Updated Facebook plans to open up members' inboxes and notifications to developers have drawn fire from security experts as an unacceptable privacy risk.

The social network site published plans to release a notification and Mailbox API in a post on a developers' forum last month. The development has received little attention since, despite marking a huge shift in how much confidential data software applications on the social networking might be able to access.

Users who sign up to applications that make use of the feature give the green-light for software to scan the contents of messages sent through the social networking website, as Facebook explains.

The Mailbox API allows you to access your users' messages, once they grant your application the new read_mailbox extended permission. This lets your applications provide an interface for users to view their messages. For example, your application could pop up an alert when the user receives a new message.

The social networking site suggests the technology might be used to develop desktop applications that allow users to check their company's Page stream, as well as read messages and receive notifications, all via their desktop. As well as the contents of messages, the recipients of a thread, time and date information would also be exposed.

About the only thing not enabled by the Mailbox API is the opportunity to send messages.

The notifications API, meanwhile, allows applications developers to retrieve users' notifications and use this data within apps they develop. Notifications cover everything from status updates through whether one of their contacts has taken a quiz through application-related activity.

But it's the mail software hook feature that has security watchers worried.

"The idea of Facebook applications being given free rein to mine users' inboxes and sent folders sends a shiver down my spine," Graham Cluley, senior technology consultant at Sophos told El Reg.

Even with safeguards and clear warnings, Cluley is wary about the technology.

"Obviously we have to hope that Facebook does not enable this functionality by default, and presents a clearly worded warning to its users if they try and add an application which insists on users waiving the rights to a private mailbox to third parties," he said.

"But my worry is that many of Facebook's 300 million users will be so keen to see what Sex and the City character they are, or to send a Best Friend Forever ecard to their online buddies, that they'll glaze over the rights they are signing away when they add an app. Even if you do decide to add an Facebook app that you trust to access your mailbox, there have been occasions in the past where app developers have used a 'bait and switch' trick to change the nature of their app overnight."

Facebook users are likely to include more detail in messages than in status updates or the like, a factor that increases the privacy risk.

Gmail users allow Google to scan the content of emails to display targeted adverts, a privacy concern for some. Facebook's approach goes even further, Cluley warned.

"Some people are nervous enough of Google using computers to examine the content of email to display targeted adverts in Gmail, but the idea of handing over this information to third party external developers sounds like a privacy nightmare waiting to happen," Cluley told El Reg.

"An additional worry is that it appears Facebook is saying it may in the future integrate functionality which would allow application developers to send mails from their users' inboxes. It doesn't take a great imagination to picture how this could be abused by spammers and malware authors."

Update

Facebook defended its plans to open up members' inboxes to application developers as a technology that will spur innovation in areas such as mobile messaging. The social networking networking argues that its Mailbox API has fewer security concerns than Gmail for the privacy conscious in a response analysed in a follow-up story here. ®