chore: Add guidance on dynamic object key handling in nodes (#31369)

cubic.yaml

@@ -99,6 +99,72 @@ reviews:
         - Decorator order should be: route decorator → `@Licensed` → scope decorators
         - Enterprise features not properly isolated in modules (newer features should use the module system)
         - Enterprise code accessible outside of `*.ee.ts` files or licensed modules
+    - name: Prototype Pollution via Node Parameters
+      description: |-
+        - Rule Statement:
+        In node code, a value derived from user-controlled input must never be
+        used as a computed object key in an **assignment** to a plain object.
+        User input reaches nodes mainly via `this.getNodeParameter(...)` (and
+        incoming `item.json` keys), and a workflow author can set such a value
+        to `__proto__`, `constructor`, or `prototype`, polluting the prototype
+        chain (denial of service, and worse depending on the sink).
+
+        Use `setSafeObjectProperty(target, key, value)` /
+        `isSafeObjectProperty(key)` from `n8n-workflow`, a `Map`, or a
+        null-prototype object (`Object.create(null)`) instead.
+
+        - Detection Criteria:
+        Flag NEW code in `packages/nodes-base/**` or
+        `packages/@n8n/nodes-langchain/**` when a value that traces back to
+        `this.getNodeParameter(...)` (directly, via a variable, via a
+        `.map`/`.reduce`/`.forEach` callback param, or via a `getNodeParam`
+        function passed into a helper such as `createTableStruct`) is used as a
+        computed key to BUILD A NESTED OR CONTAINER STRUCTURE on a plain object
+        (an `IDataObject` / object literal — not a `Map`, not `Object.create(null)`):
+          1. Container creation: `obj[key] = {}` or `obj[key] = []`
+          2. Nested write where the untrusted value is a key: `obj[k1][k2] = value`
+          3. The same via `obj[key] ??= {}` / `obj[key] ||= []`
+        This nested/container shape is what actually pollutes `Object.prototype`
+        (typically the dangerous pattern is building a shared accumulator, e.g.
+        `if (acc[table] === undefined) acc[table] = {}` then `acc[table][key] = []`).
+
+        - Do NOT flag:
+          - Reads: `const x = obj[key]`, `if (obj[key] === undefined)`
+            (reads alone do not pollute)
+          - Single-level writes of a concrete (non-object) value, e.g.
+            `item.json[field] = value`, `item.binary[prop] = data`,
+            `body[target] = value`. Assigning a primitive to `__proto__` is a
+            no-op, and setting one property on a fresh per-item object is not
+            prototype pollution. This is the overwhelmingly common, benign case
+            in nodes — flagging it is noise.
+          - Keys that are string/number literals, or are validated by
+            `isSafeObjectProperty(key)` before the write
+          - Writes routed through `setSafeObjectProperty(...)`
+          - Targets that are a `Map` or created with `Object.create(null)`
+          - Existing unchanged code (review only new or modified lines)
+
+        - Example Violation:
+          ```typescript
+          const table = this.getNodeParameter('table', i) as string;
+          const key = this.getNodeParameter('deleteKey', i) as string;
+          if (acc[table] === undefined) acc[table] = {}; // acc['__proto__'] = {}
+          if (acc[table][key] === undefined) acc[table][key] = [];
+          ```
+
+        - Example Allowed:
+          ```typescript
+          import { setSafeObjectProperty, isSafeObjectProperty } from 'n8n-workflow';
+
+          if (isSafeObjectProperty(table) && acc[table] === undefined) {
+            setSafeObjectProperty(acc, table, {});
+          }
+          // or: const acc = new Map<string, ...>();
+          ```
+
+        - Recommendation:
+        Prefer `setSafeObjectProperty` / `isSafeObjectProperty` from
+        `n8n-workflow`. See `nodes/Google/GSuiteAdmin/GSuiteAdmin.node.ts` and
+        `nodes/HttpRequest/V3/HttpRequestV3.node.ts` for reference usage.
     - name: Use DTOs for Request Body Validation
       description: |-
         - Rule Statement:

packages/nodes-base/AGENTS.md

@@ -94,6 +94,37 @@ Change parameter type to `'resourceLocator'`, define modes (list, id, url), add
 - Use `NodeApiError` for API-related errors
 - Support `continueOnFail` option when appropriate
 
+### Security
+
+User input is untrusted. In nodes it arrives mainly through
+`this.getNodeParameter(...)` (and incoming `item.json`), and a workflow author
+controls these values.
+
+**Never use an untrusted value as a computed object key in an assignment.** A
+value such as `__proto__`, `constructor`, or `prototype` pollutes the prototype
+chain:
+
+```ts
+// UNSAFE — `table`/`key` come from this.getNodeParameter(...)
+if (acc[table] === undefined) acc[table] = {};
+acc[table][key] = value;
+```
+
+Route dynamic-key writes through the `n8n-workflow` helpers, or build the
+accumulator as a `Map` / `Object.create(null)`:
+
+```ts
+import { setSafeObjectProperty, isSafeObjectProperty } from 'n8n-workflow';
+
+if (isSafeObjectProperty(table) && acc[table] === undefined) {
+	setSafeObjectProperty(acc, table, {});
+}
+```
+
+This only applies to dynamic-key **writes** (grouping/aggregating rows by a
+user-chosen column is the common case). Reads like `const x = obj[key]` are
+safe. Reference usage: `nodes/Google/GSuiteAdmin/GSuiteAdmin.node.ts`.
+
 ### Code Organization
 - Separate operation/field descriptions into separate files
 - Create reusable API request helpers in GenericFunctions