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CVE-2026-123566

8.7

High

EPSS n/aPatch available

devalue <5.9.3 CPU-amplification DoS via sparse arrays to uneval

exploited

No known exploitation not in CISA KEV as of our last sync

fix

Fixed in 5.9.3

likelihood

No EPSS score yet

Published 8 Nov 2026Analysed here 8 Nov 2026CVSS scored by the publisher (CNA)

do firstUpgrade to a fixed release: 5.9.3.

common questions

Is CVE-2026-123566 being exploited?

Not confirmed. CVE-2026-123566 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked.

How severe is CVE-2026-123566?

CVE-2026-123566 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.

Is there a patch for CVE-2026-123566?

Yes. A fix has been recorded for CVE-2026-123566. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.

What does CVE-2026-123566 affect?

CVE-2026-123566 affects sveltejs/devalue <5.9.3, Node.js apps using devalue, Svelte-based applications, build tools bundling devalue. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.

What should I do about CVE-2026-123566?

Upgrade devalue to >=5.9.3; avoid uneval on untrusted data.

Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.

executive summary

technical analysis

affected

sveltejs/devalue <5.9.3Node.js apps using devalueSvelte-based applicationsbuild tools bundling devalue

impact

Denial of Service: attacker-supplied sparse arrays to uneval cause CPU amplification and block the Node.js event loop, freezing affected apps.

action required

Patch when possible

affected versions

sveltejs devalue

affected< 5.9.3

vendor says fixed in5.9.3

As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.

timeline

8 Nov 2026Published
8 Nov 2026CVE record last updated · same day later
8 Nov 2026Analysed by vulneraibility.com · same day later

how to fix

Remediation steps are tailored for users whose stack is affected. Add this technology to your stack to see the fix checklist.

references

NVD CVE.org CVE Details Source

2 references in the CVE record: 2 advisories

Advisorygithub.com /sveltejs/devalue/security/advisories/GHSA-hx4r-w6wj-j8fg
Advisoryvulncheck.com /advisories/devalue-before-5.9.3-cpu-amplification-denial-of-service-via-uneval

sources

CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
NVD: CVSS and CPE data
FIRST EPSS: exploit probability and percentile
Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.

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