VulnerabilityPublished 29 Nov 2025Updated 1 Dec 2025

CVE-2025-66221: pallets werkzeug · Improper Handling of Windows Device Names

Affects pallets werkzeugSource CVE Database V5

Not known to be exploited · CVSS 6.3 Medium : upgrade to 3.1.4 when convenient.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 4.0 base score

6.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.5% chance of exploitation in 30 days – higher than 43% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.1.4

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

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: Present
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: Low
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the Vulnerable System are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 29 Nov 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade pallets werkzeug to 3.1.4.

Affected software

Product	Affected	Fixed in
PyPIwerkzeug`pkg:pypi/werkzeug`Other threats	Affected <3.1.4	Fixed in 3.1.4

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

European organizations should immediately upgrade Werkzeug to version 3.1.4 or later to apply the official patch that properly handles Windows device names in path segments. Until upgrade is possible, organizations can implement input validation on user-supplied file paths to reject requests containing Windows device names such as CON, AUX, PRN, NUL, COM1–COM9, LPT1–LPT9. Web application firewalls (WAFs) can be configured to detect and block HTTP requests with suspicious path segments matching these device names. Monitoring and alerting on application hangs or increased response times can help detect exploitation attempts. Where feasible, consider deploying Werkzeug-based applications on non-Windows platforms to avoid this class of vulnerabilities. Conduct thorough testing of file-serving endpoints to ensure they do not accept or process special device names. Finally, maintain an inventory of affected applications and ensure patch management processes prioritize this update.

Description

Werkzeug is a comprehensive WSGI web application library. Prior to version 3.1.4, Werkzeug's safe_join function allows path segments with Windows device names. On Windows, there are special device names such as CON, AUX, etc that are implicitly present and readable in every directory.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 patched in version 3.1.4.

Impact

For European organizations, this vulnerability primarily poses a risk of denial of service on web applications using vulnerable Werkzeug versions on Windows servers. Exploitation can cause server processes to hang indefinitely, leading to degraded service availability or outages. This can disrupt business operations, especially for public-facing services or internal tools relying on Werkzeug for file serving. While it does not directly compromise confidentiality or integrity, the availability impact can affect customer trust and operational continuity. Organizations in sectors with high reliance on web services, such as finance, healthcare, and government, may face increased operational risk. Additionally, incident response and remediation efforts can incur costs and resource allocation. Since the vulnerability is exploitable remotely without authentication, it increases the attack surface for opportunistic attackers or automated scanning. However, the impact is limited to Windows deployments, so Linux-based Werkzeug applications are not affected.

CWE-67Improper Handling of Windows Device Names (opens in a new tab)
Variant weaknessLikelihood of exploit: High
The product constructs pathnames from user input, but it does not handle or incorrectly handles a pathname containing a Windows device name such as AUX or CON. This typically leads to denial of service or an information exposure when the application attempts to process the pathname as a regular file.
Implementation
Be familiar with the device names in the operating system where your system is deployed. Check input for these device names.
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 6 Dec 2025, 04:32 UTC

WeaknessesCWE-67

Technical details

References

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