Microsoft's Internet Explorer 6, 7, and 8 beta 1 appear to contain a security flaw that could subject users who visit a malicious Web site or open a malicious e-mail message to arbitrary code.

U.S. CERT has published a vulnerability note indicating Internet Explorer doesn't handle document frames securely.

Document frames can be used to subdivide Web pages such that the content associated with each division comes from a different server or domain. These "iframes," or inline frames, often are used for serving ads, which typically come from a different domain than content that appears on the same Web page.

The problem, as U.S. CERT describes it, is that "Microsoft Internet Explorer fails to properly restrict access to a document's frames, which may allow an attacker to modify the contents of frames in a different domain."

IE includes restrictions that limit the information one frame can get from another, but it appears that certain events, such as the "onmousedown" event, are not associated with a specific domain and thus are not sufficiently restricted.

Secunia, a computer security firm, on Thursday published a security advisory warning that Internet Explorer 7.x is vulnerable to a frame location handling vulnerability. It said that other versions of IE may also be affected.

The U.S. CERT vulnerability note says that Internet Explorer 6, 7, and 8 beta 1 are vulnerable and that proof-of-concept code has been published.

The proof-of-concept code was first published in May at a Web site maintained by "sirdarckcat." It can be used to hijack an iframe window and capture keystrokes.

According to Bill Sisk, communications manager for Microsoft security response, Microsoft is investigating reports of a possible vulnerability in Internet Explorer. The company is unaware of any attacks trying to use the claimed vulnerability or of customer impact.