Parse Server is an open source backend that can be deployed to any infrastructure that can run Node.js. Prior to 9.9.1‑alpha.11 and 8.6.81, the default fileUpload.fileExtensions blocklist could be bypassed by uploading a file with a non‑standard or compound extension and a dangerous content type, allowing storage adapters such as S3 and GCS to serve attacker‑supplied active content and enable stored cross‑site scripting. This issue is fixed in versions 9.9.1‑alpha.11 and 8.6.81.

The affected product is Parse Server from parse‑community. All releases prior to 9.9.1‑alpha.11 and 8.6.81 are vulnerable, regardless of the deployment environment, since the issue resides in the open‑source code executed on Node.js infrastructure.

The CVSS score of 2.1 indicates a low severity risk, reflecting that the vulnerability requires an attacker to already have a file upload vector to the Parse Server instance. The EPSS score of < 1% (approximately 0.4%) indicates a very low likelihood of exploitation, and the issue is not listed in the CISA KEV catalog, suggesting limited active exploitation. The likely attack vector is that an attacker with access to the file upload endpoint can supply a compound extension and dangerous content type to bypass the blocklist, enabling stored XSS against downstream consumers of the file stream.