If you haven't done already you should update your Ledger Nano S to firmware-version 1.4.1 (MCU 1.5).
If you're using an older firmware your device may be potentially insecure and you should update it as soon as possible.
This is the official update-guide provided by the manufacturer:
https://support.ledgerwallet.com/hc/en-us/articles/360001340473
More information about the security-issue can be found on the discoverer's blog:
https://saleemrashid.com/2018/03/20/breaking-ledger-security-model/
How to check your device's firmware-version: (taken from the official update-guide)
Once you’ve typed in your pin code, go to the “Settings” tab
Press both buttons to open the tab. Then, go to the “Device” menu
Press both buttons to open the tab. Then, go to “Firmware” tab
Press both button to open tab and check that “Secure Element” version is 1.4.1
Click on the right button, and check that “MCU” version is 1.5 (screenshots below shows 1.4 but you should see 1.5)
Source: https://support.ledgerwallet.com/hc/en-us/articles/360001340473
Regards,
xuNsh1ne (Bitcointalk: Nestade)