The danger stems from a Cross-Site Scripting (XSS) exploit of Shockwave Flash (SWF) files generated by most of the programs that create Flash applets that allows attackers to access data on targeted websites; such as usernames and passwords, or even performing unauthorized online banking transactions.
The problem may be particularly acute for adult website operators, who have increasingly made use of Flash technology in advertisements and video files and often rely on Adobe's popular DreamWeaver software for website development – one of the tools that generate the vulnerable files.
"If a web application is vulnerable to XSS, and an attacker lures a user of the vulnerable web application to click on a link, then the attacker gains complete control of the user's session in the web application," Google's Rich Cannings wrote. "The attacker can use JavaScript to perform any action on behalf of the user (for example, perform a transaction on an online banking system) or change the way the website appears to the user (for example, perform a phishing attack)."
While security experts have warned of additional vulnerabilities, the XSS exploit was made public after companies such as Adobe updated their software to eliminate the bug.
Now, experts are recommending that all existing Flash files be removed from websites until they can be regenerated with the newest versions of these tools to address the issue.
Cannings also recommends that SWF files be served from numbered IP addresses or from separate domains from the site that features the Flash files.
"If there's an issue on a bank, the impact of an XSS is pretty large," Cannings said. "In other words, it's a huge amount of work, but well worth it for trusted sites that want to remain that way."
Expanding on the causes of the vulnerability, Cannings reported that DreamWeaver's "skinName" parameter can be used to load URLs containing the "asfunction" handler; while Adobe Acrobat Connect makes files that do not validate the "baseurl" parameter, which can allow malicious scripts to be injected into targeted websites.
The complete report can be read here.