CVE-2025-66456: 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:

• https://github.com/elysiajs/elysia/pull/1564/commits/26935bf76ebc43b4a43d48b173fc853de43bb51e
• https://github.com/elysiajs/elysia/pull/1564/commits/3af978663e437dccc6c1a2a3aff4b74e1574849e

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
			})
	})