Raw vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:HSummary
CVE-2026-27571 is a medium-severity Data Amplification (CWE-409) vulnerability in Linuxfoundation Nats-Server. Its CVSS base score is 5.9 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Network Denial of Service (T1498); ranked at the 39th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to AC-10 (Concurrent Session Control) and SC-6 (Resource Availability) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-27571 is a denial-of-service vulnerability in NATS-Server, a high-performance server for the NATS.io cloud and edge native messaging system. The issue affects the WebSockets handling of compressed NATS messages, where the implementation bounds the memory size of a NATS message but fails to independently bound the memory consumption of the decompression stream during message construction. This allows a compression bomb attack that triggers excessive memory usage, often resulting in the operating system terminating the server process. The vulnerability impacts versions prior to 2.11.2 and 2.12.3 and only affects deployments using WebSockets with the network port exposed to untrusted endpoints.
Any remote attacker can exploit this vulnerability over the network without authentication, as WebSockets compression negotiation occurs before credentials are required. By sending a specially crafted compressed message, the attacker causes the server to allocate unbounded memory during decompression, even if the final message would fail size validation. This leads to high availability impact with a CVSS v3.1 base score of 5.9 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H), mapped to CWEs 409 (Improper Handling of Highly Compressed Data) and 770 (Allocation of Resources Without Limits or Throttling).
The NATS-Server security advisory (GHSA-qrvq-68c2-7grw) and related GitHub references detail the fix implemented in versions 2.11.2 and 2.12.3, which bounds decompression to fail immediately if the message exceeds size limits rather than continuing. Security practitioners should upgrade to these patched releases and review WebSockets configurations to ensure ports are not exposed to untrusted networks. The commit f77fb7c4535e6727cc1a2899cd8e6bbdd8ba2017 provides the specific changes.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8538
Vulnerability Data
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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- 5 hardening rules · 3 OS baselines
V15.4.4
Mitigating Controls (NIST 800-53 r5) AI
Directly enforces a hard limit on concurrent sessions, structurally preventing unbounded resource allocation.
Requires explicit allocation of resources by priority or quota, directly stopping unlimited allocation.
Input validation can reject or limit decompression of data whose expansion ratio exceeds safe thresholds.
Imposes a limit on consecutive invalid attempts, preventing one specific class of unbounded resource consumption.
DoS protection limits the resource-exhaustion impact when a decompression bomb is processed.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Monitoring capacity and taking action to maintain availability directly reduces unchecked resource allocation.
Secure-development practices include input-validation and resource-limit checks that prevent improper handling of compressed data.
Runtime monitoring of compute resources can detect exhaustion caused by decompression bombs.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Baseline comparison of CPU, memory and bandwidth usage helps surface uncontrolled resource allocations before they cause service degradation.
Security testing can uncover decompression-bomb vulnerabilities before release.
Capacity projections and elasticity measures ensure that allocation requests are bounded and can be throttled, reducing the window in which an attacker can force unbounded resource reservations.
Defining retention periods and deletion schedules for backup copies prevents indefinite accumulation of data on storage media without corresponding resource-management controls.
Redundancy helps availability but does not address the root cause of the weakness.
Secure development lifecycle includes input validation and resource-limit checks that mitigate data-amplification attacks.
Hardening callouts derived
Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).
Oracle Linux 8 (2 rules)
- V-248552 OL 8 must be configured so that all network connections associated with SSH traffic terminate after becoming unresponsive. prevents CWE-770
- V-248553 OL 8 must be configured so that all network connections associated with SSH traffic are terminated after 10 minutes of becoming unresponsive. prevents CWE-770
Oracle Linux 9 (2 rules)
- V-271710 OL 9 must be configured so that all network connections associated with SSH traffic are terminated after 10 minutes of becoming unresponsive. prevents CWE-770
- V-271709 OL 9 must be configured so that all network connections associated with SSH traffic terminate after becoming unresponsive. prevents CWE-770
RHEL 8 (1 rule)
- V-230244 RHEL 8 must be configured so that all network connections associated with SSH traffic terminate after becoming unresponsive. prevents CWE-770