{"id":"CVE-2026-55778","title":"parse-server: Stored XSS via non-standard file extension bypassing file upload extension blocklist","summary":"parse-server: Stored XSS via non-standard file extension bypassing file upload extension blocklist","severity":"low","cwe":["CWE-434"],"vendor":"parse-server","product":"parse-server","ecosystem":"npm","affected":["parse-server >= 9.0.0, < 9.9.1-alpha.11","parse-server <= 8.6.80"],"patched":["parse-server 9.9.1-alpha.11","parse-server 8.6.81"],"published":"2026-06-19","updated":"2026-06-19","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-v8x7-r927-cc93","references":[{"url":"https://github.com/parse-community/parse-server/security/advisories/GHSA-v8x7-r927-cc93"},{"url":"https://github.com/parse-community/parse-server/pull/10505"},{"url":"https://github.com/parse-community/parse-server/pull/10506"},{"url":"https://github.com/advisories/GHSA-v8x7-r927-cc93"}],"tags":["ghsa","npm"],"ingestedAt":"2026-06-22T15:52:21.033Z","epss":0.00553,"epssPercentile":0.45074,"slug":"CVE-2026-55778","body":"## Overview\n\n### Impact\n\nParse 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.\n\nThis 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.\n\n### Patches\n\nThe 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.\n\n### Workarounds\n\nConfigure `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.\n\n## Affected packages\n\n- `parse-server >= 9.0.0, < 9.9.1-alpha.11`\n- `parse-server <= 8.6.80`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `parse-server 9.9.1-alpha.11`\n- `parse-server 8.6.81`","depth":"sunlit","depthScore":14,"depthScoreParts":{"impact":13.8,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}