CVE-2026-102598 Werkzeug safe_join() allows Windows special device names
An attacker can cause indefinite hangs in applications using Werkzeug on Windows by requesting paths with special device names and empty ADS markers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-102598?
CVE-2026-102598 is a medium-severity denial of service vulnerability in pallets werkzeug, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-67
Which versions are affected by CVE-2026-102598?
3 open source packages ship an affected version, including werkzeug and Werkzeug.
| Package | Affected | Fixed in |
|---|---|---|
werkzeug PyPI | All versions | 3.1.9 |
werkzeug PyPI | All versions | 3.1.9 |
Werkzeug PyPI | All versions | 3.1.9 |
How do I fix CVE-2026-102598?
Upgrade werkzeug to 3.1.9 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-102598 being exploited?
There is no known exploitation of CVE-2026-102598 in the wild and no public exploit code.
Sources
Also known as AZL-105654, AZL-105681, AZL-105921, CLEANSTART-2026-IZ97021, CLEANSTART-2026-JH53704, CLSA-2026-1791363861 and 15 more.