CVE-2026-123561 Sveltejs devalue before 5.9.3 __proto__ Rejection Bypass via Property-Key Coercion
Medium 6.3
Published
Updated
Check this codebase for CVE-2026-123561 and fix it if it is affected.
What it is: CVE-2026-123561 is a medium-severity vulnerability in sveltejs devalue, published on 8 Nov 2026.
Exploitation: There is no known exploitation of CVE-2026-123561 in the wild and no public exploit code.
Affected packages and fixed versions:
- devalue (npm): affected all versions; fixed in 5.9.3
Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.
Reference: https://insights.sbomify.com/CVE-2026-123561/ (Markdown: https://insights.sbomify.com/CVE-2026-123561/index.md)
- Exploited in the wild
- Not known
- Fix
- None listed
What is CVE-2026-123561?
CVE-2026-123561 is a medium-severity vulnerability in sveltejs devalue, published on 8 Nov 2026.
Official description
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.
Weakness: CWE-1321
Which versions are affected by CVE-2026-123561?
1 open source package ship an affected version, including devalue.
| Package | Affected | Fixed in |
|---|---|---|
devalue npm | All versions | 5.9.3 |
How do I fix CVE-2026-123561?
Upgrade devalue to 5.9.3 or later.
Is CVE-2026-123561 being exploited?
There is no known exploitation of CVE-2026-123561 in the wild and no public exploit code.
Sources
Also known as CLSA-2026-1790888237, CLSA-2026-1791626600, GHSA-2fcf-p6fg-p2wf, GHSA-4q55-j62x-fr9h.