# CVE-2026-123564: Sveltejs devalue memory exhaustion in uneval sparse arrays

Source: https://junglewise.ai/threats/cve-2026-123564-sveltejs-devalue-from-1-0-0-before-5-9-3-contains-a-memory  
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-2026-123564 · Severity: high · CVSS 7.5 · Published 2026-11-08

Sveltejs devalue is a serialization library used to send data from servers to browsers. The library's uneval function generates code that allocates memory proportional to a sparse array's declared length when evaluated, allowing attackers who control serialized data to exhaust browser or runtime memory and cause a denial of service.

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

## Quick answers

**Is CVE-2026-123564 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-123564 being exploited in the wild.

**How severe is CVE-2026-123564?** High severity, with a CVSS score of 7.5, published 8 November 2026.

**Which products and versions are affected?** Sveltejs Devalue 1.0.0 before 5.9.3.

**Is there a fix for CVE-2026-123564?** 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-2026-123564 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-123564?** 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 generates code for sparse arrays that eagerly allocates memory based on the array's declared length during evaluation, not during serialization. An attacker who can influence a very large sparse array serialized by an application can cause excessive memory allocation in any browser or runtime that evaluates the output, leading to denial of service. The vulnerability is fixed in version 5.9.3.

## Affected products

- Sveltejs devalue 1.0.0 before 5.9.3

## Timeline

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

## References

- [https://github.com/sveltejs/devalue/security/advisories/GHSA-wf3x-273g-mvxv](https://github.com/sveltejs/devalue/security/advisories/GHSA-wf3x-273g-mvxv)
- [https://www.vulncheck.com/advisories/sveltejs-devalue-before-5.9.3-memory-exhaustion-via-uneval-sparse-arrays](https://www.vulncheck.com/advisories/sveltejs-devalue-before-5.9.3-memory-exhaustion-via-uneval-sparse-arrays)

## 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 resource exhaustion in uneval](https://junglewise.ai/threats/cve-2026-123565-sveltejs-devalue-before-5-9-3-contains-a-resource-exhaustion)
- [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)