CVE-2026-33888: ApostropheCMS: publicApiProjection Bypass via `project` Query Builder in Piece-Type REST API

Summary

The getRestQuery method in the @apostrophecms/piece-type module checks whether a MongoDB projection has already been set before applying the admin-configured publicApiProjection. An unauthenticated attacker can supply a project query parameter in the REST API request to pre-populate the projection state, causing the security-enforced publicApiProjection to be skipped entirely. This allows disclosure of fields that the site administrator explicitly restricted from public access.

Details

When an unauthenticated user queries the piece-type REST API, the getRestQuery method processes the request at modules/@apostrophecms/piece-type/index.js:1120:

javascript // piece-type/index.js:1120-1137 getRestQuery(req, omitPermissionCheck = false) { const query = self.find(req).attachments(true); query.applyBuildersSafely(req.query); // [1] attacker input applied first if (!omitPermissionCheck && !self.canAccessApi(req)) { if (!self.options.publicApiProjection) { query.and({ id: null }); } else if (!query.state.project) { // [2] checks if projection already set query.project({ ...self.options.publicApiProjection, cacheInvalidatedAt: 1 }); } } return query; },

At [1], applyBuildersSafely iterates over all query string parameters and invokes their corresponding builder methods. The project builder exists in @apostrophecms/doc-type with a launder method (doc-type/index.js:1876) that sanitizes values to booleans:

javascript // doc-type/index.js:1875-1889 project: { launder (p) { if (!p || typeof p !== 'object' || Array.isArray(p)) { return {}; } const projection = Object.entries(p).reduce((acc, [ key, val ]) => { return { ...acc, [key]: self.apos.launder.boolean(val) }; }, {}); return projection; },

When a request includes ?project[someField]=1, the builder sets query.state.project to {someField: true}. At [2], the conditional !query.state.project evaluates to false because the state is already populated, so the publicApiProjection is never applied.

For comparison, the @apostrophecms/page module's equivalent method (page/index.js:2953) unconditionally applies the projection:

javascript // page/index.js:2953-2958 } else { query.project({ ...self.options.publicApiProjection, cacheInvalidatedAt: 1 }); }

PoC

Prerequisites: An ApostropheCMS 4.x instance with a piece-type (e.g., article) that has publicApiProjection configured to restrict fields. For example:

javascript // modules/article/index.js module.exports = { extend: '@apostrophecms/piece-type', options: { publicApiProjection: { title: 1, url: 1 } } };

Step 1: Normal request — observe restricted fields are hidden:

bash curl 'http://localhost:3000/api/v1/article'

Response returns only title and url fields per the configured projection.

Step 2: Bypass projection by supplying project query parameter:

bash curl 'http://localhost:3000/api/v1/article?project[internalNotes]=1&project[title]=1&project[slug]=1&project[createdAt]=1'

Response now includes internalNotes, slug, createdAt, and any other requested fields — bypassing the admin-configured publicApiProjection restriction.

Step 3: Request all default fields by projecting inclusion of sensitive fields:

bash curl 'http://localhost:3000/api/v1/article?project[id]=1&project[title]=1&project[slug]=1&project[visibility]=1&project[type]=1&project[createdAt]=1&project[updatedAt]=1'

All requested fields are returned, confirming the publicApiProjection is fully bypassed.

Impact

- Information Disclosure: An unauthenticated attacker can read any field on documents that are already publicly queryable, bypassing administrator-configured field restrictions. This may expose internal notes, draft content, metadata, or other sensitive fields the administrator intentionally hid from the public API. - Scope: Affects all piece-type modules with publicApiProjection configured. The attacker cannot access documents they wouldn't otherwise be able to query (document-level permissions still apply), but they can read any field on accessible documents. - Exploitability: Trivial — requires only appending query parameters to a public URL. No authentication, special tools, or chaining required.

Recommended Fix

Remove the conditional check on query.state.project in piece-type/index.js, matching the page module's unconditional behavior. The admin-configured publicApiProjection should always override any user-supplied projection for unauthenticated users:

javascript // modules/@apostrophecms/piece-type/index.js:1123-1134 // BEFORE (vulnerable): if (!omitPermissionCheck && !self.canAccessApi(req)) { if (!self.options.publicApiProjection) { query.and({ id: null }); } else if (!query.state.project) { query.project({ ...self.options.publicApiProjection, cacheInvalidatedAt: 1 }); } }

// AFTER (fixed): if (!omitPermissionCheck && !self.canAccessApi(req)) { if (!self.options.publicApiProjection) { query.and({ id: null }); } else { query.project({ ...self.options.publicApiProjection, cacheInvalidatedAt: 1 }); } }