Extensibility Implementation Error

[fabltd]

Hi All

Sign ups have started to fail on an applicaton.

I have no idea where to start to look.

Thanks

The error is:

{
“date”: “2025-11-20T20:17:52.356Z”,
“type”: “fs”,
“description”: “ExtensibilityImplementationError on pre-user-registration: Unexpected undefined error response payload from extension URL https://webtask-router.space.local/api/run/redacted/check-user, missing / invalid .message or .statusCode”,
“connection”: “Username-Password-Authentication”,
“connection_id”: “con_VH4k6nPcebIxZnoM”,
“client_id”: “tvEFZqJfrtitgDdM976fnmBPkFZvTfmk”,
“client_name”: “Alexa”,
“ip”: “2603:9000:9a02:1aea:f688:334d:9bdd:6d3d”,
“user_agent”: “Chrome Mobile 142.0.0 / Android 10.0.0”,
“details”: {
“body”: {
“connection”: “Username-Password-Authentication”,
“client_id”: “redacted”,
“email”: “redacted”,
“password”: “*****”,
“is_signup”: true,
“tenant”: “redacted”,
“transaction”: {
“id”: “redacted”,
“locale”: “en”,
“protocol”: “oidc-basic-profile”,
“requested_scopes”: [
“openid”,
“email”,
“offline_access”,
“profile”
],
“acr_values”: ,
“ui_locales”: ,
“redirect_uri”: “ Account Linking ,
“prompt”: ,
“state”:
“response_type”: [
“code”
]
},
“form”: {},
“request_language”: “en-US,en;q=0.9”
},
“authentication_methods”: [
“pwd”
]
},
“user_id”: “”,
“user_name”: “redacted”,
“strategy”: “auth0”,
“strategy_type”: “database”,
“$event_schema”: {
“version”: “1.0.0”
},
“environment_name”: “prod-eu-1”,
“log_id”: “redacted”,
“tenant_name”: “redacted”,
“_id”: “90020251120201752545211000000000000001223372142986860210”,
“isMobile”: true,
“location_info”: {},
“id”: “redacted”
}

[remus.ivan]

Hello @fabltd,

Welcome back to the Auth0 Community!

The error message received in the above mentioned log is most certainly related to a Pre-User-Registration action. In order to see exactly is the issue is related to a missconfiguration in your action code, you can try disabling the action to see if users are able to sign up again.

If that is the case, even though the description of this error is often not clear enough as of what the exact reason is, this points clearly to an incorrect method of stopping the registration process in the Auth0 Pre-User-Registration Action.

What I would recommend is to make sure that you are calling the correct methods from within the Pre-user Registration Trigger, especially methods such as the api.access.deny(reason, userMessage). Additionally, if you have an older configuration you can also check the Migrating from Rules to Actions article since it might help.

Hope this helps, and if you have further questions please let me know.
Thank you,
Remus