nats-server websockets are vulnerable to pre-auth memory DoS
Medium5.9CVE-2026-27571 · Published Feb 24, 2026 · updated Sep 10, 2026
### 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 rep...
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| github.com/nats-io/nats-server Go | <= 1.4.1 | No fix yet |
Details and references
### 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).
- CVSS 3.1
- CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-409, CWE-770
- Also known as
- BIT-nats-2026-27571, CVE-2026-27571, GO-2026-4533
- github.com/nats-io/nats-server/security/advisories/GHSA-qrvq-68c2-7grw
- nvd.nist.gov/vuln/detail/CVE-2026-27571
- github.com/nats-io/nats-server/commit/f77fb7c4535e6727cc1a2899cd8e6bbdd8ba2017
- github.com/nats-io/nats-server
- github.com/nats-io/nats-server/releases/tag/v2.11.12
- github.com/nats-io/nats-server/releases/tag/v2.12.3
- pkg.go.dev/vuln/GO-2026-4533
More nats-server advisories
All nats-server| Date | Advisory | Severity | Fixed in |
|---|---|---|---|
| Mar 24 | NATS JetStream has an authorization bypass through its Management API Medium4.9Mar 24 | Medium4.9 | No fix yet |
| Mar 24 | NATS is vulnerable to pre-auth DoS through WebSockets client serviceMedium5.3Mar 24 | Medium5.3 | No fix yet |
| Mar 24 | NATS has pre-auth server panic via leafnode handlingHigh7.5Mar 24 | High7.5 | No fix yet |
| Mar 24 | NATS allows MQTT clients to bypass ACL checksHigh7.1Mar 24 | High7.1 | No fix yet |
| Mar 24 | NATS has MQTT plaintext password disclosureHigh8.6Mar 24 | High8.6 | No fix yet |
| Mar 24 | NATS Server panic via malicious compression on leafnode portHigh7.5Mar 24 | High7.5 | No fix yet |