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

Quick assessment

Affected
nats-io nats-server
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Nats-Server是Nats开源的一个用于 Nats.io、云和边缘本机消息传递系统的高性能服务器。 NATS-Server 2.11.2之前版本和2.12.3之前版本存在安全漏洞，该漏洞源于WebSockets处理NATS消息时未限制解压缩内存消耗，可能导致拒绝服务攻击。

CVSS 5.9 · Medium EPSS 0.75% · P54

Possible ATT&CK Techniques 1 AI

T1496 · Resource Hijacking
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I. Basic Information for CVE-2026-27571

Vulnerability Information
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Vulnerability Title
nats-server websockets are vulnerable to pre-auth memory DoS
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
对高度压缩数据的处理不恰当(数据放大攻击)
Source: CVE Program / CVE List V5
Vulnerability Title
Nats-Server 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Nats-Server是Nats开源的一个用于 Nats.io、云和边缘本机消息传递系统的高性能服务器。 NATS-Server 2.11.2之前版本和2.12.3之前版本存在安全漏洞，该漏洞源于WebSockets处理NATS消息时未限制解压缩内存消耗，可能导致拒绝服务攻击。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
nats-io nats-server < 2.11.12 -

II. Public POCs for CVE-2026-27571

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III. Intelligence Information for CVE-2026-27571

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Other References for CVE-2026-27571 (1)

IV. Related Vulnerabilities

Same product: nats-server
Same vendor: nats-io
Same weakness: CWE-409

V. Comments for CVE-2026-27571

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