# CVE-2021-47857: Moodle 3.10.3 - 'label' Persistent Cross Site Scripting

> Attackers can inject malicious scripts into Moodle calendar event subtitles, leading to arbitrary code execution when users view the event.

- **Severity:** Medium 5.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 21 Jan 2026
- **Updated:** 5 Mar 2026
- **Also known as:** BIT-moodle-2021-47857, GHSA-px56-6vfj-h8xp

## What is CVE-2021-47857?

CVE-2021-47857 is a medium-severity cross-site scripting vulnerability in Moodle Moodle, published on 21 Jan 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

Moodle 3.10.3 contains a persistent cross-site scripting vulnerability in the calendar event subtitle field that allows attackers to inject malicious scripts. Attackers can craft a calendar event with malicious JavaScript in the subtitle track label to execute arbitrary code when users view the event.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2021-47857?

1 open source package ship an affected version, including moodle. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `moodle` | Bitnami | From 3.10.3 to 3.10.3 | None |

Products: Moodle Moodle.

## How do I fix CVE-2021-47857?

No fix has been published for CVE-2021-47857 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-47857 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-47857 exists.

## Other vulnerabilities in Moodle Moodle

- [CVE-2006-4935](https://insights.sbomify.com/CVE-2006-4935/): File upload in n/a (critical 10.0)
- [CVE-2006-4936](https://insights.sbomify.com/CVE-2006-4936/): Moodle does not properly validate module instance id (critical 10.0)
- [CVE-2004-2233](https://insights.sbomify.com/CVE-2004-2233/): Unknown "front page vulnerability with Moodle servers" for Moodle before 1.3.2 has unknown impact and attack vectors. (critical 10.0)
- [CVE-2004-2235](https://insights.sbomify.com/CVE-2004-2235/): Cross-site scripting in n/a (critical 10.0)
- [CVE-2004-2236](https://insights.sbomify.com/CVE-2004-2236/): Other in n/a (critical 10.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-47857 and fix it if it is affected.

What it is: CVE-2021-47857 is a medium-severity cross-site scripting vulnerability in Moodle Moodle, published on 21 Jan 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-47857 exists.

Affected packages and fixed versions:
- moodle (Bitnami): affected from 3.10.3 up to 3.10.3; 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-2021-47857/ (Markdown: https://insights.sbomify.com/CVE-2021-47857/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47857
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47857
- OSV: https://osv.dev/vulnerability/CVE-2021-47857

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47857/
Summaries are generated and may contain mistakes; the official description above is the record.