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Werkzeug CVE-2026-27199

MEDIUM
Improper Handling of Windows Device Names (CWE-67)
2026-02-21 security-advisories@github.com GHSA-29vq-49wr-vm6x
Windows Werkzeug Suse Palletsprojects
5.3
CVSS 3.1 · GitHub Advisory
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Vulnerability Assessment

Weakness
CWE
CWE-67
OWASP Top 10:2025
A01:2025 - Broken Access Control
ASVS 5.0
V5 - File Handling Improper handling of Windows device names
Impact

CVSS 5.3 (MEDIUM). Attack vector: Network. Complexity: Low. Privileges required: None. User interaction: None.
Exploit ScenarioDerived

An attacker could exploit this vulnerability to compromise the affected system.
This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).
Patch available 0.1% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 12, 2026

Werkzeug versions 3.1.5 and below on Windows fail to properly filter reserved device names in the safe_join function when paths contain multiple segments, allowing attackers to craft requests that trigger indefinite hangs by targeting special device names like NUL. Remote attackers can exploit this denial-of-service vulnerability against applications using send_from_directory to serve user-specified files. A patch is available in version 3.1.6.

Earlier versions of this analysis (1)
v1 Mar 12, 2026 - 22:04 CVSS 5.3

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. [CVSS 5.3 MEDIUM]

RemediationAI

A vendor patch is available — apply it immediately. Fixed in version 3.1.6.. Restrict network access to the affected service where possible.

Affected Packages

pypi werkzeug fix: 3.1.6

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in pypi

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Technical ContextAI · show details

Affects Werkzeug. 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

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Severity by source

GitHub Advisory PRIMARY 5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
SUSE MEDIUM

Primary rating from GitHub Advisory.

CVSS VectorGitHub Advisory

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

Lifecycle Timeline

2
Patch released
Mar 03, 2026 - 17:30 nvd
Patch available
CVE Published
Feb 21, 2026 - 06:17 nvd
MEDIUM 5.3

DescriptionGitHub Advisory

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.

Blast Radius

ecosystem impact
† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth
  • 37 pypi packages depend on werkzeug (33 direct, 4 indirect)

Ecosystem-wide dependent count for version 3.1.6.

Executive ViewAI

Why It Matters

Organizations using Werkzeug should be aware of moderate risk from CVE-2026-27199 rated CVSS 5.3. Exploitation could compromise the security of affected deployments. A patch is available and should be applied during regular vulnerability management.

Business Impact: Low
Compensating Controls

Apply vendor patch. If patching is delayed: restrict network access and enable monitoring.

Threat Intelligence Overview

EPSS Score
0.1%
Top 80% exploitable
Data Quality
4 sources
Confidence: Medium
NVD CVE.org EPSS Vendor GitHub

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity MEDIUM
CISA KEV No
Public POC No
EPSS 0.1%
Patch available Yes
Active threat No
Deciding rule
MEDIUM severity (CVSS 4.0-6.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation· derived
None
System Exposure· derived
Open
Automatable· derived
Yes
Human Impact
Low

Priority Score

26
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +26
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Affected Products (CPE)GitHub Advisory

cpe:2.3:a:palletsprojects:werkzeug:*:*:*:*:*:*:*:*

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Same weakness CWE-67 – Improper Handling of Windows Device Names

View all
CVE-2026-17545 MEDIUM
6.9
CVE-2024-35197 MEDIUM
5.4
CVE-2024-51745 LOW
2.3

Vendor StatusVendor

SUSE

Severity: Medium
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Fixed
SUSE Linux Enterprise High Performance Computing 12 Fixed
SUSE Linux Enterprise High Performance Computing 15 SP7 Fixed
SUSE Linux Enterprise Module for Basesystem 15 SP7 Fixed
SUSE Linux Enterprise Module for Package Hub 15 SP7 Fixed

References

Advisories & patches
PATCH https://github.com/pallets/werkzeug/commit/f407712fdc60a09c2b3f4fe7db557703e5d9338d ADVISORY https://github.com/pallets/werkzeug/security/advisories/GHSA-29vq-49wr-vm6x REF https://github.com/pallets/werkzeug/releases/tag/3.1.6

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