# CVE-2026-123565: Sveltejs devalue resource exhaustion in uneval

Source: https://junglewise.ai/threats/cve-2026-123565-sveltejs-devalue-before-5-9-3-contains-a-resource-exhaustion  
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-2026-123565 · Severity: high · CVSS 7.5 · Published 2026-11-08

Sveltejs devalue is a serialization library used in web applications to convert data structures into code. A resource exhaustion vulnerability allows attackers to submit a small crafted input that, when processed, expands into an enormous serialized string, consuming available memory or CPU and causing the application to become unresponsive or crash.

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

## Quick answers

**Is CVE-2026-123565 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-123565 being exploited in the wild.

**How severe is CVE-2026-123565?** 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-2026-123565?** 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-2026-123565 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-123565?** 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 vulnerability exists in the uneval function, where repeated primitive strings trigger quadratic expansion in the serialization process. An attacker can craft a small payload, have it parsed and passed to uneval, which generates a disproportionately large output. This is a denial-of-service attack requiring only network access with no authentication or user interaction needed.

## 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-mcm9-63f2-9j32](https://github.com/sveltejs/devalue/security/advisories/GHSA-mcm9-63f2-9j32)
- [https://www.vulncheck.com/advisories/devalue-before-5.9.3-quadratic-expansion-denial-of-service-via-uneval](https://www.vulncheck.com/advisories/devalue-before-5.9.3-quadratic-expansion-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 denial of service in uneval](https://junglewise.ai/threats/cve-2026-123566-sveltejs-devalue-before-5-9-3-contains-a-denial-of-service)
- [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)