> ## Documentation Index
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> Describes common sign up and login errors that you might see when you authenticate users using Auth0 libraries.

# Common Auth0 Library Authentication Errors

The actions or input data of your users, during the sign up or the log in processes, might trigger errors. Here is a list of the most common errors that you might get if you use any of the Auth0 libraries for authentication.

## Sign up

In the case of a failed signup, the most common errors are:

| **Error** | **Description** |
| - | - |
| **invalid\_password** | If the password used doesn't comply with the password policy for the connection |
| **invalid\_signup** | The user you are attempting to sign up is invalid |
| **password\_dictionary\_error** | The chosen password is too common |
| **password\_no\_user\_info\_error** | The chosen password is based on user information |
| **password\_strength\_error** | The chosen [password is too weak](/docs/authenticate/database-connections/password-strength) |
| **unauthorized** | If you cannot sign up for this application. May have to do with the violation of a specific rule |
| **user\_exists** | The user you are attempting to sign up has already signed up |
| **username\_exists** | The username you are attempting to sign up with is already in use |

## Log in

In the case of a failed login, the most common errors are:

| **Error** | **Description** |
| - | - |
| **access\_denied** | When using web-based authentication, the resource server denies access per OAuth2 specifications |
| **invalid\_user\_password** | The username and/or password used for authentication are invalid |
| **mfa\_invalid\_code** | The <Tooltip tip="Multi-factor authentication (MFA): User authentication process that uses a factor in addition to username and password such as a code via SMS." cta="View Glossary" href="/docs/glossary?term=multi-factor+authentication+%28MFA%29">multi-factor authentication (MFA)</Tooltip> code provided by the user is invalid/expired |
| **mfa\_registration\_required** | The administrator has required [multi-factor authentication](/docs/secure/multi-factor-authentication), but the user has not enrolled |
| **mfa\_required** | The user must provide the [multi-factor authentication](/docs/secure/multi-factor-authentication) code to authenticate |
| **password\_leaked** | If the password has been leaked and a different one needs to be used |
| **PasswordHistoryError** | The password provided for sign up/update has already been used (reported when [password history](/docs/authenticate/database-connections/password-options#password-history) feature is enabled) |
| **PasswordStrengthError** | The password provided does not match the connection's [strength requirements](/docs/authenticate/database-connections/password-strength) |
| **too\_many\_attempts** | The account is blocked due to too many attempts to sign in |
| **unauthorized** | The user you are attempting to sign in with is blocked |

## Learn more

* [Attack Protection](/docs/secure/attack-protection)