A security hole in the widely used Macromedia Shockwave Flash file format used with Web browsers can allow an attacker to execute code of their choice on affected systems, according to a new security alert.

The vulnerability is limited, however, to Shockwave Flash files edited by hand with a binary editor, meaning that the Flash application will not produce files that contain the vulnerability on its own, according to a separate security alert from Macromedia, which is based in San Francisco, Calif.

The vulnerability is serious because it affects Web browsers, which are trusted by firewalls to receive incoming traffic, and because it affects all versions of Shockwave Flash used in the Internet Explorer and Netscape Navigator Web browsers running on both Windows and Unix, eEye said.

The flaw comes as the result of a problem in the data header of Shockwave Flash files which allows an attacker to supply more data to the file decoder than is expected and in turn can eventually lead to code execution.

Because the vulnerability is browser-based, it can be exploited in any situation in which a Web browser views a Shockwave Flash file, such as on Web pages, in e-mail or newsgroups.

For more information and an updated version of Flash go here.