# CVE-2021-29621: Observable Response Discrepancy in Flask-AppBuilder

> An unauthenticated attacker can enumerate valid user accounts by analyzing server response times during login attempts in Flask-AppBuilder.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 Jun 2021
- **Updated:** 3 Aug 2024
- **Also known as:** BIT-airflow-2021-29621, GHSA-434h-p4gx-jm89, PYSEC-2021-90

## What is CVE-2021-29621?

CVE-2021-29621 is a medium-severity vulnerability in Apache Airflow, published on 7 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Flask-AppBuilder is a development framework, built on top of Flask. User enumeration in database authentication in Flask-AppBuilder <= 3.2.3. Allows for a non authenticated user to enumerate existing accounts by timing the response time from the server when you are logging in. Upgrade to version 3.3.0 or higher to resolve.

Weakness: CWE-203 (Observable Discrepancy)

## Which versions are affected by CVE-2021-29621?

4 open source packages ship an affected version, including airflow, Flask-AppBuilder and flask-appbuilder. It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `airflow` | Bitnami | From 1.10.0 | 1.10.1 |
| `Flask-AppBuilder` | PyPI | All versions | 3.3.0 |
| `flask-appbuilder` | PyPI | All versions | 3.3.0 |
| `flask-appbuilder` | PyPI | All versions | 3.3.0 |

Products: Apache Airflow, Dpgaspar Flask-Appbuilder.

## How do I fix CVE-2021-29621?

Upgrade airflow to 1.10.1 or later.

## Is CVE-2021-29621 being exploited?

There is no known exploitation of CVE-2021-29621 in the wild and no public exploit code.

## Other vulnerabilities in Apache Airflow

- [CVE-2020-13927](https://insights.sbomify.com/CVE-2020-13927/): Apache Airflow's Experimental API Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2020-11978](https://insights.sbomify.com/CVE-2020-11978/): Apache Airflow Command Injection (high 8.8, exploited in the wild)
- [CVE-2026-33264](https://insights.sbomify.com/CVE-2026-33264/): Apache Airflow: DAG author RCE on webserver via unrestricted import_string() in BaseSerialization.deserialize() (critical 9.8)
- [CVE-2023-25754](https://insights.sbomify.com/CVE-2023-25754/): Apache Airflow: Privilege escalation using airflow logs (critical 9.8)
- [CVE-2023-22884](https://insights.sbomify.com/CVE-2023-22884/): Apache Airflow, Apache Airflow MySQL Provider: Arbitrary file read via MySQL provider in Apache Airflow (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29621 and fix it if it is affected.

What it is: CVE-2021-29621 is a medium-severity vulnerability in Apache Airflow, published on 7 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-29621 in the wild and no public exploit code.

Affected packages and fixed versions:
- airflow (Bitnami): affected from 1.10.0; fixed in 1.10.1
- Flask-AppBuilder (PyPI): affected all versions; fixed in 3.3.0
- flask-appbuilder (PyPI): affected all versions; fixed in 3.3.0
- flask-appbuilder (PyPI): affected all versions; fixed in 3.3.0

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-2021-29621/ (Markdown: https://insights.sbomify.com/CVE-2021-29621/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29621
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29621
- OSV: https://osv.dev/vulnerability/CVE-2021-29621

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29621/
Summaries are generated and may contain mistakes; the official description above is the record.