@rvf/set-get has a prototype pollution issue that's reachable via @rvf/core preprocessFormData (HTTP form data)

Summary

setPath in @rvf/set-get (used by @rvf/core to flatten incoming form data into a nested object) does not block the keys __proto__, constructor, or prototype when walking a path. Because field names in submitted form data are passed directly to setPath via preprocessFormData (and through parseFormData / validate), an attacker who can submit a form to a Remix / React Router app using the library can set arbitrary properties on Object.prototype of the running server process.

This is a default-reachable prototype pollution primitive: no special configuration is required. Any endpoint that accepts a form via parseFormData or runs a validator created with createValidator is affected.

Affected versions

• @rvf/set-get < 7.0.2 (7.x line)
• @rvf/set-get < 6.0.4 (6.x line)

Reached through @rvf/core versions that depend on a vulnerable @rvf/set-get (current 8.1.0 resolves to 7.0.1 without the override).

Patched

• @rvf/set-get 7.0.2
• @rvf/set-get 6.0.4

The fix adds a REJECT_KEYS blocklist (__proto__, constructor, prototype) and throws when one is encountered while walking a path inside setPath.

Proof of concept

Install a vulnerable resolution and run on Node 18+:

{
  "dependencies": { "@rvf/core": "8.1.0" },
  "overrides": { "@rvf/set-get": "7.0.1" }
}

const { preprocessFormData } = require('@rvf/core');

const form = new FormData();
form.append("username", "alice");
form.append("__proto__[polluted]", "yes");

preprocessFormData(form);
console.log(({}).polluted); // -> 'yes'

The field name __proto__[polluted] is the kind of value an attacker can submit from any HTML form or HTTP client. After the call, every plain object in the process inherits polluted = 'yes'.

A second working payload is constructor.prototype.<key>=<value>, which goes through setPath walking constructor then prototype.

Impact

• Any property assignable on Object.prototype of the server process, set by a single unauthenticated HTTP request.
• Persists for the life of the worker process and affects every subsequent request handled by the same process.
• Direct downstream consequences depend on the host application and the rest of its dependency tree, but typical risks include: bypassing if (obj.isAdmin) style checks, injecting unintended config values into objects merged with user input, breaking template rendering, and crashing the worker by polluting properties used by other libraries (DoS).
• Worth noting: the visible output of preprocessFormData does not contain the malicious key, so the attack leaves no obvious trace in request logs that show parsed bodies.

CVSS

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L (8.2, High)

Integrity is High because the primitive lets the attacker change the meaning of property reads on every object in the process. Confidentiality is None and Availability is Low without a named downstream gadget; both could be higher in a specific consuming app.

Remediation for users

Upgrade to @rvf/set-get 7.0.2 or 6.0.4. If you cannot upgrade @rvf/core directly, an npm / pnpm override on @rvf/set-get works.

Credit

Reported by Mohamed Bassia (@0xBassia).