CVE-2018-6873: Security Vulnerability in the Auth0 Authentication Service

Published: April 4, 2018

CVE number: CVE-2018-6873

Credit: Cinta Infinita


A vulnerability was identified in the Auth0 authentication service. It affected all Auth0 tenants, and was patched on October 15th, 2017 within four hours of report for all public cloud customers. This notice is informational and is intended to explain the vulnerability and the mitigation taken.

As part of an user authentication flow employed by the Auth0 service, a JSON Web Token (JWT) is passed to the /login/callback endpoint identifying the user. This token contains a reference to which audience - an Auth0 tenant - it is intended for. A flaw in the Auth0 service did not properly validate this audience, and therefore allowed tokens intended for one tenant to be used at another. Further, the custom database functionality available to all Auth0 tenants allows for authenticated tokens to be generated with any desired identifier. Therefore, could an attacker learn the user identifier of an intended victim at a target tenant - which is generally considered public information - they could construct a token with that identifier. Due to the improper audience checking the target tenant would accept it, and establish a login session recognizing the attacker as the victim. This allowed for privilege escalation, among other possible attack vectors.

A specific concern was the potential use of this attack on the Auth0 management service. Auth0 tenants are managed by tenant administrators, who have accounts on an "authority tenant" with relevant permissions. If an attacker could learn the user identifier of a tenant administrator on the tenant authority (such as by social engineering), this allowed the attacker to login as the administrator by the described attack method. The attacker could then undertake administration actions and view all information at the tenant.

The attack was never effective if the user had multifactor authentication enabled, which is recommended.

Am I affected?

All Auth0 tenants were affected, but have been patched. Public cloud tenants were patched within four hours of the vulnerability report.

The attack was never effective if the user had multifactor authentication enabled.

How to fix that?

The vulnerability was fixed by Auth0, accomplished by adding proper validation of the audience parameter. No additional actions are required of our customers.

Will this update impact my users?

The fix was invisible to all users.