What’s Vulnerable › Vulnerabilities

CVE-2026-21860: Werkzeug safe_join() allows Windows special device names with compound extensions

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-01-08 · updated 2026-06-17
• Weakness: CWE-67

Description

Werkzeug is a comprehensive WSGI web application library. Prior to version 3.1.5, 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 issue has been patched in version 3.1.5.

Affected products

• pallets werkzeug: before 3.1.5 — fixed in 3.1.5
• Pypi Werkzeug: before 3.1.5; 0.1, 0.10, 0.10.1, 0.10.2, 0.10.3, 0.10.4 (+96) — fixed in 3.1.5

How to fix

Upgrade to a fixed version: pallets werkzeug 3.1.5; Pypi Werkzeug 3.1.5. Check the vendor advisory for mitigations.

References

• https://github.com/pallets/werkzeug/commit/7ae1d254e04a0c33e241ac1cca4783ce6c875ca3
• https://github.com/pallets/werkzeug/security/advisories/GHSA-87hc-h4r5-73f7

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