# Werkzeug safe_join() allows Windows special device names with compound extensions

Advisory record · `GHSA-87hc-h4r5-73f7`

- **Identifiers:** `GHSA-87hc-h4r5-73f7` · `CVE-2026-21860` · `PYSEC-2026-2044`
- **State:** Confirmed
- **Severity:** CVSS 6.3 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-67](https://cwe.mitre.org/data/definitions/67.html)
- **Tags:** [CWE-67](https://agentprime.io/advisories/tag/cwe-67.md)
- **Published:** 8 Jan 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 8 Jan 2026, 19:51 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `werkzeug` (PyPI): affected `<3.1.5`; fixed in `3.1.5`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Werkzeug versions before 3.1.5 have a flaw in safe_join, which does not reject Windows device names such as CON or AUX when they carry a file extension or trailing spaces. The source says that when an application on Windows uses send_from_directory and a requested path ends in such a name, reading the file hangs indefinitely; the attacker needs only to be able to request a path from that application. Upgrade to Werkzeug 3.1.5 or later.

Written 5 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 6.3 Medium, CVSS 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`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0047, the 41st percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:34 UTC: First seen by agentprime
- 31 Oct 2026, 14:34 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-87hc-h4r5-73f7)

## Sources

- [GitHub Advisory Database: GHSA-87hc-h4r5-73f7](https://github.com/advisories/GHSA-87hc-h4r5-73f7)
- [OSV: GHSA-87hc-h4r5-73f7](https://osv.dev/vulnerability/GHSA-87hc-h4r5-73f7)
- <https://github.com/pallets/werkzeug/security/advisories/GHSA-87hc-h4r5-73f7>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-21860>
- <https://github.com/pallets/werkzeug/commit/7ae1d254e04a0c33e241ac1cca4783ce6c875ca3>
- <https://github.com/pallets/werkzeug>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-87hc-h4r5-73f7>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-87hc-h4r5-73f7/>. Everything for agents: <https://agentprime.io/llms.txt>.