GHSA-J756-F273-XHP4

(This advisory is canonically https://advisories.nats.io/CVE/CVE-2021-3127.txt)

Problem Description

The NATS server provides for Subjects which are namespaced by Account; all Subjects are supposed to be private to an account, with an Export/Import system used to grant cross-account access to some Subjects. Some Exports are public, such that anyone can import the relevant subjects, and some Exports are private, such that the Import requires a token JWT to prove permission.

The JWT library's validation of the bindings in the Import Token incorrectly warned on mismatches, instead of outright rejecting the token.

As a result, any account can take an Import token used by any other account and re-use it for themselves because the binding to the importing account is not rejected, and use it to import any Subject from the Exporting account, not just the Subject referenced in the Import Token.

The NATS account-server system treats account JWTs as semi-public information, such that an attacker can easily enumerate all account JWTs and retrieve all Import Tokens from those account JWTs.

The CVE identifier should cover the JWT library repair and the nats-server containing the fixed JWT library, and any other application depending upon the fixed JWT library.

Affected versions

JWT library

• all versions prior to 2.0.1
• fixed after nats-io/jwt#149 landed (2021-03-14)

NATS Server

• Version 2 prior to 2.2.0
  • 2.0.0 through and including 2.1.9 are vulnerable
• fixed with nats-io/nats-server@423b79440c (2021-03-14)

Workaround

Deny access to clients to update their account JWT in the account server.

In deployments with untrusted accounts able to update the Account Server with imports, a malicious account can access any Subject from an account which provides Exported Subjects.

Abuse of this facility requires the malicious actor to upload their tampered Account JWT to the Account Server, providing the service operator with a data-store which can be scanned for signs of abuse.

The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.

GHSA-J756-F273-XHP4 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

Upgrade the JWT dependency in any application using it.

Upgrade the NATS server if using NATS Accounts (with private Exports; Account owners can create those at any time though).

Audit all accounts JWTs to scan for exploit attempts; a Python script to audit the accounts can be found at https://gist.github.com/philpennock/09d49524ad98043ff11d8a40c2bb0d5a.

1. What is GHSA-J756-F273-XHP4? GHSA-J756-F273-XHP4 is a high-severity incorrect authorization vulnerability in github.com/nats-io/nats-server/v2 (go), affecting versions < 2.2.0. It is fixed in 2.2.0. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
2. How severe is GHSA-J756-F273-XHP4? GHSA-J756-F273-XHP4 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/nats-io/nats-server/v2 are affected by GHSA-J756-F273-XHP4? github.com/nats-io/nats-server/v2 (go) versions < 2.2.0 is affected.
4. Is there a fix for GHSA-J756-F273-XHP4? Yes. GHSA-J756-F273-XHP4 is fixed in 2.2.0. Upgrade to this version or later.
5. Is GHSA-J756-F273-XHP4 exploitable, and should I be worried? Whether GHSA-J756-F273-XHP4 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-J756-F273-XHP4 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-J756-F273-XHP4? Upgrade github.com/nats-io/nats-server/v2 to 2.2.0 or later.