Vulnerability · Published Feb 24, 2026 · Assigned by GitHub_M
CVE-2026-27571nats-server websockets are vulnerable to pre-auth memory DoS
Medium 5.9
In plain English
A medium-severity resource allocation without limits in nats-server from nats-io. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 46% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | nats-server | no fix listed | DEBIAN-CVE-2026-27571 |
| Debian:13 | nats-server | no fix listed | DEBIAN-CVE-2026-27571 |
| Debian:14 | nats-server | 2.12.4-1 | DEBIAN-CVE-2026-27571 |
| Go | github.com/nats-io/nats-server | no fix listed | GO-2026-4533 |
| Go | github.com/nats-io/nats-server | no fix listed | GHSA-qrvq-68c2-7grw |
| Go | github.com/nats-io/nats-server/v2 | 2.11.12 | GHSA-qrvq-68c2-7grw |
| Go | github.com/nats-io/nats-server/v2 | 2.12.3 | GHSA-qrvq-68c2-7grw |
| Go | github.com/nats-io/nats-server/v2 | 2.11.12, 2.12.3 | GO-2026-4533 |
| Ubuntu:24.04:LTS | nats-server | no fix listed | UBUNTU-CVE-2026-27571 |
| Ubuntu:25.10 | nats-server | no fix listed | UBUNTU-CVE-2026-27571 |
| Ubuntu:26.10 | nats-server | no fix listed | UBUNTU-CVE-2026-27571 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| nats-io | nats-server | < 2.11.12>= 2.12.0-RC.1, < 2.12.3 |
Weakness: CWE-409, CWE-770 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- https://github.com/nats-io/nats-server/security/advisories/GHSA-qrvq-68c2-7grw x_refsource_CONFIRM
- https://github.com/nats-io/nats-server/commit/f77fb7c4535e6727cc1a2899cd8e6bbdd8ba2017 x_refsource_MISC
- https://github.com/nats-io/nats-server/releases/tag/v2.11.12 x_refsource_MISC
- https://github.com/nats-io/nats-server/releases/tag/v2.12.3 x_refsource_MISC