Google’s browser Chrome may have just suffered its most serious security setback yet. Security firm VUPEN today announced that its researchers had found a way to have a webpage run any code or program it wants on a Chrome user’s Windows computer.

A video shows how visiting the attacking webpage enables it to download and then execute a calculator program–a standard method of demonstrating how an attack works. If the vulnerability were exploited in the wild it could be used to install programs that steal passwords, or make a computer part of a network of infected computers used to attack websites.

VUPEN hasn’t made the exact details of the hack public, saying only that it required “one of the most sophisticated codes we have seen” and that it will relay the details to government agencies on its customer list. According to the company’s statement:

“This code and the technical details of the underlying vulnerabilities will not be publicly disclosed. They are shared exclusively with our Government customers as part of our vulnerability research services.”

VUPEN provides law enforcement and intelligence agencies with what it dubs “weaponized” exploits for surveillance or other covert operations. The company has made no mention of whether Google was warned of today’s announcement, or how it will be helped to fix the exploit.

Google has promoted Chrome since its launch as offering superior security to other browsers. Chrome has never been defeated at the annual Pwn2Own contest that challenges the world’s best hackers to compromise popular software. At this year’s event in March, Firefox and Chrome were the only two popular browsers not defeated.

VUPEN’s attack is the first to beat a feature of Chrome called “sandboxing” that carefully isolates web code in different tabs from each other, and the rest of a person’s computer. The new attack is able to somehow bust out of the sandbox to download and execute any code it wishes. Another of Chrome’s security features will help it minimize the risk to users, though. Unlike most software, the browser silently upgrades itself to the most recent version without a person’s permission whenever possible. That habit will enable the fix to Google’s new security hole to spread fast, once it has been worked out.