# CVE-2026-40873: mailcow: dockerized vulnerable to stored XSS in Quarantine attachment filenames

> Attackers can execute arbitrary JavaScript in admin browsers via crafted attachment filenames in the quarantine interface, leading to account takeover.

- **Severity:** High 8.9 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/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:** No
- **Published:** 21 Apr 2026
- **Updated:** 21 Apr 2026
- **Also known as:** GHSA-2xjc-rg88-jvpp

## What is CVE-2026-40873?

CVE-2026-40873 is a high-severity cross-site scripting vulnerability in mailcow-dockerized, published on 21 Apr 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

mailcow: dockerized is an open source groupware/email suite based on docker. In versions prior to 2026-03b, the Quarantine details modal injects attachment filenames into HTML without escaping, allowing arbitrary HTML/JS execution. An attacker can deliver an email with a crafted attachment name so that when an admin views the quarantine item, JavaScript executes in their browser, taking over their account. Version 2026-03b fixes the vulnerability.

Weakness: CWE-80 (Improper Neutralization of Script-Related HTML Tags in a Web Page), CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-40873?


## How do I fix CVE-2026-40873?

No fix has been published for CVE-2026-40873 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-40873 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-40873 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40873 and fix it if it is affected.

What it is: CVE-2026-40873 is a high-severity cross-site scripting vulnerability in mailcow-dockerized, published on 21 Apr 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-2026-40873 exists.

Affected product: mailcow-dockerized. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-40873/ (Markdown: https://insights.sbomify.com/CVE-2026-40873/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40873
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40873
- OSV: https://osv.dev/vulnerability/CVE-2026-40873

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40873/
Summaries are generated and may contain mistakes; the official description above is the record.