# CVE-2026-54889: Unsanitized URL schemes in MDEx Quill Delta output allow javascript: injection (XSS)

> Attackers can inject JavaScript via unsanitized URL schemes in Markdown links, leading to XSS when rendered by downstream clients.

- **Severity:** Medium 5.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:L/VI:L/VA:N/SC:L/SI:L/SA: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:** 29 Jun 2026
- **Updated:** 30 Jun 2026
- **Also known as:** EEF-CVE-2026-54889, GHSA-4383-7xfp-gpph

## What is CVE-2026-54889?

CVE-2026-54889 is a medium-severity cross-site scripting vulnerability in leandrocp mdex, published on 29 Jun 2026. 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

Improper Neutralization of Input During Web Page Generation (XSS) vulnerability in leandrocp mdex allows cross-site scripting via unsanitized URL schemes in Quill Delta output.

'Elixir.MDEx':to_delta/2 converts Markdown into a Quill Delta. 'Elixir.MDEx.DeltaConverter':default_convert_node/3 in lib/mdex/delta_converter.ex copies the URL of a link, wikilink, or image node directly from the parsed Markdown into the Delta "link" or "image" attribute without applying a scheme allowlist or any normalization.

An attacker who controls the Markdown text can supply a javascript: URL (for example [click](javascript:alert(document.cookie))) that survives verbatim into the Delta attribute. When the Delta is rendered to HTML by a downstream renderer (such as quill-delta-to-html or the Quill client), the attribute becomes an <a href> or <img src>, and the javascript: scheme executes in the browser of anyone who views the rendered content. The link and wikilink cases are the strongest vectors because javascript: in an href executes on click; the image case is lower impact because javascript: in <img src> generally does not execute in modern browsers.

This issue affects mdex: from 0.8.3 before 0.13.2.

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

## Which versions are affected by CVE-2026-54889?

1 open source package ship an affected version, including mdex.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mdex` | Hex | From 0.8.3 | 0.13.2 |

## How do I fix CVE-2026-54889?

Upgrade mdex to 0.13.2 or later.

## Is CVE-2026-54889 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54889 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54889 and fix it if it is affected.

What it is: CVE-2026-54889 is a medium-severity cross-site scripting vulnerability in leandrocp mdex, published on 29 Jun 2026. An attacker can exploit it over the internet without logging in, 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-2026-54889 exists.

Affected packages and fixed versions:
- mdex (Hex): affected from 0.8.3; fixed in 0.13.2

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-2026-54889/ (Markdown: https://insights.sbomify.com/CVE-2026-54889/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54889
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54889
- OSV: https://osv.dev/vulnerability/CVE-2026-54889

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-54889/
Summaries are generated and may contain mistakes; the official description above is the record.