parse-server: Stored XSS via non-standard file extension bypassing file upload extension blocklist
Impact
Parse Server's default fileUpload.fileExtensions blocklist is intended to prevent uploading files that browsers render as active content (such as HTML and SVG), which can be used to perform stored cross-site scripting (XSS) attacks against other users. The blocklist could be bypassed by uploading a file whose extension is not an exact match of a blocked extension (for example a non-standard or compound extension) together with a dangerous content type. On storage adapters that persist and serve the uploaded content type (such as S3 and GCS), the file is then served with the attacker-supplied content type, enabling stored XSS against users who open the file URL.
This affects the default configuration, in which authenticated users are allowed to upload files. The default GridFS/filesystem adapter sets the X-Content-Type-Options: nosniff response header, which mitigates browser rendering on that adapter, but the upload restriction itself is still bypassed. This is an incomplete-fix follow-up of GHSA-vr5f-2r24-w5hc and GHSA-7wqv-xjf3-x35v.
Patches
The file upload extension validation now also evaluates the request content type against the configured blocklist whenever the filename's extension is not a recognized type. As a result, a dangerous content type can no longer be preserved by uploading a file with a non-standard extension, and such uploads are rejected.
Workarounds
Configure fileUpload.fileExtensions as a strict allowlist of only the file extensions your application needs (for example ["^(png|jpe?g|gif|pdf)$"]) instead of relying on the default blocklist. Additionally, serve uploaded files from a separate domain than the application, so that any executed content is isolated from the application's origin.