An alleged vulnerability on American Express site exposed customers to a serious security risk before the credit card giant closed down a portion of its site on Thursday afternoon.

Researcher Niklas Femerstrand claimed the problem arose because the debug mode of the americanexpress.com site had inexplicably been left on, thus providing access to vulnerable debug tools. The security shortcoming creating a possible mechanism to harvest users' authentication cookies, according to Femerstrand.

American Express said the issue was confined to a test page it took down on Thursday afternoon. In a statement, issued to Financial News Network, it stressed that customer information was never at risk.

We learned this morning that an internal test page created to update promotional offers was temporarily accessible on our US website. The page did not contain CM information such as card number, name or address. The page in question has been taken down. We are not aware of any information at this time that this vulnerability was used for malicious purposes but we are continuing to investigate.

Femerstrand went public with his findings on Wednesday – posting what appears to be a harmless proof-of-concept illustration of the bug – after he struggled to report the bug directly to the credit card giant.

"The debugging tool is vulnerable to XSS [cross-site scripting] and it quickly becomes an issue when the debugging tools are called through unprotected GET parameters," he said.

"The debug window refreshes itself so that injected code that doesn’t break the loop will execute infinitely. An attacker could inject a cookie stealer combined with jQuery’s .hide() and harvest cookies – which can, ironically enough, be exploited by using the admin panel provided by sloppy American Express developers."

Femerstrand told El Reg on Thursday that the security vulnerability was still present hours after he went public about the flaw.

"The admin page is supposedly limited but debugging is still on and the vuln is still active," he claimed via Twitter.

Reg reader David, who brought the issue to our attention, said any fix ought to have been easy to apply. "This should be simple to fix, but the potential for cookie harvesting and further exploration/exploitation is still there," he said.

We asked American Express directly for comment on this story, which we'll update as and when we hear more. ®