How secure is a NFC tag?

If a tag is unlocked anyone is able to 'over write' your previous encoding. Essentially re-encode the tag with new information. They are then also able to lock your tag with this new information. If the tag is already locked, no one can re-encode the tags, not even us.

On some chips you are able to use password protection, the NTAG213 and ICODE SLIX 2 .These chips allows you to set a password which from the app can unlock the tag and allow you to change the data. Its not exactly easy to use and tends to be quite temperamental but its technically possible.

NTAG424 and NTAG223/224 are authentication chips that are unable to be copied. This will provide the tags the extra security as every time a user scan this tag it will produce a new code. and the code will expire as soon as it reaches its destination.

Depending on what you are doing you might prefer to use a tag management system. This would essentially mean that all the tags are pre-encoded and locked.
Seritag - 19 Jan 2023