AusCERT 2012: Flash drive problem — even deleted data can leak

Securing data on Flash media is harder than most people believe, Peter Jamieson of UL subsidiary Witham Laboratories, told AusCERT delegates.

“The only way to be absolutely certain that data can’t be recovered from a USB key is to hit it with a hammer… and to get medieval about it,” he said. Even encrypted keys, he said, have vulnerabilities that might expose data the user believes is safe.

Jamieson highlighted a combination of how Flash controllers manage the storage device for the longest possible life, the physical behavior of the Flash cells, and how encryption is managed.

“Flash media is portable, robust, ubiquitous – and therefore losable”, he said.

One problem lies in how the Flash controller behaves, with two particular characteristics of the Flash Transfer Layer (FTL) that are designed to cope with some of the shortcomings of Flash.

Flash controllers, Jamieson explained, have a particular lifecycle consideration in mind: each block on the drive can only be physically deleted between 10,000 and 100,000 times before it begins to degrade. Moreover, the block – which is the smallest unit that Flash controllers delete – will have data other than what you’re trying to alter or delete.

An alteration to a small file (say, an e-mail signature) will, he explained, result in the drive’s controller creating a new copy of the block containing that file – but to avoid an unnecessary erasure, the FTL will mark the block as unusable, but only erase it when it’s necessary.

“When you ‘delete’ a file on a USB flash drive,” he said, “you don’t overwrite the file, you just delete the filesystem’s representation of the file.”

The electrical properties of Flash cells are also problematic: “deleting” the contents of the cell only requires that its charge falls below the arbitrary value that represents “zero”. Various experiments, he noted, suggest that operating a Flash drive at lower than its nominal voltage will cause residual charge remaining in the cells to be readable.

He recommends that all data stored on USBs is encrypted (with a preference for software encryption or a combination of hardware and software), and strongly suggests multiple-pass overwrites as a data deletion strategy.


Follow @CSO_Australia and sign up to the CSO Australia newsletter.

Join the CSO newsletter!

Error: Please check your email address.

More about CSO

Show Comments

Featured Whitepapers

Editor's Recommendations

Solution Centres

Stories by Richard Chirgwin

Latest Videos

  • 150x50

    CSO Webinar: Will your data protection strategy be enough when disaster strikes?

    Speakers: - Paul O’Connor, Engagement leader - Performance Audit Group, Victorian Auditor-General’s Office (VAGO) - Nigel Phair, Managing Director, Centre for Internet Safety - Joshua Stenhouse, Technical Evangelist, Zerto - Anthony Caruana, CSO MC & Moderator

    Play Video

  • 150x50

    CSO Webinar: The Human Factor - Your people are your biggest security weakness

    ​Speakers: David Lacey, Researcher and former CISO Royal Mail David Turner - Global Risk Management Expert Mark Guntrip - Group Manager, Email Protection, Proofpoint

    Play Video

  • 150x50

    CSO Webinar: Current ransomware defences are failing – but machine learning can drive a more proactive solution

    Speakers • Ty Miller, Director, Threat Intelligence • Mark Gregory, Leader, Network Engineering Research Group, RMIT • Jeff Lanza, Retired FBI Agent (USA) • Andy Solterbeck, VP Asia Pacific, Cylance • David Braue, CSO MC/Moderator What to expect: ​Hear from industry experts on the local and global ransomware threat landscape. Explore a new approach to dealing with ransomware using machine-learning techniques and by thinking about the problem in a fundamentally different way. Apply techniques for gathering insight into ransomware behaviour and find out what elements must go into a truly effective ransomware defence. Get a first-hand look at how ransomware actually works in practice, and how machine-learning techniques can pick up on its activities long before your employees do.

    Play Video

  • 150x50

    CSO Webinar: Get real about metadata to avoid a false sense of security

    Speakers: • Anthony Caruana – CSO MC and moderator • Ian Farquhar, Worldwide Virtual Security Team Lead, Gigamon • John Lindsay, Former CTO, iiNet • Skeeve Stevens, Futurist, Future Sumo • David Vaile - Vice chair of APF, Co-Convenor of the Cyberspace Law And Policy Community, UNSW Law Faculty This webinar covers: - A 101 on metadata - what it is and how to use it - Insight into a typical attack, what happens and what we would find when looking into the metadata - How to collect metadata, use this to detect attacks and get greater insight into how you can use this to protect your organisation - Learn how much raw data and metadata to retain and how long for - Get a reality check on how you're using your metadata and if this is enough to secure your organisation

    Play Video

  • 150x50

    CSO Webinar: How banking trojans work and how you can stop them

    CSO Webinar: How banking trojans work and how you can stop them Featuring: • John Baird, Director of Global Technology Production, Deutsche Bank • Samantha Macleod, GM Cyber Security, ME Bank • Sherrod DeGrippo, Director of Emerging Threats, Proofpoint (USA)

    Play Video

More videos

Blog Posts

Market Place