Adobe's Flash Player has been updated, with seven fixes tackling vulnerabilities in the graphics and video software widely used for interactive web pages and banner advertisements.
Adobe classifies the patches as 'critical' and advises people upgrade to the latest version, 188.8.131.52. All of the vulnerabilities could allow a hacker to execute code on a machine.
One of the vulnerabilities allowed Shane Macaulay to win a laptop in the PWN 2 OWN hacking contest at last month's CanSecWest conference in Vancouver.
Macaulay, a researcher with the Security Objectives consultancy, used the Flash flaw to break into a machine running Windows Vista. He later said 90 percent of computers worldwide were vulnerable.
Exploiting vulnerabilities in Flash software has become an increasingly popular may for hackers to compromise machines for two reasons. Most web browsers have the Flash Player installed, and malicious banner advertisements - which can achieve wide distribution on websites pulling ads from a network - can take advantage of those vulnerabilities.
"These vulnerabilities could be accessed through content delivered from a remote location via the user's web browser, email client, or other applications that include or reference the Flash Player," Adobe wrote in its advisory.
If a malicious banner advertisement is widely distributed, a hacker has the potential to take control of many PCs. Lately, these 'malvertisements' have been popping up everywhere, wrote Sandi Hardmeier, a Microsoft Most Valued Professional and security blogger.
On Sunday, Hardmeier wrote that she observed a fake FedEx banner ad that causes a user to be redirected to a website selling dodgy security software.
On Tuesday, security vendor Websense blogged about a malicious banner ad on the website of USA Today. Websense wrote that if a user simply viewed the malicious ad, the person's browser window is immediately minimised, and a warning appears saying the computer is infected with malware, according to a description of the attack. Even if the user hits 'cancel', the browser is redirected to another website selling spyware, which tries to download code to the PC.
In January, Adobe and other software vendors fixed some of their Flash development tools to stop hackers from creating malicious Shockwave Flash (.swf) files that enabled cross-site scripting attacks. That style of attack makes a browser execute malicious code via security weaknesses in a website.
At least 10,000 buggy websites were still serving up buggy Flash files around mid-March, as developers worked to fix the problem.
The latest fixes focus solely on the Flash Player. One fix adds a feature Adobe calls a 'cross-domain policy check'. The Flash Player uses policy files, which allow it to use content from other domains. The feature allows for more richer capabilities in the player, wrote Deneb Meketa, a Flash engineer for Adobe, on the company's developer site.
But hackers can also build a policy file. If the policy file is accepted by the server, the hacker can then write a ".swf" file and load other data from outside the particular server's domain, which could lead to a security problem.