# CVE-2026-123566: Devalue before 5.9.3 CPU Amplification Denial of Service via uneval

Status: published. Published: 2026-11-08. Updated: 2026-11-09.

Sveltejs devalue before 5.9.3 contains a denial of service vulnerability that allows attackers to cause CPU amplification by supplying sparse arrays to uneval. Attackers who influence sparse values passed to uneval can force synchronous work proportional to the declared array length, blocking the event loop.

## Remediation

- fix: devalue (npm): OSV reports a fixed version 5.9.3. Check the advisory for supported release branches. (source: OSV GHSA-hx4r-w6wj-j8fg) — https://github.com/advisories/GHSA-hx4r-w6wj-j8fg

## Affected products and versions

- sveltejs / devalue (devalue): ≥ 0; < 5.9.3 (affected, semver); 5.9.3 (unaffected, semver). Source: CNA: VulnCheck
-  / devalue (devalue): ≥ 0; < 5.9.3 (affected, semver); OSV events: introduced 0; fixed 5.9.3. Source: OSV GHSA-hx4r-w6wj-j8fg

## Severity and prioritization

- CVSS 4.0: 8.7 (high); CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H; source: CNA: VulnCheck

- CVSS 4.0: 8.7 (high); CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: disclosure@vulncheck.com

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H; source: NVD: disclosure@vulncheck.com

- CISA KEV: No listing in collected sources

- EPSS: 0.34% as of 2026-11-08; percentile 28.07%

- SSVC: exploitation none; automatable yes; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-400

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/sveltejs/devalue/security/advisories/GHSA-hx4r-w6wj-j8fg (CNA: VulnCheck; vendor-advisory)

- https://www.vulncheck.com/advisories/devalue-before-5.9.3-cpu-amplification-denial-of-service-via-uneval (CNA: VulnCheck; third-party-advisory)

- https://github.com/sveltejs/devalue/security/advisories/GHSA-hx4r-w6wj-j8fg (disclosure@vulncheck.com)

- https://www.vulncheck.com/advisories/devalue-before-5.9.3-cpu-amplification-denial-of-service-via-uneval (disclosure@vulncheck.com)

- https://github.com/advisories/GHSA-hx4r-w6wj-j8fg (OSV / GitHub Security Advisories; advisory)

- https://osv.dev/vulnerability/GHSA-hx4r-w6wj-j8fg (OSV / GitHub Security Advisories; osv)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-123566; retrieved 2026-11-09; source updated 2026-11-09

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-123566; retrieved 2026-11-08; source updated 2026-11-08

- OSV / GitHub Security Advisories: https://osv.dev/vulnerability/GHSA-hx4r-w6wj-j8fg; retrieved 2026-11-09; source updated 2026-11-08

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-123566; retrieved 2026-11-09; source updated Not reported

Revision: e40eed62b916ea4cb1cc9c749f3d87ab3ee5614eb118af03978e7485927d7ebe

Canonical: https://y2.dev/cve/CVE-2026-123566

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