{"id":"CVE-2026-21860","aliases":["GHSA-87hc-h4r5-73f7","PYSEC-2026-2044"],"title":" Werkzeug safe_join() allows Windows special device names with compound extensions","summary":" Werkzeug safe_join() allows Windows special device names with compound extensions","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","vendor":"werkzeug","product":"werkzeug","ecosystem":"pip","affected":["werkzeug < 3.1.5"],"patched":["werkzeug 3.1.5"],"published":"2026-01-08","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:50:32.862525778Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-87hc-h4r5-73f7","references":[{"url":"https://github.com/pallets/werkzeug/security/advisories/GHSA-87hc-h4r5-73f7"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-21860"},{"url":"https://github.com/pallets/werkzeug/commit/7ae1d254e04a0c33e241ac1cca4783ce6c875ca3"},{"url":"https://github.com/pallets/werkzeug"}],"tags":["osv","pip"],"epss":0.00482,"epssPercentile":0.40709,"ingestedAt":"2026-07-08T18:25:47.322Z","slug":"CVE-2026-21860","body":"## Overview\n\nWerkzeug's `safe_join` function allows path segments with Windows device names that have file extensions or trailing spaces. On Windows, there are special device names such as `CON`, `AUX`, etc that are implicitly present and readable in every directory. Windows still accepts them with any file extension, such as `CON.txt`, or trailing spaces such as `CON `.\n\nThis was previously reported as https://github.com/pallets/werkzeug/security/advisories/GHSA-hgf8-39gv-g3f2, but the fix failed to account for compound extensions such as `CON.txt.html` or trailing spaces. It also missed some additional special names.\n\n`send_from_directory` uses `safe_join` to safely serve files at user-specified paths under a directory. If the application is running on Windows, and the requested path ends with a special device name, the file will be opened successfully, but reading will hang indefinitely.\n\n## Affected packages\n\n- `werkzeug < 3.1.5`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `werkzeug 3.1.5`","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}