[onError][reply] hide sensitive data in the error response

[xr0master]

Hi guys, sorry if this is trivial, couldn't figure out how to do it.

💬 Question here

1. Errors are passed to the onError hook, where there is no way to change the body of the reply, but only the status. Am I missing something?

// call it into onError hook
reply.status(555).send('my error'); // the status is changed, but 'my error' is ignored.

Yes, I can mutate the error itself, but that will affect the logs as well.

  error.message = 'my error';

2. Following the 1st problem, I thought that sensitive information can be filtered through the JSON scheme, is there an option to specify the response JSON schema for all outgoing traffic? In other words, how to implement the code below to each response without adding this block to each router?

response: {
  '5xx': {
    type: 'object',
    properties: {
      error: {
        type: 'string',
      },
    },
  },
},

Perhaps there is an easier solution to hide the error message in the response.

Thank you.

[climba03003]

https://www.fastify.io/docs/latest/Reference/Hooks/#onerror

It is not intended for changing the error, and calling reply.send will throw an exception.

Define your own error handler to override the default behavior.
https://www.fastify.io/docs/latest/Reference/Server/#seterrorhandle

[xr0master]

@climba03003 as you can see below, the onError hook runs at the end and requires an error instance, so it cannot be changed in the custom error handler.

[xr0master]

On another thought, there is no special requirement to hold on to this hook. Just the meaning of this onError hook is questionable. If we use it, then we can not customize the answer, so the error message is publicly available, which is a security issue. And if we do customization the reply, then the hook won't call and store the error in the logs.

I think it would be more logical to call this hook before calling setErrorHandler (or in parallel), then it would really be possible to save the error in the logs and then customize the reply to the client through the error handler.

[mcollina]

This was significantly refactored in fastify/fastify#3261. Can you check out the next branch of fastify and see if it solves your problem?

Not that for redacting sensitive info in the logs you should use https://getpino.io/#/docs/redaction