Little-Known Hole Lets Attacker Hit Main Website Domain Via Its Subdomains

By Kelly Jackson Higgins
DarkReading
Nov 05, 2009

Turns out an exploit on a Website's subdomain can be used to attack the main domain: A researcher has released a proof-of-concept showing how cookies can be abused to execute such an insidious attack.

Michael Bailey, senior researcher for Foreground Security, published a paper this week that demonstrates how an exploit in a subdomain, such as mail.google.com, could be used to hack the main production domain, google.com, all because of the way browsers handle cookies.

"There's no specific vulnerability here, but it's widening the attack surface for any large organization that has more than one [Web] server set up. A [vulnerability] in any one of those servers can affect all the rest," Bailey says.

Most Web developers aren't aware that a vulnerability in a subdomain could be used to target the main domain. "We're trying to get the message out that now you have to treat everything [in the domain] as though someone can compromise your crown jewels," says Michael Murray, CSO for Foreground. "You have to realize that every vulnerability, every attack vector in those subdomains, can be used to compromise [other areas of the domain]," he says.

It all boils down to the browsers themselves. Within the DNS architecture, the main domain -- fortune500company.com, for instance -- has control over its subdomains, such as development.fortune500company.com. Development.fortune500company.com has no authority to change anything on the main fortune500company.com site.

But browsers do the reverse, Murray says.
Development.fortune500company.com can set cookies for fortune500company.com, the main domain. That leaves the door open for cookie-tampering, he says, when the subdomain has an exploitable vulnerability, such as cross-site scripting (XSS) or cross-site request forgery (CSRF).