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

## Advisory

The management of JetStream assets happens with messages in the `$JS.` subject namespace in the system account; this is partially exposed into regular accounts to allow account holders to manage their assets.

Some of the JS API requests were missing access controls, allowing any user with JS management permissions in any account to perform certain administrative actions on any JS asset in any other account. At least one of the unprotected APIs allows for data destruction. None of the affected APIs allow disclosing stream contents.

### Affected versions

NATS Server: * Version 2 from v2.2.0 onwards, prior to v2.11.1 or v2.10.27

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## Original Report

(Lightly edited to confirm some supposition and in the summary to use past tense)

### Summary

nats-server did not include authorization checks on 4 separate admin-level JetStream APIs: account purge, server remove, account stream move, and account stream cancel-move.

In all cases, APIs are not properly restricted to system-account users. Instead, _any_ authorized user can execute the APIs, including across account boundaries, as long as the current user merely has permission to publish on `$JS.>`.

Only the first seems to be of highest severity. All are included in this single report as they seem likely to have the same underlying root cause.

Reproduction of the `ACCOUNT.PURGE` case is below. The others are like it.

### Details & Impact

#### Issue 1: `$JS.API.ACCOUNT.PURGE.*`

Any user may perform an…