# CVE-2026-27571: nats-server websockets are vulnerable to pre-auth memory DoS

CVE-2026-27571 is fixed in 2 products on 1 platform, first by DEBIAN-CVE-2026-27571, released February 24, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-27571
- Priority: High
- CVSS: 7.5 (Moderate)
- EPSS: 0.0054
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Feb 24, 2026
- Weakness: Improper Handling of Highly Compressed Data (Data Amplification)
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-27571

## Description

NATS-Server is a High-Performance server for NATS.io, a cloud and edge native messaging system. The WebSockets handling of NATS messages handles compressed messages via the WebSockets negotiated compression. Prior to versions 2.11.2 and 2.12.3, 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, present in versions 2.11.2 and 2.12.3, was to bounds the decompression to fail once the message was too large, instead of continuing on. The vulnerability only affects deployments which use WebSockets and which expose the network port to untrusted end-points.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | govulncheck-vulndb opensuse-leap 15.6 | 0.0.20260226T182644-150000.1.149.1 | [SUSE-SU-2026:0757-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-0757-1.md) | Mar 3, 2026 |
| Linux | govulncheck-vulndb opensuse-leap 16.0 | 0.0.20260723T184607-160000.1.1 | [OPENSUSE-SU-2026:21483-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21483-1.md) | Jul 30, 2026 |
| Linux | nats-server Debian 14 | 2.12.4-1 | [DEBIAN-CVE-2026-27571](https://patchblox.com/security/data/fix/linux/debian-cve-2026-27571.md) | Feb 24, 2026 |

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Data as of 2026-11-09T07:25:28Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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