---
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: 0.38875
ingestedAt: '2026-07-08T18:25:47.322Z'
---

## Overview

Werkzeug'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 `.

This 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.

`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.

## Affected packages

- `werkzeug < 3.1.5`

## Remediation

Upgrade to a patched release:

- `werkzeug 3.1.5`