VulnerabilityPublished 8 Jan 2026

CVE-2026-21860: 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 3.1.5 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: 0.4% chance of exploitation in 30 days – higher than 40% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.1.5

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 8 Jan 2026
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 3.1.5.

Affected software

Product	Affected	Fixed in
palletswerkzeug	Affected <3.1.5	Fixed in 3.1.5
PyPIwerkzeug`pkg:pypi/werkzeug`Other threats	Affected <3.1.5	Fixed in 3.1.5

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

AI-suggested

Upgrade Werkzeug to version 3.1.5 or later immediately to apply the official patch that correctly handles Windows device names in path segments.
Audit all Python web applications running on Windows servers to identify usage of Werkzeug and verify versions.
Implement additional input validation and sanitization on file path parameters at the application level to detect and reject suspicious device name patterns (e.g., CON, AUX, PRN) with extensions or trailing spaces.
Employ runtime application self-protection (RASP) or web application firewalls (WAFs) configured to detect and block requests containing Windows device names in path inputs.
Monitor application logs for unusual file access attempts involving reserved device names.
Educate development teams about Windows-specific path handling quirks and the importance of secure path joining functions.
Consider isolating Windows-hosted web applications in hardened environments with strict access controls to limit impact if exploitation occurs.

Description

Werkzeug's `safe_join` function allows path segments with Windows device names that have file extensions or trailing spaces. On Windows, there are special device names such as `CON`, `AUX`, etc that are implicitly present and readable in every directory. Windows still accepts them with any file extension, such as `CON.txt`, or trailing spaces such as `CON `. This was previously reported as https://github.com/pallets/werkzeug/security/advisories/GHSA-hgf8-39gv-g3f2, but the fix failed to account for compound extensions such as `CON.txt.html` or trailing spaces. It also missed some additional special names. `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.

Impact

For European organizations, this vulnerability primarily affects web applications running on Windows servers that utilize Werkzeug versions prior to 3.1.5. Exploitation could allow attackers to bypass path validation mechanisms, potentially leading to unauthorized access or manipulation of special device files. This can result in denial of service conditions or unintended behavior in web applications, impacting availability. While confidentiality and integrity impacts are not explicitly noted, the ability to manipulate file paths could be leveraged in complex attack chains. Organizations relying on Werkzeug in Windows environments, especially those hosting critical web services, may face service disruptions or exploitation attempts. The medium severity rating reflects a moderate risk, but the absence of known exploits reduces immediate threat. However, given the common use of Werkzeug in Python web frameworks and the prevalence of Windows servers in enterprise environments, the vulnerability warrants prompt remediation to prevent potential exploitation.

Summary

Werkzeug is a widely used WSGI web application library in Python environments. The vulnerability identified as CVE-2026-21860 (CWE-67) affects Werkzeug versions prior to 3.1.5 in its safe_join function, which is designed to securely concatenate file system paths. On Windows, certain device names such as CON, AUX, PRN, NUL, and others are reserved system devices that can be accessed implicitly in any directory. Windows also permits these device names to be used with file extensions or trailing spaces (e.g., CON.txt or CON ). Werkzeug's safe_join function did not correctly handle these special device names with extensions or trailing spaces, allowing crafted path segments to bypass intended path validation and normalization. This improper handling could enable an attacker to reference or manipulate special device files, potentially leading to unauthorized file access, denial of service, or other impacts depending on application logic. The vulnerability is exploitable remotely over the network without requiring authentication or user interaction, as indicated by the CVSS vector (AV:N/AC:L/PR:N/UI:N). The impact is limited to availability (VA:L) with no confidentiality or integrity impact reported. The issue was patched in Werkzeug version 3.1.5, which properly sanitizes and rejects such Windows device name path segments. No public exploits have been observed, but the vulnerability poses a risk for Windows-hosted web applications using vulnerable Werkzeug versions.

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 8 Jan 2026, 19:09 UTC

WeaknessesCWE-67

Technical details

References

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