USB Security: How to Prevent Data Loss and Stop Malware
Graeme Batsman, a Senior Security Consultant with Fortra’s Professional Services team, gives a primer on all things USB: How to limit data ingress, egress, and other technical controls that can be implemented to mitigate risk.
Jump back to the mid to late 1990s – for some, a distant memory – to the release of optical drives for PCs. At the time, paying more than $500 per drive was not uncommon; the blank CDs or DVDs that went with them aren’t dollars apiece like they are now, either.
These days, it's rare to see optical drives in desktops or laptops. In fact, few modern laptops or desktops are even shipped with CD, DVD or Blu-ray drives. That’s why worrying about leaks or malware infections via CD or DVD isn’t top of mind for CISOs.
The same cannot be said for USB ports - which are less popular than 10 years ago due to increases in internet speed and stability as well as cloud-based options like Microsoft OneDrive, Google Drive etc. - but still used. While less popular, they’re fairly inexpensive, especially compared to what they cost in year 2000. These days, for under $40, you can buy a 256GB USB stick, something that can easily eat up an entire modern solid state drive (SSD) or hard disk drive (HDD).
When it comes to your organization and how you approach USB security, it's all about your risk appetite. Chances are pretty good a defense contractor will approach how USB devices are used differently compared to a hair dresser franchise.
What are the dangers of USB sticks or external USB hard drives?
Few people think about how data is ingested - how data files are imported - from a USB stick to a laptop or desktop. Assuming the data is non-malicious, then what’s the problem? For one thing, music tracks, films and software all have license agreements around them. Consider CCleaner, a well-known utility used to clean potentially unwanted files from machines. It may be free but it’s licensed for home use, not company use. If an employee was to bring it to the office, it could get the company in trouble for breaching the license agreement.
The same applies to bootleg music or videos. Imagine Disney or Sony suing a Fortune 500 company because one of their employees has been caught with stolen music or the latest unreleased films.
Consider the high stakes auto racing industry. Rivals regularly develop cutting edge hardware under tight regulations. Imagine if an employee stole IP (intellectual property) from the other company, ingested it via USB and implemented it into their latest cars? The opposition would sue them, something that almost certainly would ignite a powder keg of controversy.
When it comes to data egress - when data leaves a network and is transferred to an external location - the actions (and consequences) usually fall under two camps: on purpose and accidental. Let’s say an employee saves data to a USB stick for genuine reasons and the USB stick isn’t encrypted by software or hardware methods. Then, let's say that USB stick falls out of a coat pocket and lands on a packed London Underground train. While the USB stick isn’t technically lost or stolen, once the data has left the company, it can be further egressed to home computers where it could be infected or not disposed of correctly.
When you consider information egressed on purpose, you’re talking usually about an insider threat scenario. Perhaps an employee was asked by an OCG (organized crime group) or nation state to steal files. Rumors going back years suggest Russia and China have hundreds of agents planted into all sorts of organizations within the United Kingdom. Moving away from the likes of Mr. Bond, even genuine staff who move on can have a tendency to log into Salesforce, download client lists and load them on to a USB stick before serving their notice period.
Machine/network infection or exploitation
Write “staff salaries” or “pay rises 2023” on a USB stick and there’s a high likeliness that if someone finds it outside of an office entrance, they're going to insert it into a machine. Stuxnet, which targeted supervisory control and data acquisition (SCADA) systems at a nuclear enrichment/power plant in Iran in the late 2000s, used a similar infection method. A simple file that looks like a .DOCX file can be a executable file (.EXE) that can go on to infect a target computer. Similarly, actual Portable Document Format (.PDF) files, Word, Excel, and PowerPoint files can be rigged with zero days exploits. Just by opening an “innocent” file, it can exploit the application and download an .EXE from the network in seconds. If the file contains a zero-day vulnerability, software defenses may not be able to nab it in time.
What are the soft options to control USBs?
Implement zero technical controls and perhaps have a policy saying do not use USB devices. As an example, consider speed limits. 70 MPH is the UK limit and while there may not always be cameras or traffic officers, there's nothing to physically stop you from driving over 70 MPH. It's just like how people don’t always explicitly follow data protection policies or laws.
2. Write Blocker
Permit USB storage devices from mounting and simply block write which means that files can be opened and copied but not written to disk. Here, data egress is not a problem but data ingress is, along with an increased chance of malware infections or application/operating system exploits running.
3. Mount blocker
The strictest of options. If an employee inserts a USB storage device, it does not get a drive letter, something which makes it invisible to the device its inserted into. No user access equals no data ingress/egress or infection chances.
4. Forced encryption
A compromise for employees. Allow USB storage read/write and enforce encryption on data written to the external drive. Depending on your organization's risk appetite, you can encrypt based on system generated key or allow the user to select a password which generates a key in the background. The latter option increases useability/portability by allowing the USB stick to be used on non-organizational devices without the requirement for special software to be installed on each device. That said, it could open up the device to password cracking since the authentication is done via the user rather than a transparent key.
USB Best Practices
Options number 2 & 4 are an improvement over number 1 but infections and exploits can unfortunately still be a concern for organizations. Software, such as endpoint data loss prevention solutions can block when applications are launched from removable media drives and block the copying/opening of certain file types like: Windows PowerShell script files (.PS1), Java ARchive files (.JAR), batch files (.BAT), Windows command files (.CMD), HTML application files (.HTA), and so on. Blocking by the hexadecimal file header is typically safer as attackers can easily rename the malicious file to trick users or bypass technical controls.