# CVE-2026-123567: Sveltejs devalue before 5.6.4 Prototype Injection via parse and unflatten


- **Severity:** Medium 6.3 (`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`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 8 Nov 2026
- **Updated:** 8 Nov 2026
- **Also known as:** GHSA-8f8h-hxr2-jrh3, GHSA-mwv9-gp5h-frr4

## What is CVE-2026-123567?

CVE-2026-123567 is a medium-severity vulnerability in sveltejs devalue, published on 8 Nov 2026. 

### Official description

Sveltejs devalue before 5.6.4 contains a prototype injection vulnerability that allows attackers to inject __proto__ own properties into objects returned by devalue.parse and devalue.unflatten. Attackers can supply crafted serialized input that, when downstream code merges results via Object.assign, alters the target object's prototype.

Weakness: CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-123567?

1 open source package ship an affected version, including devalue.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `devalue` | npm | From 4.0.0 | 5.6.4 |

## How do I fix CVE-2026-123567?

Upgrade devalue to 5.6.4 or later.

## Is CVE-2026-123567 being exploited?

There is no known exploitation of CVE-2026-123567 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-123567 and fix it if it is affected.

What it is: CVE-2026-123567 is a medium-severity vulnerability in sveltejs devalue, published on 8 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-123567 in the wild and no public exploit code.

Affected packages and fixed versions:
- devalue (npm): affected from 4.0.0; fixed in 5.6.4

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-2026-123567/ (Markdown: https://insights.sbomify.com/CVE-2026-123567/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-123567
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-123567
- OSV: https://osv.dev/vulnerability/CVE-2026-123567

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-123567/
Summaries are generated and may contain mistakes; the official description above is the record.