CVE-2026-27571
nats-server websockets are vulnerable to pre-auth memory DoS
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.
INFO
Published Date :
Feb. 24, 2026, 5:29 p.m.
Last Modified :
Feb. 26, 2026, 7:06 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 3.1 | HIGH | [email protected] |
Solution
- Update NATS-Server to version 2.11.2 or later.
- Update NATS-Server to version 2.12.3 or later.
- Restrict network access to NATS-Server WebSocket port.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-27571.
| URL | Resource |
|---|---|
| https://github.com/nats-io/nats-server/commit/f77fb7c4535e6727cc1a2899cd8e6bbdd8ba2017 | Patch |
| https://github.com/nats-io/nats-server/releases/tag/v2.11.12 | ProductRelease Notes |
| https://github.com/nats-io/nats-server/releases/tag/v2.12.3 | ProductRelease Notes |
| https://github.com/nats-io/nats-server/security/advisories/GHSA-qrvq-68c2-7grw | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-27571 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-27571
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-27571 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-27571 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Feb. 26, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Added CPE Configuration OR *cpe:2.3:a:linuxfoundation:nats-server:*:*:*:*:*:*:*:* versions up to (excluding) 2.11.12 *cpe:2.3:a:linuxfoundation:nats-server:*:*:*:*:*:*:*:* versions from (including) 2.12.0 up to (excluding) 2.12.3 Added Reference Type GitHub, Inc.: https://github.com/nats-io/nats-server/commit/f77fb7c4535e6727cc1a2899cd8e6bbdd8ba2017 Types: Patch Added Reference Type GitHub, Inc.: https://github.com/nats-io/nats-server/releases/tag/v2.11.12 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/nats-io/nats-server/releases/tag/v2.12.3 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/nats-io/nats-server/security/advisories/GHSA-qrvq-68c2-7grw Types: Vendor Advisory -
New CVE Received by [email protected]
Feb. 24, 2026
Action Type Old Value New Value Added 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H Added CWE CWE-409 Added CWE CWE-770 Added Reference https://github.com/nats-io/nats-server/commit/f77fb7c4535e6727cc1a2899cd8e6bbdd8ba2017 Added Reference https://github.com/nats-io/nats-server/releases/tag/v2.11.12 Added Reference https://github.com/nats-io/nats-server/releases/tag/v2.12.3 Added Reference https://github.com/nats-io/nats-server/security/advisories/GHSA-qrvq-68c2-7grw