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Nats ServerApplication · Linuxfoundation

CVE-2026-33219

MEDIUM · 5.3 CVSS v3.1 Published 2026-03-25
Fix available
A fix is available. Upgrade to 2.11.15 / 2.12.6 or later.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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, a malicious client which can connect to the WebSockets port can cause unbounded memory use in the nats-server before authentication; this requires sending a corresponding amount of data. This is a milder variant of CVE-2026-27571. That earlier issue was a compression bomb, this vulnerability is not. Attacks against this new issue thus require significant client bandwidth. Versions 2.11.15 and 2.12.6 contain a fix. As a workaround, disable websockets if not required for project deployment.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

NATS-Server versions prior to 2.11.15 and 2.12.6 contain an unauthenticated denial-of-service vulnerability where a malicious client connected to the WebSocket port can cause unbounded memory consumption by sending large amounts of data before authentication completes. Unlike the related CVE-2026-27571 (compression bomb), this attack requires significant client bandwidth rather than compression.

MitigationUpgrade to NATS-Server version 2.11.15 or 2.12.6, or disable WebSocket functionality if not required for deployment.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Nats ServerApplication
Affected:< 2.11.15>= 2.12.0, < 2.12.6

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify NATS-Server installation
    Run 'nats-server --version' or check the binary version via 'nats-server -v' to obtain the installed version string
    Affected if Version is below 2.11.15, or version is 2.12.0 through 2.12.5 inclusive
  2. Locate NATS-Server configuration file
    Common paths include /etc/nats-server.conf, /etc/nats/nats-server.conf, or check for -c or --config flag when starting the server process
    Affected if A configuration file exists and the server is running with it
  3. Verify WebSocket port is enabled
    Search the configuration file for 'websocket' block or 'ws_port' directive. Also run 'nats-server --help' to check if WebSocket support is compiled in, or inspect running process with 'ps aux | grep nats-server' and check for websocket-related flags
    Affected if WebSocket is enabled (ws_port or websocket block is present in config, or server was started with WebSocket flags)
  4. Confirm server is listening on WebSocket port
    Run 'netstat -tlnp | grep nats' or 'ss -tlnp | grep nats' to list listening ports, then cross-reference with the WebSocket port configured (default 6222 or 8080)
    Affected if Server has a WebSocket port in LISTEN state

A user is affected if their NATS-Server version falls below 2.11.15 or between 2.12.0-2.12.5 AND the WebSocket port is enabled and exposed, regardless of whether authentication is required.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 2.11.15 / 2.12.6 or later
Fixed in 2.11.152.12.6
Interim mitigation

Upgrade to NATS-Server version 2.11.15 or 2.12.6, or disable WebSocket functionality if not required for deployment.

Recommended fix High confidence

2.11.15 or 2.12.6 (depending on current version branch)

  1. Identify the current Nats Server version using `nats-server --version` or checking the service configuration
  2. Determine which version branch is in use (2.11.x or 2.12.x)
  3. Download the appropriate fixed release: version 2.11.15 for 2.11.x branch, or version 2.12.6 for 2.12.x branch from the NATS GitHub releases page
  4. Stop the Nats Server service safely
  5. Replace the existing nats-server binary with the new version
  6. Restart the Nats Server service
  7. Verify the new version is running using `nats-server --version`
  8. If WebSockets is not required, consider disabling it in the configuration as an additional hardening measure

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Nats Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-33219 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data