Internet Explorer ignores cookies on some domains (cannot read or set cookies)

Question

I have a site, e.g. example.com, where users can set their own subdomains (one user - one subdomain) and upload their own scripts, e.g. http://somedomain.example.com/xyzzy.php would map to /www/somedomain/xyzzy.php

Now, on some of those domains, Internet Explorer 7 won't/can't accept cookies. Checked with Fiddler: the server sends Set-Cookie response correctly, yet the cookie never shows up in IE - for JS or Developer Tools. On request, IE7 doesn't send the Cookie header either.

The cookies are set for the user's domain (e.g. somedomain.example.com), path is /, tried different expiration options (past, future, current, "0"), are not HttpOnly, are not secure.

FF, Opera, Safari and Chrome all work without problems.

Why does IE ignore the cookies?

Answer 1

Does one of the subdomains use an underscore? IE has problems accepting cookies from subdomains that don't follow the URI RFC.