# CVE-2021-28363: Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection

> Attackers can bypass SSL certificate validation when using HTTPS proxies without a custom SSLContext, allowing man-in-the-middle attacks.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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:** 15 Mar 2021
- **Updated:** 14 Oct 2026
- **Also known as:** ALPINE-CVE-2021-28363, CLEANSTART-2026-AK39245, CLEANSTART-2026-MO57102, DEBIAN-CVE-2021-28363, GHSA-5phf-pp7p-vc2r, PYSEC-2021-59, RHBA-2021:1147, ROOT-APP-PYPI-CVE-2021-28363

## What is CVE-2021-28363?

CVE-2021-28363 is a medium-severity vulnerability in Fedoraproject Fedora, published on 15 Mar 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies. The initial connection to the HTTPS proxy (if an SSLContext isn't given via proxy_config) doesn't verify the hostname of the certificate. This means certificates for different servers that still validate properly with the default urllib3 SSLContext will be silently accepted.

Weakness: CWE-295 (Improper Certificate Validation)

## Which versions are affected by CVE-2021-28363?

3 open source packages ship an affected version, including urllib3. It affects 3 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `urllib3` | PyPI | From 1.26.0 | 1.26.4 |
| `urllib3` | PyPI | From 1.26.0 | 1.26.4 |
| `urllib3` | PyPI | From 1.26.0 | 1.26.4 |
| `python-urllib3` | Red Hat:openshift:3.11::el7 | All versions | 1:1.24.3-2.el7 |
| `python2-urllib3` | Red Hat:openshift:3.11::el7 | All versions | 1:1.24.3-2.el7 |

Products: Fedoraproject Fedora, Oracle Peoplesoft Enterprise Peopletools, Python Urllib3.

## How do I fix CVE-2021-28363?

Upgrade urllib3 to 1.26.4 or later.

## Is CVE-2021-28363 being exploited?

There is no known exploitation of CVE-2021-28363 in the wild and no public exploit code.

## Other vulnerabilities in Fedoraproject Fedora

- [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild)
- [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild)
- [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-28363 and fix it if it is affected.

What it is: CVE-2021-28363 is a medium-severity vulnerability in Fedoraproject Fedora, published on 15 Mar 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-2021-28363 in the wild and no public exploit code.

Affected packages and fixed versions:
- urllib3 (PyPI): affected from 1.26.0; fixed in 1.26.4
- urllib3 (PyPI): affected from 1.26.0; fixed in 1.26.4
- urllib3 (PyPI): affected from 1.26.0; fixed in 1.26.4

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-2021-28363/ (Markdown: https://insights.sbomify.com/CVE-2021-28363/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-28363
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-28363
- OSV: https://osv.dev/vulnerability/CVE-2021-28363

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-28363/
Summaries are generated and may contain mistakes; the official description above is the record.