nats-io nats-server up to 2.11.11/2.12.2 WebSockets data amplification

Summaryinfo

A vulnerability marked as problematic has been reported in nats-io nats-server up to 2.11.11/2.12.2. The affected element is an unknown function of the component WebSockets Handler. This manipulation causes data amplification. This vulnerability is tracked as CVE-2026-27571. The attack is possible to be carried out remotely. No exploit exists. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic was found in nats-io nats-server up to 2.11.11/2.12.2. Affected by this vulnerability is an unknown code of the component WebSockets Handler. The manipulation with an unknown input leads to a data amplification vulnerability. The CWE definition for the vulnerability is CWE-409. The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. As an impact it is known to affect availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-27571 since 02/20/2026. The exploitation appears to be difficult. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1499 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 299911 (Linux Distros Unpatched Vulnerability : CVE-2026-27571), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 2.11.12 or 2.12.3 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (299911). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• nats-io

Name

• nats-server

Version

• 2.11.0
• 2.11.1
• 2.11.2
• 2.11.3
• 2.11.4
• 2.11.5
• 2.11.6
• 2.11.7
• 2.11.8
• 2.11.9
• 2.11.10
• 2.11.11
• 2.12.0
• 2.12.1
• 2.12.2

Website

• Product: https://github.com/nats-io/nats-server/

CPE 2.3info

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CPE 2.2info

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Entryinfo

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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