New versions of the Mozilla Foundation's browsers have re-introduced a seven-year-old flaw that makes them vulnerable to spoofing attacks, Secunia said on Monday.

Secunia first publicised the flaw last summer, warning that a feature that had been built into most browsers for years was in fact a security liability. The firm argued that a feature allowing one Web page to load arbitrary content into a frame of another page could allow an attacker to, for example, substitute his own login window on a bank's website. The feature was found in IE, Mozilla, Opera, Safari and Mozilla derivatives such as Konqueror.

"We believe that it is important that Microsoft and the other vendors seriously consider the minor gains from such 'functionality' against the possible consequences for their customers," said Secunia CTO Thomas Kristensen at the time. "In our opinion, this is a vulnerability and should be treated as such, whether the vendors implemented this intentionally or not."

Most browser vendors, including Mozilla, agreed and updated their products to remove the feature. But it has been re-introduced in Firefox 1.0.4, Mozilla 1.7.8 and Camino 0.x, according to the firm. Secunia has published an online demonstration of the flaw here.

The new vulnerability is a slight variation of the flaw fixed last year, Secunia said.

The Mozilla Project said it is investigating the report, and a moderator of the organisation's online support site said the flaw had not been exploited. "To protect yourself, close all other windows/tabs before accessing a site where you routinely put in a secure password (your bank or PayPal account), or your bank or credit card details (e.g. Amazon), or other sensitive data," the moderator said.

Only a handful of other flaws have had an impact reaching across browsers and platforms. Another example is a spoofing flaw involving the use of international domain names, discovered in browsers such as Mozilla, Firefox and Opera - though not IE - in February.