The FIDO U2F PIN

Both the FIDO U2F and FIDO2 standards specify that a device have a "test of user presence" in order to perform registration and authentication operations. This test is described as follows. "The user touches a button (or sensor of some kind) to 'activate' the U2F device".

With the YubiKey, there is the option to specify a PIN in order to use the U2F application as well as touch. However, this option is available only for YubiKey 4 FIPS series. If you try to set a PIN for the U2F application on a non-FIPS version 4 YubiKey, or a YubiKey 5 (FIPS or non-FIPS) series, or a YubiKey Security Key series, it will not work.

Note that with a version 5 FIPS series YubiKey, it is possible to set a PIN on the FIDO2 application. Version 4 series YubiKeys do not have the FIDO2 application, and it is not possible to set a FIDO2 PIN on a non-FIPS version 5 series YubiKey.

YubiKey	U2F Available	FIDO2 Available	U2F PIN	FIDO2 PIN
v 4	yes	no	no	-
v 4 FIPS	yes	no	yes	-
v 5	yes	yes	no	no
v 5 FIPS	yes	yes	no	yes
SKY	yes	yes	no	no

To learn more about the U2F PIN, see the article on FIDO U2F and FIPS.