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Fencer Vulns
medium 5.9

CVE-2026-27571 json

nats-server websockets are vulnerable to pre-auth memory DoS

published 2026-02-24 8 mo ago modified 2026-09-10 1 mo ago also BIT-nats-2026-27571 , EUVD-2026-8538 , GHSA-qrvq-68c2-7grw , GO-2026-4533 , JLSEC-2026-1120

cvss
5.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 45%
public exploits
0
none known

Summary

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.

Patch / fix

  • go github.com/nats-io/nats-server/v2 2.11.12, 2.12.3
  • bitnami nats 2.11.12, 2.12.3
  • julia nats_server_jll 2.14.3+0
  • linuxfoundation nats-server 2.11.12, 2.12.3
  • nats nats_server 2.11.12, 2.12.3
  • nats nats_streaming_server 2.11.12, 2.12.3
  • nats-server Debian 14 2.12.4-1

debian advisories DEBIAN-CVE-2026-27571

  • govulncheck-vulndb openSUSE Leap 15.6 0.0.20260226T182644-150000.1.149.1

suse advisories SUSE-SU-2026:0757-1

  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

  • telegraf CBL-Mariner 2.0 1.29.4-21
  • telegraf Azure Linux 3.0 1.31.0-15

azure linux advisories AZL-78372AZL-78374

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-27571 updated cvss 7.5

nats-server: WebSockets pre-auth memory DoS

  • Red Hat Advanced Cluster Management for Kubernetes 2 rhacm2/observatorium-rhel9
    affected: workaround, no fix available
  • Red Hat Ceph Storage 7 rhceph/snmp-notifier-rhel9 Red Hat Ceph Storage 8 rhceph/snmp-notifier-rhel9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Multicluster Global Hub 1.6.5 multicluster-globalhub 1773650060
    fixed: vendor fix RHSA-2026:6226
  • Red Hat multicluster global hub 1.5.3 multicluster-globalhub 1773650767
    fixed: vendor fix RHSA-2026:5110
  • Red Hat Multicluster Global Hub 1.6.5 multicluster-globalhub 1773649712, multicluster-globalhub 1774245716, multicluster-globalhub 1774245790, multicluster-globalhub 1774362315, multicluster-globalhub 1774364330 Red Hat multicluster global hub 1.5.3 multicluster-globalhub 1773649705, multicluster-globalhub 1773649850, multicluster-globalhub 1773650627, multicluster-globalhub 1773650749, multicluster-globalhub 1773652587 Red Hat OpenShift Container Platform 4 openshift4/oc-mirror-plugin-rhel9
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
8 mo ago

Exploit prediction

0.54% chance of exploitation in the next 30 days, higher than 45% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
OSV 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
go github.com/nats-io/nats-server/v2
<2.11.12
>=2.12.0-RC.1, <2.12.3
2.11.12
2.12.3
go github.com/nats-io/nats-server
<=1.4.1
all versions
 no fix listed
bitnami nats
<2.11.12
>=2.12.0, <2.12.3
2.11.12
2.12.3
julia nats_server_jll
<2.14.3+0
2.14.3+0

Affected products

  • nats-io nats-server

Classification

Weakness
  • CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)
  • CWE-770 Allocation of Resources Without Limits or Throttling

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.