CVE-2023-47090: Adding accounts for just the system account adds auth bypass

(This advisory is canonically https://advisories.nats.io/CVE/secnote-2023-01.txt)

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 came from encouraging and documenting a switch to using only "accounts", instead of "authorization".

In the fixed versions, using an “authorization” block will inhibit the implicit creation of a “$G” user and setting it as the no_auth_user target. In unfixed versions, just creating a second account, with no users, will also inhibit this behavior.

Affected versions

NATS Server:

• 2.2.0 up to and including 2.9.22 and 2.10.1
• Fixed with nats-io/nats-server: 2.10.2 and backported to 2.9.23

Workarounds

In the “accounts” block, define a second non-system account, leave it empty.

accounts {
    SYS: {
        users: [
            { user: sysuser, password: makemeasandwich }
        ]
    }
    DUMMY: {}  # for security, before 2.10.2
}
system_account: SYS

Solution

Any one of these:

1. Upgrade the NATS server to at least 2.10.2 (or 2.9.23)
2. Or define a dummy account
3. Or complete the migration of authorization entries to be inside a named account in the “accounts” block

Credits

Problem reported by Alex Herrington.
Addressed publicly in a GitHub Discussion prior to this advisory.