Adds warning about PUK being default

README.md

@@ -4,6 +4,8 @@ Keys stored on YubiKey are [non-exportable](https://support.yubico.com/support/s
 
 **New!** [drduh/Purse](https://github.com/drduh/Purse) is a password manager which uses GPG and YubiKey.
 
+**Security Note**: If you followed this guide before Jan 2021, your PUK (Pin Unblock Key) may be set to its default value of `12345678`. An attacker can use this to reset your PIN and use your Yubikey. Please see the [Change PUK](#change-puk) section for details on how to change your PUK.
+
 If you have a comment or suggestion, please open an [Issue](https://github.com/drduh/YubiKey-Guide/issues) on GitHub.
 
 - [Purchase](#purchase)
@@ -326,7 +328,7 @@ From YubiKey firmware version 5.2.3 onwards - which introduces "Enhancements to
 ## YubiKey
 
 To feed the system's PRNG with entropy generated by the YubiKey itself, issue:
-```console 
+```console
 $ echo "SCD RANDOM 512" | gpg-connect-agent | sudo tee /dev/random | hexdump -C
 ```
 This will seed the Linux kernel's PRNG with additional 512 bytes retrieved from the YubiKey.