NATS nats-server before 2.9.23 and 2.10.x before 2.10.2 has an authentication bypass. An implicit $G user in an authorization block can sometimes be used for unauthenticated access, even when the intention of the configuration was for each user to have an account. The earliest affected version is 2.2.0.

Affected: Go: github.com/nats-io/nats-server/v2

## Background

NATS.io is a high performance open source pub-sub distributed communication technology, built for the cloud, on-premise, IoT, and edge computing.

NATS users exist within accounts, and once using accounts, the old authorization block is not applicable.

## Problem Description

Without any authorization rules in the nats-server, users can connect without authentication.

Before nats-server 2.2.0, all authentication and authorization rules for a nats-server lived in an "authorization" block, defining users. With nats-server 2.2.0 all users live inside accounts. When using the authorization block, whose syntax predates this, those users will be placed into the implicit global account, "$G". Users inside accounts go into the newer "accounts" block.

If an "accounts" block is defined, in simple deployment scenarios this is often used only to enable client access to the system account. When the only account added is the system account "$SYS", the nats-server would create an implicit user in "$G" and set it as the `no_auth_user` account, enabling the same "without authentication" logic as without any rules.

This preserved the ability to connect simply, and then add one authenticated login for system access.

But with an "authorization" block, this is wrong. Users exist in the global account, with login rules. And in simple testing, they might still connect fine without administrators seeing that authentication has been disabled.

The blind-spot on our part ca…

Withdrawn by the publisher.

Affected: Ubuntu:24.04:LTS: nats-server

NATS nats-server before 2.9.23 and 2.10.x before 2.10.2 has an authentication bypass. An implicit $G user in an authorization block can sometimes be used for unauthenticated access, even when the intention of the configuration was for each user to have an account. The earliest affected version is 2.2.0.