile operating system, and at least three had been addressed by Google in its Chrome browser as far back as mid-August.
Chrome and Safari share the open-source WebKit browser engine. Apple identified all 27 vulnerabilities it patched today as within WebKit.
Most of the vulnerabilities addressed in the Safari updates -- Apple also patched the older Safari 4 that runs in Mac OS X 10.4, aka Tiger -- were accompanied by the phrase "arbitrary code execution," which is Apple's way of saying "critical."
Unlike other browser makers, including Google, Microsoft and Mozilla, Apple doesn't assign severity labels to vulnerabilities.
According to Apple, the 23 critical bugs can be exploited by "drive-by" attacks that launch as soon as a victim browses to a malicious Web site.
Among the non-critical vulnerabilities patched today was one that could be used by unscrupulous site owners to secretly track users' browsing habits, even when Safari has disabled cookies. Another flaw could let identity thieves spoof the URL showing in Safari's address bar, a common tactic of phishers who feed bogus sites to users in the hope of capturing passwords to online bank accounts.
Apple also fixed several stability bugs, boosted the reliability of its pop-up ad blocker and improved the accuracy of the choices displayed in Top Sites, the navigational feature that posts thumbnails of frequently-visited sites.
As has been the case before, Apple credited a wide range of researchers who work for rivals for finding flaws it fixed today: A third of the vulnerabilities were reported by Google developers, one was submitted by an engineer in Microsoft's bug research team, and another was filed by someone working for Opera Software, the Norwegian company that creates the browser by the same name.
Today's update was the first since Sept. 7, and the third since Apple rolled out Safari 5 in June.
Safari 5.0.3 can be downloaded from Apple's site for Mac OS X 10.5 (Leopard), Mac OS X 10.6 (Snow Leopard), Windows XP, Windows Vista and Windows 7.
Mac OS X users will be notified of the new version automatically, while Windows users already running Safari will be alerted by the Apple Software Update tool.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is firstname.lastname@example.org.