tRPC allows users to build and consume fully typesafe APIs without schemas or code generation. Starting in version 10.27.0 and prior to versions 10.45.3 and 11.8.0, a 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. Note that this vulnerability is only present when using `experimental_caller` / `experimental_nextAppDirCaller`. Versions 10.45.3 and 11.8.0 fix the issue.

Affected: npm: @trpc/server

> 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

```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); // ← Recursiv…