# CVE-2026-123566: Sveltejs devalue denial of service in uneval

Source: https://junglewise.ai/threats/cve-2026-123566-sveltejs-devalue-before-5-9-3-contains-a-denial-of-service  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-08. Updated: 2026-11-08  
License: CC BY 4.0 with a link back

CVE-2026-123566 · Severity: high · CVSS 7.5 · Published 2026-11-08

Sveltejs devalue is a JavaScript serialization library used to safely convert data structures into code. A flaw in the uneval function allows attackers to cause excessive CPU consumption by passing specially crafted sparse arrays, which can freeze the application and deny service to legitimate users. Exploiting this requires the attacker to influence the data being serialized, which is difficult but possible in certain application architectures.

Technologies: [Sveltejs Devalue](https://junglewise.ai/threats/technologies/sveltejs-devalue). Vendors: [Sveltejs](https://junglewise.ai/threats/vendors/sveltejs).

## Quick answers

**Is CVE-2026-123566 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-123566 being exploited in the wild.

**How severe is CVE-2026-123566?** High severity, with a CVSS score of 7.5, published 8 November 2026.

**Which products and versions are affected?** Sveltejs Devalue before 5.9.3.

**Is there a fix for CVE-2026-123566?** Yes: fixed in 5.9.3. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-123566 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-123566?** Check whether you run Sveltejs Devalue at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 5.9.3

## Technical details

The uneval function performs synchronous work proportional to a sparse array's declared length rather than its actual element count, creating a CPU amplification vector. An attacker who can influence sparse array values passed to uneval can force the event loop to block for extended periods. The vulnerability is difficult to exploit in practice due to the challenge of creating sparse arrays from untrusted input, but feasible in applications that deserialize or process attacker-controlled data structures.

## Affected products

- Sveltejs devalue before 5.9.3

## Timeline

- 2026-09-18: disclosed
- 2026-11-08: advisory

## References

- [https://github.com/sveltejs/devalue/security/advisories/GHSA-hx4r-w6wj-j8fg](https://github.com/sveltejs/devalue/security/advisories/GHSA-hx4r-w6wj-j8fg)
- [https://www.vulncheck.com/advisories/devalue-before-5.9.3-cpu-amplification-denial-of-service-via-uneval](https://www.vulncheck.com/advisories/devalue-before-5.9.3-cpu-amplification-denial-of-service-via-uneval)

## Related threats

- [devalue sparse array denial of service via CPU and memory exhaustion](https://junglewise.ai/threats/cve-2026-123569-the-devalue-package-for-npm-before-5-6-3-can-consume-excessive)
- [Sveltejs devalue prototype pollution in uneval](https://junglewise.ai/threats/cve-2026-123568-sveltejs-devalue-before-5-6-3-contains-a-prototype-pollution)
- [Sveltejs devalue resource exhaustion in uneval](https://junglewise.ai/threats/cve-2026-123565-sveltejs-devalue-before-5-9-3-contains-a-resource-exhaustion)
- [Sveltejs devalue memory exhaustion in uneval sparse arrays](https://junglewise.ai/threats/cve-2026-123564-sveltejs-devalue-from-1-0-0-before-5-9-3-contains-a-memory)
- [devalue unhandled promise rejection in stringifyAsync](https://junglewise.ai/threats/cve-2026-123562-devalue-sveltejs-devalue-5-8-0-before-5-9-3-contains-an)