{"id":"CVE-2026-55778","aliases":["BIT-parse-2026-55778","GHSA-v8x7-r927-cc93"],"url":"https://o3.security/vulnerability/CVE-2026-55778","summary":"Parse Server: Stored XSS via non-standard file extension bypassing file upload extension blocklist","details":"### 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.","published":"2026-07-08T20:50:52.531Z","modified":"2026-10-21T02:58:39.622486921Z","cvss":null,"epss":{"score":0.00553,"percentile":0.44552,"asOf":"2026-09-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"parse-server","fixedVersion":"9.9.1-alpha.11"},{"ecosystem":"npm","name":"parse-server","fixedVersion":"8.6.81"}],"fix":{"url":"https://github.com/parse-community/parse-server/commit/97c6a78d19f976ec756c1295f08a8fccab90799a","label":"parse-community/parse-server@97c6a78"},"references":[{"type":"WEB","url":"https://github.com/parse-community/parse-server/releases/tag/8.6.81"},{"type":"WEB","url":"https://github.com/parse-community/parse-server/releases/tag/9.9.1-alpha.11"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55778.json"},{"type":"ADVISORY","url":"https://github.com/parse-community/parse-server/security/advisories/GHSA-v8x7-r927-cc93"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55778"},{"type":"FIX","url":"https://github.com/parse-community/parse-server/commit/97c6a78d19f976ec756c1295f08a8fccab90799a"},{"type":"FIX","url":"https://github.com/parse-community/parse-server/commit/be12a60d65b6e140481882037fb896b1f951df50"},{"type":"FIX","url":"https://github.com/parse-community/parse-server/pull/10505"},{"type":"FIX","url":"https://github.com/parse-community/parse-server/pull/10506"},{"type":"PACKAGE","url":"https://github.com/parse-community/parse-server"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:58:39.622486921Z"}}