Bug 2442401 (CVE-2026-27571)

Summary: CVE-2026-27571 nats-server: WebSockets pre-auth memory DoS
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: alcohan, amctagga, aoconnor, bniver, flucifre, gmeno, gparvin, groman, jbalunas, mbenjamin, mhackett, pahickey, rhaigner, sostapov, vereddy
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in nats-server. The WebSockets implementation fails to enforce a memory allocation limit during the decompression of WebSocket messages. A malicious compressed payload allows an attacker to cause an excessive memory consumption, eventually resulting in a server crash and a complete denial of service.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On: 2443319, 2443320    
Bug Blocks:    

Description OSIDB Bzimport 2026-02-24 17:04:35 UTC 
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.