# CVE-2026-123561: Sveltejs devalue before 5.9.3 __proto__ Rejection Bypass via Property-Key Coercion

Status: published. Published: 2026-11-08. Updated: 2026-11-10.

devalue before 5.9.3 allows malformed null-prototype object keys to bypass parse's __proto__ rejection through property-key coercion, an incomplete fix for GHSA-mwv9-gp5h-frr4. Attackers can supply crafted payloads that yield objects with a __proto__ own property, which alone does not cause prototype pollution.

## Remediation

- fix: devalue (npm): OSV reports a fixed version 5.9.3. Check the advisory for supported release branches. (source: OSV GHSA-4q55-j62x-fr9h) — https://github.com/advisories/GHSA-4q55-j62x-fr9h

## 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-4q55-j62x-fr9h

## Severity and prioritization

- CVSS 4.0: 6.3 (medium); CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 3.7 (low); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N; source: CNA: VulnCheck

- CVSS 4.0: 6.3 (medium); CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/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: 3.7 (low); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N; source: NVD: disclosure@vulncheck.com

- CISA KEV: No listing in collected sources

- EPSS: 0.25% as of 2026-11-10; percentile 16.31%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-1321

## Exploit evidence

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

## References

- https://github.com/sveltejs/devalue/security/advisories/GHSA-4q55-j62x-fr9h (CNA: VulnCheck; vendor-advisory)

- https://www.vulncheck.com/advisories/sveltejs-devalue-before-5.9.3-proto-rejection-bypass-via-property-key-coercion (CNA: VulnCheck; third-party-advisory)

- https://github.com/sveltejs/devalue/security/advisories/GHSA-4q55-j62x-fr9h (disclosure@vulncheck.com)

- https://www.vulncheck.com/advisories/sveltejs-devalue-before-5.9.3-proto-rejection-bypass-via-property-key-coercion (disclosure@vulncheck.com)

- https://github.com/advisories/GHSA-4q55-j62x-fr9h (OSV / GitHub Security Advisories; advisory)

- https://osv.dev/vulnerability/GHSA-4q55-j62x-fr9h (OSV / GitHub Security Advisories; osv)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-123561; retrieved 2026-11-10; source updated 2026-11-10

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-123561; retrieved 2026-11-08; source updated 2026-11-08

- OSV / GitHub Security Advisories: https://osv.dev/vulnerability/GHSA-4q55-j62x-fr9h; retrieved 2026-11-10; source updated 2026-11-08

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-123561; retrieved 2026-11-10; source updated Not reported

Revision: d1f11d3c9d0f6ed47663a103819f79e33709c232c16ab2868c35d11c4daec37e

Canonical: https://y2.dev/cve/CVE-2026-123561

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