Users of Internet Explorer on Windows platforms are being advised to turn off JavaScript to prevent their cookie files from being read by hostile Web sites.

Thousands of electronic commerce sites use cookies to authenticate users or store private information.

However, those cookies could be exposed by IE and intercepted by a third-party Web site, according to an Internet privacy watchdog group. has demonstrated that by using a specially constructed URL, a Web site can read IE cookies.

For example, to read an cookie, a site might direct the user's browser to

Peacefire points out that if the "%2f"'s are replaced with "/" characters, and the "%3F" with "?", this URL is actually

This hack confuses IE into thinking the page is located in the domain and allows the page to read the user's cookie. Normally, only the site that issued a cookie has permission to read data within that cookie.

According to Peacefire, all known versions of Internet Explorer for Windows 95, 98, and NT are affected. The organisation reports that IE for the Macintosh and Unix do not appear to be affected, and no version of Netscape Navigator or any other browser is vulnerable.

Quick Fix?

Peacefire says the safest workaround for Windows IE users is to disable JavaScript. When the browser loads a URL like, the cookie is only available to JavaScript code on the page; it is not submitted to the server in an HTTP header.

A spokesperson for Microsoft says that the company is working on a patch for the IE cookie issue to be released shortly. A security bulletin will be published at to discuss the issue and advise customers how to obtain and apply the patch.

Sensitive Cookies

Microsoft acknowledges that the vulnerability could allow a malicious Web site operator to read, change, or delete cookies that belong to another Web site.

However, according to the company, the vulnerability could not be used by a malicious Web site operator to "inventory" what cookies a person has.

Instead, the hacker would need to randomly try to recover cookies from various sites.
However, Peacefire's Jamie McCarthy says that a number of popular sites deploy cookies that collect sensitive information.

He pointed out that intercepting a cookie set by Hotmail, Yahoo Mail, or any other free Web-based e-mail sites that use cookies for authentication could allow the operator of a hostile Web site to break into a visitor's Hotmail account and read the contents of the user's inbox.

McCarthy also points out that intercepting a user's cookie could allow a hacker to visit impersonating that user, and access their real name and e-mail address.

Credit-card numbers or actual lists of previous orders can't be accessed because viewing this information requires a password not contained in the cookie, says McCarthy.