# CVE-2021-23401: Open Redirect

> Attackers can redirect users to arbitrary URLs via malformed slashes in Flask-User, requiring non-default WSGI server configurations.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 5 Jul 2021
- **Updated:** 16 Sep 2024
- **Also known as:** GHSA-4298-89hc-6rfv, PYSEC-2021-337, SNYK-PYTHON-FLASKUSER-1293188

## What is CVE-2021-23401?

CVE-2021-23401 is a medium-severity vulnerability in Flask-User, published on 5 Jul 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

This affects all versions of package Flask-User. When using the make_safe_url function, it is possible to bypass URL validation and redirect a user to an arbitrary URL by providing multiple back slashes such as /////evil.com/path or \\\evil.com/path. This vulnerability is only exploitable if an alternative WSGI server other than Werkzeug is used, or the default behaviour of Werkzeug is modified using 'autocorrect_location_header=False.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2021-23401?

3 open source packages ship an affected version, including flask-user and Flask-User. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `flask-user` | PyPI | All versions | None |
| `flask-user` | PyPI | All versions to 1.0.2.2 | None |
| `Flask-User` | PyPI | All versions to 1.0.2.2 | None |

Products: Flask-User Project Flask-User.

## How do I fix CVE-2021-23401?

No fix has been published for CVE-2021-23401 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-23401 being exploited?

There is no known exploitation of CVE-2021-23401 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-23401 and fix it if it is affected.

What it is: CVE-2021-23401 is a medium-severity vulnerability in Flask-User, published on 5 Jul 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-23401 in the wild and no public exploit code.

Affected packages and fixed versions:
- flask-user (PyPI): affected all versions; no fix listed
- flask-user (PyPI): affected all versions up to 1.0.2.2; no fix listed
- Flask-User (PyPI): affected all versions up to 1.0.2.2; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-23401/ (Markdown: https://insights.sbomify.com/CVE-2021-23401/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-23401
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-23401
- OSV: https://osv.dev/vulnerability/CVE-2021-23401

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-23401/
Summaries are generated and may contain mistakes; the official description above is the record.