CVE-2025-66456: Elysia vulnerable to prototype pollution with multiple standalone schema validation

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.6%, percentile 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes elysia (npm) >= 1.4.0, < 1.4.17, fixed in 1.4.17. Check your lockfile, including transitive dependencies.

What it is

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.

From the GitHub advisory

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.

Impact Routes with more than 2 standalone schema validation, eg. zod

Example vulnerable code:

Patches Patched by 1.4.17 (https://github.com/elysiajs/elysia/pull/1564)

Weakness type: CWE-1321.

Product: elysia (npm) · Assigned by: GitHub_M · Published: 2025-12-09

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.56% probability of exploitation activity in the next 30 days; percentile 46.

Timeline

• 2025-12-09 — CVE record last updated
• 2025-12-09 — CVE record published by GitHub_M
• 2025-12-09 — GitHub-reviewed advisory GHSA-hxj9-33pp-j2cc published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-hxj9-33pp-j2cc github.com
2. CVE.org record CVE-2025-66456 cve.org
3. NVD entry CVE-2025-66456 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com
6. github.com github.com

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