Event 6273 with reason code 262 — "The supplied message is incomplete. The signature was not verified." The RADIUS request arrived, but NPS could not verify its integrity. Because the Message-Authenticator and password-hiding mechanisms are keyed by the shared secret, this is almost always a shared-secret mismatch between the switch/controller and the NPS RADIUS client entry.
Reason code 262
NPS 6273 reason 262: message signature not verified
What it means
Common causes
The shared secret on the NAS (switch, wireless controller, VPN box) differs from the one in the NPS RADIUS client entry — including trailing-whitespace and copy/paste errors.
The request came from a different source IP than expected (NAT, controller failover), matching a different RADIUS client entry with a different secret.
Load balancers or proxies rewriting packets between NAS and NPS.
How to fix it
Re-enter the shared secret on both sides — NPS console > RADIUS Clients > (client) and the NAS config — rather than trying to compare them visually.
Confirm the source IP in the event matches the RADIUS client entry you think is in play; with controller HA or NAT, add entries for every source address.
Prefer long random secrets generated once and stored in your secret manager; per-NAS secrets make mismatches easy to spot.