# CVE-2021-21333: HTML injection in email and account expiry notifications

> Attackers can inject HTML into Synapse notification emails, potentially forging content or bypassing security controls in missed message notifications.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N`)
- **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:** Yes
- **Published:** 26 Mar 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-c5f8-35qr-q4fm, PYSEC-2021-134, UBUNTU-CVE-2021-21333

## What is CVE-2021-21333?

CVE-2021-21333 is a medium-severity cross-site scripting vulnerability in Fedoraproject Fedora, published on 26 Mar 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

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component)

## Which versions are affected by CVE-2021-21333?

3 open source packages ship an affected version, including matrix-synapse. It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `matrix-synapse` | PyPI | All versions | 1.27.0 |
| `matrix-synapse` | PyPI | All versions | 1.27.0 |
| `matrix-synapse` | PyPI | All versions | 1.27.0 |

Products: Fedoraproject Fedora, Matrix Synapse.

## How do I fix CVE-2021-21333?

Upgrade matrix-synapse to 1.27.0 or later.

## Is CVE-2021-21333 being exploited?

There is no known exploitation of CVE-2021-21333 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-2021-21333 and fix it if it is affected.

What it is: CVE-2021-21333 is a medium-severity cross-site scripting vulnerability in Fedoraproject Fedora, published on 26 Mar 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-2021-21333 in the wild and no public exploit code.

Affected packages and fixed versions:
- matrix-synapse (PyPI): affected all versions; fixed in 1.27.0
- matrix-synapse (PyPI): affected all versions; fixed in 1.27.0
- matrix-synapse (PyPI): affected all versions; fixed in 1.27.0

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-2021-21333/ (Markdown: https://insights.sbomify.com/CVE-2021-21333/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21333
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21333
- OSV: https://osv.dev/vulnerability/CVE-2021-21333

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21333/
Summaries are generated and may contain mistakes; the official description above is the record.