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CVE-2025-66456

Elysia vulnerable to prototype pollution with multiple standalone schema validation

9.1 CriticalFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
High
Availability
None

Exploit likelihood

0.56%Up 0.5 points since Dec 10, 2025
90%50%10%1%

What each source reports

CVE List Score used 9.1

Elysia vulnerable to prototype pollution with multiple standalone schema validation

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.

Record
CVE-2025-66456
CVSS 4.0
9.1 CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
Weakness
CWE-1321
Updated
2026-11-02
EUVDEUVD-2025-202180 9.1

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.

Record
EUVD-2025-202180
CVSS 4.0
9.1 CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-hxj9-33pp-j2cc 9.1

Elysia vulnerable to prototype pollution with multiple standalone schema validation

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:

import { Elysia } from "elysia"
import * as z from "zod"

const app = new Elysia()
	.guard({
		schema: "standalone",
		body: z.object({
			data: z.any()
		})
	})
	.post("/", ({ body }) => ({ body, win: {}.foo }), {
		body: z.object({
			data: z.object({
				messageId: z.string("pollute-me"),
			})
		})
	})

Patches

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

Reference commit:

Workarounds

Remove __proto__ key from body

Example plugin for removing __proto__ from body

new Elysia()
	.onTransform(({ body, headers }) => {
		if (headers['content-type'] === 'application/json')
			return JSON.parse(JSON.stringify(body), (k, v) => {
				if (k === '__proto__') return

				return v
			})
	})
Record
GHSA-hxj9-33pp-j2cc
CVSS 4.0
9.1 CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
Weakness
CWE-1321
Updated
2026-11-02
NVD 9.1

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.

Record
CVE-2025-66456
CVSS 4.0
9.1 CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS 3.1
9.8 CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Weakness
CWE-1321
Updated
2026-11-02
OSV 9.1

Elysia vulnerable to prototype pollution with multiple standalone schema validation

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.

Record
CVE-2025-66456
CVSS 4.0
9.1 CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
Weakness
CWE-1321
Updated
2026-11-02
OSVGHSA-hxj9-33pp-j2cc 9.1

Elysia vulnerable to prototype pollution with multiple standalone schema validation

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:

import { Elysia } from "elysia"
import * as z from "zod"

const app = new Elysia()
	.guard({
		schema: "standalone",
		body: z.object({
			data: z.any()
		})
	})
	.post("/", ({ body }) => ({ body, win: {}.foo }), {
		body: z.object({
			data: z.object({
				messageId: z.string("pollute-me"),
			})
		})
	})

Patches

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

Reference commit:

Workarounds

Remove __proto__ key from body

Example plugin for removing __proto__ from body

new Elysia()
	.onTransform(({ body, headers }) => {
		if (headers['content-type'] === 'application/json')
			return JSON.parse(JSON.stringify(body), (k, v) => {
				if (k === '__proto__') return

				return v
			})
	})
Record
GHSA-hxj9-33pp-j2cc
CVSS 4.0
9.1 CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
Weakness
CWE-1321
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
elysia
npm
 1.4.0 up to 1.4.17
1.4.17

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2025-66456 is a critical vulnerability (CVSS 9.1) in Elysiajs Elysia, published December 9, 2025. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.56%, the 46th percentile. Fixed in elysia 1.4.17.

References

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