Werkzeug safe_join() allows Windows special device names

Description

Werkzeug's safe_join function allows Windows device names as filenames if when preceded by other path segments.

This was previously reported as https://github.com/pallets/werkzeug/security/advisories/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.

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.

Basic information

Type
reviewed
Severity
medium
Advisory on GitHub
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Repository advisory
Open repository advisory ↗
Source code
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Published (advisory)
2026-02-19 20:32:45 UTC
Updated
2026-02-23 22:27:39 UTC
GitHub reviewed
2026-02-19 20:32:45 UTC
NVD published
2026-02-21

EPSS Score

Score Percentile
0.02% 5.24%

CVSS Scores

Base score Version Severity Vector
6.3 4.0
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 Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network.
Attack complexity (AC:L)
Exploitation conditions are straightforward and stable.
Attack requirements (AT:P)
Additional preconditions must be present for exploitation.
Privileges required (PR:N)
No privileges are required.
User interaction (UI:N)
No user interaction is required.
Vulnerable system confidentiality impact (VC:N)
No confidentiality impact on the vulnerable system.
Vulnerable system integrity impact (VI:N)
No integrity impact on the vulnerable system.
Vulnerable system availability impact (VA:L)
Limited availability impact on the vulnerable system.
Subsequent system confidentiality impact (SC:N)
No confidentiality impact on subsequent systems.
Subsequent system integrity impact (SI:N)
No integrity impact on subsequent systems.
Subsequent system availability impact (SA:N)
No availability impact on subsequent systems.

Identifiers

CWEs

CWE id Name
CWE-67 Improper Handling of Windows Device Names

Credits

  • alimezar (reporter)

Affected packages (1)

Vulnerable version ranges and first patched releases as published by GitHub.

Ecosystem Package Vulnerable range First patched Vulnerable functions
pip werkzeug < 3.1.6 3.1.6

References

cvelogic Threat Intelligence