How severe is CVE-2026-27571?

CVE-2026-27571 has a CVSS score of 5.9/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-27571?

CVE-2026-27571 affects the following packages: github.com/nats-io/nats-server/v2 (Go), github.com/nats-io/nats-server/v2 (Go), github.com/nats-io/nats-server (Go). Ecosystems affected: Go.

How do I fix CVE-2026-27571?

Update github.com/nats-io/nats-server/v2 to 2.11.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27571 is resolved across your whole dependency graph.

How do I detect CVE-2026-27571 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nats-io/nats-server/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-27571 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-27571 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-27571 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-27571?

CVE-2026-27571 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 46th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-27571?

CVE-2026-27571 is classified as CWE-409 (CWE-409), CWE-770 (CWE-770). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-27571 published, and has it been updated?

CVE-2026-27571 was published on February 24, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2026-27571 — nats-server

MEDIUMFix: nats-io/nats-server@f77fb7c

CVE-2026-27571 is a medium-severity (CVSS 5.9) CWE-409 vulnerability in github.com/nats-io/nats-server/v2. A fix is available for github.com/nats-io/nats-server/v2 — see the affected versions and patch details below.

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

Also known asBIT-nats-2026-27571GHSA-qrvq-68c2-7grwGO-2026-4533

Published

Feb 24, 2026

Updated

Oct 28, 2026

Affected

3 pkgs

Patched

2 / 3

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-27571.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk+0.06%

Lower risk than most CVEs46th percentile — riskier than 46% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected

🐹github.com/nats-io/nats-server/v2🐹github.com/nats-io/nats-server/v2🐹github.com/nats-io/nats-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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).

Affected Packages

3 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/nats-io/nats-server/v2`	all versions	2.11.12`go get github.com/nats-io/nats-server/v2@v2.11.12`
🐹Go	`github.com/nats-io/nats-server/v2`	≥ 2.12.0-RC.1&&< 2.12.3	2.12.3`go get github.com/nats-io/nats-server/v2@v2.12.3`
🐹Go	`github.com/nats-io/nats-server`	all versions	No fix

Affected Products

1 product · 2 configurations

Application

nats-serverlinuxfoundation

≥ 2.12.0 && < 2.12.3

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nats-io/nats-server/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/nats-io/nats-server/v2 to 2.11.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27571 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This flaw does not require valid NATS credentials to be exploited as the use of compression is negotiated before the authentication process. However, only deployments using WebSockets and that are exposed to untrusted network endpoints are vulnerable to this issue, limiting its exposure. Due to these reasons, this…

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Source: Red Hat security advisory for CVE-2026-27571 (CC BY 4.0)

Product	Fixed in	Advisory
Multicluster Global Hub 1.6.5	`multicluster-globalhub/multicluster-globalhub-grafana-rhel9:1773650060`	RHSA-2026:6226
Red Hat multicluster global hub 1.5.3	`multicluster-globalhub/multicluster-globalhub-grafana-rhel9:1773650767`	RHSA-2026:5110

Frequently Asked Questions

### 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 credenti