Bypass of CSRF protection in the presence of predictable userInfo

Description

The CSRF protection enforced by the @fastify/csrf-protection library in combination with @fastify/cookie can be bypassed from network and same-site attackers under certain conditions.

@fastify/csrf-protection supports an optional userInfo parameter that binds the CSRF token to the user. This parameter has been introduced to prevent cookie-tossing attacks as a fix for CVE-2021-29624. Whenever userInfo parameter is missing, or its value can be predicted for the target user account, network and same-site attackers can 1. fixate a _csrf cookie in the victim's browser, and 2. forge CSRF tokens that are valid for the victim's session. This allows attackers to bypass the CSRF protection mechanism.

As a fix, @fastify/csrf-protection starting from version 6.3.0 (and v4.1.0) includes a server-defined secret hmacKey that cryptographically binds the CSRF token to the value of the _csrf cookie and the userInfo parameter, making tokens non-spoofable by attackers. This protection is effective as long as the userInfo parameter is unique for each user.

Patches

This is patched in version 6.3.0 and v4.1.0.

Workarounds

As a workaround, developers can use a random, non-predictable userInfo parameter for each user.

Credits

• Pedro Adão (@pedromigueladao), Instituto Superior Técnico, University of Lisbon
• Marco Squarcina (@lavish), Security & Privacy Research Unit, TU Wien