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A Google researcher is advising that security professionals rewrite code associated with Adobe Flash content two weeks after warning that buggy files can be exploited by attackers to gain complete control over transactions on websites belonging to banks, government agencies and other trusted organizations.

The security bug resides in SWF files created by most of the most common programs for generating the ubiquitous Flash applets that animate sites across the web. Vulnerable content opens websites up to cross-site scripting (XSS) exploits that allow an attacker to perform any action available to a user of the targeted website. Criminals could use it to pilfer a users' account details or perform withdrawals on behalf of a customer.

Adobe, Autodemo, TechSmith and InfoSoft have updated their content generation products so they no longer produce buggy SWF files, according to this post from Rich Cannings, a senior information security engineer at Google who helped discover the vulnerability. He advised security professionals to remove vulnerable SWF files from websites and regenerate them following the manufacturers' advice.

Cannings recommended security pros take other steps. They include serving automatically generated SWF files from numbered IP addresses or from domains that are separate from trusted sites that contain sensitive cookies or other credentials that could be used in phishing attacks. Or security people may opt to move or remove all third-party generated SWF files entirely, he said.

"If there's an issue on a bank, the impact of an XSS is pretty large," said Cannings. In other words, it's a huge amount of work, but well worth it for trusted sites that want to remain that way.

Security pros and Flash authors may also want to use Stafano Di Paola's SWFIntruder to check for vulnerabilities in their content.

When we first reported the Flash vulnerability two weeks ago, some Reg readers complained the article didn't detail exactly how the vulnerability worked. Now that patches are available, Cannings is willing to say a bit more.

Flash files produced by Adobe DreamWeaver contain a "skinName" parameter that can be exploited to force victims to load arbitrary URLs that include the "asfunction" protocol handler. SWF files generated with Adobe Acrobat Connect don't properly validate the "baseurl" parameter, allowing script injection. Content produced by TechSmith Camtasia contain a "csPreloader" parameter that loads an arbitrary flash file.

The vulnerability is documented in the book Hacking Exposed Web 2.0: Web 2.0 Security Secrets and Solutions, which is hitting store shelves now. In addition to Cannings, it was written by Himanshu Dwivedi, Zane Lackey, Chris Clark and Alex Stamos of iSEC Partners. It is published by The McGraw-Hill Companies.

The buggy Flash files are known to be contained on tens of thousands of websites, many belonging to banks, government agencies and major corporations, according to the authors, who relied on Google searches for their estimate. The actual number of sites could be in the hundreds of thousands because vulnerable files don't always turn up in web searches, they said. ®

Internet Security Threat Report 2014

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