CVE-2026-21860: Werkzeug Werkzeug Safe_join() Allows Windows Special Vulnerability

MODERATE
Advisory Identifier: GHSA-87hc-h4r5-73f7
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CVSS 5.3
CWE-67
Published
Advisory Summary

CVE-2026-21860 is a MODERATE (CVSS 5.3) werkzeug safe_join() allows windows special vulnerability (CWE-67: Improper Handling of Windows Device Names) affecting Werkzeug libraries in the PyPI ecosystem. The issue involves werkzeug safe_join() allows windows special device names with compound extensions. Remediation is available by upgrading affected packages to version 3.1.5.

CVE-2026-21860 at a Glance

CVE Identifier CVE-2026-21860
GHSA Identifier GHSA-87hc-h4r5-73f7
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-67: Improper Handling of Windows Device Names
Affected Product Werkzeug
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability Low (A:L)

How to Fix CVE-2026-21860

To remediate this vulnerability, upgrade all affected Werkzeug dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

Werkzeug fixed: 3.1.5 
pip install Werkzeug>=3.1.5
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show Werkzeug

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-21860, inspect your installed packages using pip:

pip show Werkzeug

Analyze the terminal output to locate any version of Werkzeug in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.1.5), your application is affected.

Note: The pip show Werkzeug command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-67: Improper Handling of Windows Device Names, where 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. In Werkzeug, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

CVE-2026-21860 Affected Werkzeug Versions

1 component(s)
Fixed Versions:
3.1.5
Product / Package Ecosystem Affected Versions Fixed Version
Werkzeug
pypi/Werkzeug
PyPI
< 3.1.5
3.1.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/pallets/werkzeug/security/advisories/GHSA-87hc-h4r5-73f7
NVD — CVE-2026-21860 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-21860
Security Fix Commit FIX COMMIT
https://github.com/pallets/werkzeug/commit/7ae1d254e04a0c33e241ac1cca4783ce6c875ca3
Werkzeug Source Repository REPOSITORY
https://github.com/pallets/werkzeug

Frequently Asked Questions

What is CVE-2026-21860?
CVE-2026-21860 is a security vulnerability in Werkzeug involving Werkzeug safe_join() allows Windows special device names with compound extensions under CWE-67: Improper Handling of Windows Device Names.
Which Werkzeug versions are affected?
Affected versions include releases prior to 3.1.5 across affected packages: Werkzeug.
What version fixes CVE-2026-21860?
Upgrading to 3.1.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is Werkzeug affected?
Yes, Werkzeug is affected in releases prior to 3.1.5. Upgrade your dependency to 3.1.5 or later.
What is the CVSS score of CVE-2026-21860?
CVE-2026-21860 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2026-21860?
CVE-2026-21860 is classified under CWE-67: Improper Handling of Windows Device Names (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.).
How can I check PyPI dependencies for CVE-2026-21860?
Run the dependency tree command: 'pip show Werkzeug'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-87hc-h4r5-73f7",
  "modified" : "2026-02-02T19:57:31Z",
  "published" : "2026-01-08T19:51:21Z",
  "aliases" : [ "CVE-2026-21860" ],
  "summary" : " Werkzeug safe_join() allows Windows special device names with compound extensions",
  "details" : "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 `.\n\nThis 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.\n\n`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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  }, {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "Werkzeug"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.1.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/pallets/werkzeug/security/advisories/GHSA-87hc-h4r5-73f7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21860"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pallets/werkzeug/commit/7ae1d254e04a0c33e241ac1cca4783ce6c875ca3"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/pallets/werkzeug"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-67" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-08T19:51:21Z",
    "nvd_published_at" : "2026-01-08T19:15:59Z"
  }
}