CVE-2026-27571

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

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).

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.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-27571 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.11.12, 2.12.3); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-27571? CVE-2026-27571 is a medium-severity allocation of resources without limits or throttling vulnerability in github.com/nats-io/nats-server/v2 (go), affecting versions < 2.11.12. It is fixed in 2.11.12, 2.12.3. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-27571? CVE-2026-27571 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-27571?
   • github.com/nats-io/nats-server/v2 (go) (versions < 2.11.12)
   • github.com/nats-io/nats-server (go) (versions <= 1.4.1)
4. Is there a fix for CVE-2026-27571? Yes. CVE-2026-27571 is fixed in 2.11.12, 2.12.3. Upgrade to this version or later.
5. Is CVE-2026-27571 exploitable, and should I be worried? Whether CVE-2026-27571 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27571 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27571?
   • Upgrade github.com/nats-io/nats-server/v2 to 2.11.12 or later
   • Upgrade github.com/nats-io/nats-server/v2 to 2.12.3 or later

Other vulnerabilities in github.com/nats-io/nats-server/v2