Auth0 authentication bypass issue exposed enterprises to hack

Pierluigi Paganini April 08, 2018

Auth0, one of the biggest identity-as-a-service platform is affected by a critical authentication bypass vulnerability that exposed enterprises to hack.

Auth0, one of the biggest identity-as-a-service platform is affected by a critical authentication bypass vulnerability that could be exploited by attackers to access any portal or application which are using it for authentication.

Auth0 implements a token-based authentication model for a large number of platforms, it managed 42 million logins every day and billions of login per month for over 2000 enterprise customers.


In September 2017, researchers from security firm Cinta Infinita discovered a flaw in Auth0’s Legacy Lock API while they were pentesting an unnamed application that used service for the authentication.

The vulnerability tracked as CVE-2018-6873 it related to improper validation of the JSON Web Tokens (JWT) audience parameter.

The experts exploited this issue to bypass login authentication using a cross-site request forgery (CSRF/XSRF) attack triggering the CVE-2018-6874 flaw against applications implementing Auth0 authentication.

The experts exploited the CSRF vulnerability to reuse a valid signed JWT generated for a separate account to access the targeted victim’s account.

The unique information needed by attackers is the victim’s user ID or email address, that could be easily obtained with social engineering attacks.

“So, now we had the ability to forge a valid signed JWT with the “email” and “user_id” of the victim.” reads the analysis of the experts.

“It worked!! Why? The audience claim was not being checked and JWTs generated from our test application were accepted by the Management Console app (same signing key / private certificate).”

Below a video PoC of the attack to demonstrate how to obtain the victim’s user id and bypass password authentication when logging into Auth0’s Management Dashboard by forging an authentication token.

The researchers explained that it is possible to use this attack against many organizations.

Could we use this attack to access arbitrary applications? Yes, as long as we know the expected fields and values for the JWT. There is no need of social engineering in most of the cases we saw. Authentication for applications that use an email address or an incremental integer for user identification would be trivially bypassed.” continues the analysis.

Security firm Cinta Infinita reported the vulnerability to the company in October 2017 and Auth0 solved the issue in a few hours but it spent several months to reach each customers using the vulnerable SDK and supported libraries of Auth0.

“We waited for six months before publicly disclosing this issue so that Auth0could update all their Private SaaS Appliances (on-premise) as well.” continues the analysis.

“Auth0 published a blog post about their internal vulnerability management and remediation process where they mention our finding and the assistance we provided:

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Pierluigi Paganini

(Security Affairs – Authentication, hacking)

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