Executive brief
Parse Server, an open-source backend framework, is vulnerable to a security flaw that allows users to upload malicious files. By using a specially crafted file type, an attacker can bypass security filters and store a file that acts as a script. If another user opens this file, the script will run in their browser, potentially allowing the attacker to steal session information or perform actions on the user's behalf.
Technical details
A stored cross-site scripting (XSS) vulnerability exists in Parse Server due to improper validation of client-supplied Content-Type headers. When a file extension is not recognized by the 'mime' package, the server preserves the client-provided Content-Type. Attackers can provide malformed types (e.g., 'image//svg+xml') that bypass the 'fileUpload.fileExtensions' blocklist. When these files are served from adapters like Amazon S3, Google Cloud Storage, or Azure Blob Storage, browsers may fail to parse the header and resort to MIME-sniffing. If the file content begins with HTML, it is executed in the application's origin. The default GridFS adapter is unaffected as it recomputes Content-Type and uses 'nosniff' headers. Fixes are available in versions 9.10.0-alpha.2 and 8.6.84.
Affected products
- parse-community Parse Server >= 9.0.0, < 9.10.0-alpha.2; <= 8.6.83
Timeline
- 2026-06-25: advisory: GitHub advisory published by maintainers
- 2026-07-11: disclosed: NVD publication date