Software Engineering Institute

Adobe Flash Player is a widely deployed application for multimedia viewing on the internet. A vulnerability exists in the way that Flash Player handles cross-domain policy files. An attacker could use a specially crafted cross-domain policy file to circumvent a site's Flash content data loading security polices.

From Security changes in Flash Player 9:

There is a possibility that a file on a server that does not appear to be a policy file may in fact be used as a policy file. For example, if a server permits uploads by users, but does not intend to open data for cross-domain access, it is possible that a user could deliberately construct a policy file but disguise it as a different type of file, such as an ordinary text, XML, or HTML file, or even as a binary type, such as a PNG or JPEG image file. A user who successfully uploaded this disguised policy file could then write a SWF file that takes advantage of the disguised policy file to load data from outside the server's domain.

A remote, unauthenticated attacker may be able to conduct cross-domain and cross-site scripting attacks on sites hosting Flash content. This could allow the attacker to read data or modify Flash content, possibly gaining access to sensitive information or spoofing web site content.

Adobe has released an update for Flash Player to address this vulnerability. Please see Adobe Security bulletin APSB07-20.

Limit access to Flash files

Limiting access to untrusted Flash files can prevent cross-site scripting attacks that use Flash files. Disable ActiveX controls in the Internet Zone in Microsoft Internet Explorer. See Securing Your Web Browser for more information. Consider using the NoScript extension to whitelist web sites that can run Flash in Mozilla browsers such as Firefox. See the NoScript FAQ for more information.