CVE-2026-63667

## Summary The `@apostrophecms/import-export` module reconstructs the on-disk source path of every imported attachment from JSON metadata contained in the uploaded archive. The archive carries an `aposAttachments.json` file whose `name` and `extension` fields are concatenated into a filesystem path with no traversal check. The zip-slip guard that the module applies during tar extraction validates tar entry names only and does not cover this second path, which is built after extraction. The file at the resulting path is read and copied into the public uploads directory, then served over HTTP without authentication. A `../` sequence in `name` makes the module read a file outside the extraction directory and publish it at an anonymous URL. Result: an authenticated contributor reads any file on the host whose name ends in an allowlisted extension (other users' uploaded documents, text or CSV dumps, PDFs) by importing a crafted archive and fetching the planted attachment anonymously. ## Affected apostrophecms/apostrophe with the `@apostrophecms/import-export` module installed and registered. Module version 3.6.1 (current latest), tested against Apostrophe 4.31.0 (monorepo HEAD 4d478d9). Requires an account with the contributor role or higher; guest and anonymous requests are rejected. The module is not part of the default starter kit, so sites that never installed it are not affected. Files whose real name lacks an accepted file-group extension are not reachable. ## Root cause The import parser builds each attachment's source path by concatenating attacker-controlled JSON fields: `lib/formats/gzip.js:46` sets `file.path = path.join(attachmentFilesPath, ${attachment._id}-${attachment.name}.${attachment.extension})` from the `aposAttachments.json` entries in the uploaded archive. That path flows unchanged through `lib/methods/import.js:832` (`insertAttachments`) into `lib/methods/import.js:1074` (`attachment.insert`), where uploadfs copies the referenced file into