How severe is CVE-2025-66456?

CVE-2025-66456 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2025-66456 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (38th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-66456?

CVE-2025-66456 affects Elysiajs Elysia. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-66456?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2025-66456 have an EU (EUVD) identifier?

Yes. CVE-2025-66456 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-202180.

When was CVE-2025-66456 published?

CVE-2025-66456 was published on 2025-12-09 and last updated on 2026-06-17.

CVE-2025-66456

CVE-2025-66456 is a critical-severity vulnerability in Elysiajs Elysia with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-1321.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v4: 9.1
EPSS exploit prediction: 0% (38th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-202180
Weakness: CWE-1321
Affected product: Elysiajs Elysia
Published: 2025-12-09
Last modified: 2026-06-17

Description

Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation and client-server communication. Versions 1.4.0 through 1.4.16 contain a prototype pollution vulnerability in `mergeDeep` after merging results of two standard schema validations with the same key. Due to the ordering of merging, there must be an any type that is set as a standalone guard, to allow for the `__proto__ prop` to be merged. When combined with GHSA-8vch-m3f4-q8jf this allows for a full RCE by an attacker. This issue is fixed in version 1.4.17. To workaround, remove the `__proto__ key` from body.

Frequently asked questions

What is CVE-2025-66456?: Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation and client-server communication. Versions 1.4.0 through 1.4.16 contain a prototype pollution vulnerability in `mergeDeep` after merging results of two standard schema validations with the same key. Due to the ordering of merging, there must be an any type that is set as a standalone guard, to allow for the `__proto__ prop` to be merged. When combined with GHSA-8vch-m3f4-q8jf this allows for a full RCE by an attacker. This issue is fixed in version 1.4.17. To workaround, remove the `__proto__ key` from body.
How severe is CVE-2025-66456?: CVE-2025-66456 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2025-66456 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (38th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-66456?: CVE-2025-66456 affects Elysiajs Elysia. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-66456?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2025-66456 have an EU (EUVD) identifier?: Yes. CVE-2025-66456 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-202180.
When was CVE-2025-66456 published?: CVE-2025-66456 was published on 2025-12-09 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:elysiajs:elysia:*:*:*:*:*:node.js:*:*

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