CVE-2026-102598: Werkzeug safe_join() allows Windows special device names

Severity: medium (CVSS 6.3 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-29 · updated 2026-09-30
Weakness: CWE-67

Description

Werkzeug is a comprehensive WSGI web application library. Prior to 3.1.9, the safe_join function used by send_from_directory can allow a NUL: special-device path because safe_join checks the Windows device name without first removing an empty NTFS ADS marker. The trigger is that an application runs on Windows with NTFS and serves a user-specified path ending in a special device name such as NUL:. The attack mechanism is that a requested path ends in a Windows special device name with an empty ADS marker. The impact is that the special device opens successfully and the file read hangs indefinitely. This issue is fixed in version 3.1.9.

Affected products

Pypi Werkzeug: before 3.1.9; 0.1, 0.10, 0.10.1, 0.10.2, 0.10.3, 0.10.4 (+100) — fixed in 3.1.9
pallets werkzeug: before 3.1.9

How to fix

Upgrade to a fixed version: Pypi Werkzeug 3.1.9. Check the vendor advisory for mitigations.

References

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