joi: object().rename() with a template target can set the validated object's prototype

Impact

Applications are affected only if a schema renames keys with a regular-expression source and a Joi.expression() / Joi.x() target that interpolates the pattern's own match data, combined with { multiple: true }, for example .rename(/^x-(.+)$/, Joi.x('{#1}'), { multiple: true }). Because the target is rendered from the matched input key, an attacker who controls input keys can send x-__proto__ with an object value and make the rename target render as __proto__, which sets the prototype of the object joi returns instead of creating a key on it. The global Object.prototype is not modified, so the effect is confined to the object returned by that one validate() call.

Schemas using a static string rename target are not affected, and neither are schemas left on the default { multiple: false }.

Patches

Versions 17.13.5 and 18.2.4 have been released to address the issue.

Workarounds

1. Replace the template rename target with a static string target.
2. Keep the template but make the capture unable to produce __proto__, using a negative lookahead: .rename(/^x-(?!__proto__$)(.+)$/, Joi.x('{#1}'), { multiple: true })
3. Drop { multiple: true } from the rename, which stops the rename before the assignment.