# CVE-2020-28724: PYSEC-2020-157 - Open redirect vulnerability in werkzeug before 0.11.6 via a double slash in the URL.

Source: https://junglewise.ai/threats/cve-2020-28724-werkzeug-open-redirect-via-double-slash-in-url  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2020-11-18. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2020-28724 · Severity: low · CVSS 3.1 · Published 2020-11-18

Werkzeug is a Python library that provides utilities for building web applications, including URL routing and request handling. This vulnerability allows an attacker to craft a URL with a double slash (//) in the path that causes the application to redirect users to an arbitrary external website, potentially leading to phishing attacks or credential theft. An attacker can trick users into clicking a malicious link that appears legitimate but redirects them to a fake login page or other malicious site.

Technologies: [werkzeug (PyPI)](https://junglewise.ai/threats/technologies/werkzeug). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2020-28724 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2020-28724 being exploited in the wild.

**How severe is CVE-2020-28724?** Low severity, with a CVSS score of 3.1, published 18 November 2020.

**Which products and versions are affected?** werkzeug (PyPI) ECOSYSTEM:introduced=0,fixed=0.11.6.

**Is there a fix for CVE-2020-28724?** Yes. werkzeug (PyPI): upgrade to 0.11.6 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2020-28724 attacked?** It is attacked over the network.

**Am I affected by CVE-2020-28724?** Check whether you run werkzeug (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- werkzeug (PyPI): upgrade to 0.11.6 or later

  ```sh
  pip install "werkzeug>=0.11.6"
  ```


## Technical details

This is an open redirect vulnerability (CWE-601) in Werkzeug's URL processing logic. The vulnerability occurs when a URL path begins with a double slash (//), which is processed incorrectly by the URL routing and redirect handling code, allowing an attacker to specify an arbitrary redirect destination. The attack requires user interaction (the victim must click the crafted link) but no authentication, and is network-accessible via HTTP requests. An attacker can craft a URL like "http://trusted-app.com//../www.attacker.com" to redirect users to attacker-controlled sites. The vulnerability was fixed in version 0.11.6 and later.

## Affected products

- Pallets Werkzeug before 0.11.6

## Identifiers

- CVE-2020-28724
- GHSA-3p3h-qghp-hvh2
- PYSEC-2020-157

## Timeline

- 2021-04-20: disclosed
- 0.11.6: patched

## References

- [https://github.com/pallets/flask/issues/1639](https://github.com/pallets/flask/issues/1639)

## Related threats

- [Werkzeug path traversal via NUL device with NTFS ADS marker](https://junglewise.ai/threats/cve-2026-102598-werkzeug-is-a-comprehensive-wsgi-web-application-library-prior)
- [PYSEC-2026-2044 - Werkzeug safe_join() allows Windows special device names with compound extensions](https://junglewise.ai/threats/cve-2026-21860-werkzeug-safe-join-allows-windows-special-device-names-with)
- [PYSEC-2026-2045 - Werkzeug safe_join not safe on Windows](https://junglewise.ai/threats/cve-2024-49766-werkzeug-safe-join-not-safe-on-windows)
- [PYSEC-2026-2043 - Werkzeug debugger vulnerable to remote execution when interacting with attacker controlled domain](https://junglewise.ai/threats/cve-2024-34069-werkzeug-debugger-remote-code-execution-via-attacker-controlled)
- [PYSEC-2026-1065 - Pallets Werkzeug vulnerable to Path Traversal](https://junglewise.ai/threats/cve-2019-14322-pallets-werkzeug-vulnerable-to-path-traversal)