nats-server websockets are vulnerable to pre-auth memory DoS

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-27571: 0.54% estimated exploitation activity in the next 30 days; percentile 45.63 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact The WebSockets handling of NATS messages handles compressed messages via the WebSockets negotiated compression. The implementation bound the memory size of a NATS message but did not independently bound the memory consumption of the memory stream when constructing a NATS message which might then fail validation for size reasons. An attacker can use a compression bomb to cause excessive memory consumption, often resulting in the operating system terminating the server process. The use of compression is negotiated before authentication, so this does not require valid NATS credentials to exploit. The fix was to bounds the decompression to fail once the message was too large, instead of continuing on. Patches This was released in nats-server without being highlighted as a security issue. It should have been, this was an oversight. Per the NATS security policy, because this does not require a valid user, it is CVE-worthy. This was fixed in the v2.11 series with v2.11.12 and in the…

Published by the source

24 Feb 2026, 16:04 UTC

Last modified by the source

27 Feb 2026, 20:03 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qrvq-68c2-7grw

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# Patch Next advisory brief: CVE-2026-27571

Advisory: nats-server websockets are vulnerable to pre-auth memory DoS
IDs: CVE-2026-27571, GHSA-qrvq-68c2-7grw
Record: https://patchnext.com/advisories/cve-2026-27571 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: github.com/nats-io/nats-server/v2 2.12.3, 2.11.12. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/nats-io/nats-server/v2: affected < 2.11.12; >= 2.12.0-RC.1, < 2.12.3; fixed in 2.11.12, 2.12.3
- Go github.com/nats-io/nats-server: affected <= 1.4.1

## Suggested inspection (read-only)
- Go github.com/nats-io/nats-server/v2: check the installed version with your package manager.
- Go github.com/nats-io/nats-server: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/nats-io/nats-server/v2, github.com/nats-io/nats-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-qrvq-68c2-7grw (GitHub-reviewed advisory GHSA-qrvq-68c2-7grw lists affected ranges: Go github.com/nats-io/nats-server/v2 >=0 =2.12.0-RC.1 =0 <=1.4.1)

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