A study has revealed that smartphone users who sell on their old Android smartphones may be leaving traces of their personal information on the devices even after doing a complete reset.

Research conducted by internet security firm Avast has determined that Android smartphones do not completely wipe out data when users resort to a 'factory reset.'

Avast's latest study states that users selling off Android smartphones presuming that all their stored data is deleted after a factory reset, should in fact exercise caution and go for third-party applications (such as Cerberus and Avast's own Anti-theft app) that implement a 'total data wipeout' mechanism.

The internet security firm conducted a real world analysis of various Android smartphones that were sold off by users. Avast's behind-the-scenes people purchased more than 20 'second hand smartphones,' whose previous owners had resorted to a factory reset to erase their stored data.

"The amount of personal data we retrieved from the phones was astounding. We found everything from a filled-out loan form to more than 250 selfies of what appear to be the previous owner's manhood," said Jude McColgan, Avast's president of Mobile, in a news release.

"We purchased a variety of Android devices from sellers across the US and used readily available recovery software to dig up personal information that was previously on the phones. The take-away is that even deleted data on your used phone can be recovered unless you completely overwrite it," he added.

Confidential data recovered by the researchers even after factory reset could be manipulated and misused by hackers. Personal and important information such as user bank account details, Facebook credentials, Google searches and photographs that are recovered even after a presumed data wipeout can lead to large-scale privacy breach.

Avast states that a factory reset leads to data deletion only at the application layer, which means that a large amount of data is still left 'uncleaned'.

Currently, Avast's Android Privacy Research is focused only on Android smartphones and doesn't mention Apple devices.