nats-server websockets are vulnerable to pre-auth memory DoS
cve-program · CVE-2026-27571
PublishedNATS-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.
Read the publisher's advisoryeuvd · EUVD-2026-8538
PublishedNATS-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.
Read the publisher's advisorynvd · CVE-2026-27571
PublishedNATS-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.
Read the publisher's advisoryosv · GHSA-qrvq-68c2-7grw
Published### Impact The WebSockets handling of NATS messages handles compressed messages via the WebSockets negotiated compression. 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 was to bounds the decompression to fail once the message was too large, instead of continuing on. ### Patches This was released in nats-server without being highlighted as a security issue. It should have been, this was an oversight. Per the NATS security policy, because this does not require a valid user, it is CVE-worthy. This was fixed in the v2.11 series with v2.11.12 and in the v2.12 series with v2.12.3. ### Workarounds This only affects deployments which use WebSockets and which expose the network port to untrusted end-points. ### References This was reported to the NATS maintainers by Pavel Kohout of Aisle Research (www.aisle.com).
Read the publisher's advisoryosv · GO-2026-4533
Publishednats-server websockets are vulnerable to pre-auth memory DoS in github.com/nats-io/nats-server
Read the publisher's advisoryAffected versions, risk and source history
Exploitation and risk
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cve-program · CVE-2026-27571
Publishednats-server websockets are vulnerable to pre-auth memory DoS
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 published
- 24 Feb 2026 15:59:17 UTC
- Source modified
- 26 Feb 2026 21:33:40 UTC
- Saved here
- 23 Oct 2026 15:48:27 UTC
- Source terms
- CVE Program Terms of Use
Original source record · Cite this exact observation · Complete original publisher JSON
Source: cve-program · CVE Program Terms of Use. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.
CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
euvd · EUVD-2026-8538
PublishedNATS-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 2.1…
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.
Additional publisher context
EUVD source date text has no stated timezone and is retained without inventing a UTC instant. Related bulletin membership is not an alias, common cause or applicability claim. A missing vector with a zero score is treated as unrated. Complete vendor/product and underlying source evidence remains in the original JSON.
Original publisher date fields
datePublished: Feb 24, 2026, 3:59:17 PM
dateUpdated: Feb 26, 2026, 9:33:40 PM
4 related source bulletins · separate from aliases
msrc_CVE-2026-27571: nats-server websockets are vulnerable to pre-auth memory DoS
CVE-2026-27571
RHSA-2026:5110: Red Hat Security Advisory: Multicluster Global Hub 1.5.4 security update
CVE-2025-61726 · CVE-2025-61728 · CVE-2025-61729 · CVE-2025-68121 · CVE-2026-25679 · CVE-2026-27137 · CVE-2026-27571
RHSA-2026:6226: Red Hat Security Advisory: Multicluster Global Hub 1.6.2 security update
CVE-2025-47907 · CVE-2025-57810 · CVE-2025-58183 · CVE-2025-58754 · CVE-2025-59343 · CVE-2025-61726 · CVE-2025-61728 · CVE-2025-61729 · CVE-2025-68121 · CVE-2026-27571
WID-SEC-W-2026-0487: NATS Server: Schwachstelle ermöglicht Denial of Service
CVE-2026-27571
- Source published
- Feb 24, 2026, 3:59:17 PM (timezone not stated)
- Source modified
- Feb 26, 2026, 9:33:40 PM (timezone not stated)
- Saved here
- 15 Nov 2026 09:26:39 UTC
- Source terms
- ENISA EUVD · source terms and attribution apply
Original source record · Cite this exact observation · Complete original publisher JSON
Source: euvd · ENISA EUVD · source terms and attribution apply. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.
EUVD CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
nvd · CVE-2026-27571
PublishedNATS-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 2.1…
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.
Additional publisher context
NVD analysis and CPE criteria are source context, not proof that a package or your environment is affected. Complete affected data, logical configurations and SSVC are retained in the original JSON.
Source analysis status: Analyzed
Weakness classifications: CWE-409 · CWE-770
Original publisher date fields
published: 2026-02-24T17:29:03.393
lastModified: 2026-06-17T10:27:19.650
1 source CPE criteria · context only
- cpe:2.3:a:linuxfoundation:nats-server:*:*:*:*:*:*:*:*
The original JSON retains the logical configuration operators, exclusions and version bounds.
- Source published
- 24 Feb 2026 17:29:03 UTC
- Source modified
- 17 Jun 2026 10:27:19 UTC
- Saved here
- 25 Oct 2026 06:17:28 UTC
- Source terms
- NIST NVD usage terms; underlying source attribution retained
Original source record · Cite this exact observation · Complete original publisher JSON
Source: nvd · NIST NVD usage terms; underlying source attribution retained. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.
CVSS 3.1 · [email protected] · Secondary · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 3.1 · [email protected] · Primary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
osv · GHSA-qrvq-68c2-7grw
Publishednats-server websockets are vulnerable to pre-auth memory DoS
### Impact The WebSockets handling of NATS messages handles compressed messages via the WebSockets negotiated compression. 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 was to bounds the decompression to fail once the message was too large, instead of continuing on. ### Patches This was released in nats-server without being highlighted as a security issue. It should have been, this was an oversight. Per the NATS security policy, because this does not require a valid user, it is CVE-worthy. This was fixed in the v2.11 series with v2.11.12 and in the v2.12 series with v2.12.3. ### Workarounds This only affects deployments which use WebSockets and which expose the network port to untrusted end-points. ### References This was reported to the NATS maintainers by Pavel Kohout of Aisle Research (www.aisle.com).
- Source published
- 24 Feb 2026 16:04:53 UTC
- Source modified
- 10 Sep 2026 03:50:59 UTC
- Saved here
- 24 Oct 2026 13:19:41 UTC
- Source terms
- GitHub Advisory Database · CC-BY-4.0
Original source record · Cite this exact observation · Complete original publisher JSON
Source: osv · GitHub Advisory Database · CC-BY-4.0. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.
CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Publisher rating · MODERATE
osv · GO-2026-4533
Publishednats-server websockets are vulnerable to pre-auth memory DoS in github.com/nats-io/nats-server
nats-server websockets are vulnerable to pre-auth memory DoS in github.com/nats-io/nats-server
- Source published
- 25 Feb 2026 23:07:04 UTC
- Source modified
- 2 Mar 2026 12:17:25 UTC
- Saved here
- 24 Oct 2026 13:19:41 UTC
- Source terms
- OSV · upstream source license applies
Original source record · Cite this exact observation · Complete original publisher JSON
Source: osv · OSV · upstream source license applies. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.