A Spanish builder stumbled on an online loophole that enables users to send Facebook messages in other users' names, prompting an alert by authorities, he said Wednesday.
Spain's Internet watchdog said it had alerted the US online networking site after Alfredo Arias, 37, warned them of the glitch that allows a hacker to use Facebook e-mail addresses when sending spoof messages.
Arias found that anyone who knows how to create an online messaging form -- a simple procedure for a website designer -- can enter another person's e-mail address in the "sender" field and send a message as if it were from them.
The procedure is a common trick of online frauds, but Spain's government Internet watchdog Inteco said it was concerned to see that Facebook's e-mail service did not have safeguards to stop its addresses being misused this way.
"It is very easy. You only have to know how to create a web page to do it," Arias, from the northern Spanish city of Leon, told AFP.
He said he alerted the institute and published details of how to carry out the procedure on his blog.
Inteco issued a warning to web users about the risk.
"We issued this message when we heard of this concrete case," a spokeswoman for the institute told AFP, adding that it had alerted Facebook.
"While the problem is common to all e-mail services, on Facebook it takes on a bigger dimension by linking in to the online social environment," said a warning to web users published by Inteco's Internet Safety Bureau.
"This flaw could be used by ill-intentioned users for example to pass themselves off as a friend of the victim and invite them to visit malicious web pages or download untrustworthy applications."
Separately, US authorities said Tuesday that Facebook agreed to tighten its privacy policies and submit to external audits in order to settle charges that it abused users' personal data.
That deal settled two-year-old accusations that Facebook -- which has some 800 million users -- had allowed advertisers access to users' personal data when users were told it was being kept private.
Explore further: Facebook abuses 'quasi-monopoly' on user data, EU lawmaker says