# CVE-2026-35539: Roundcube Webmail: Insufficient HTML attachment sanitization in preview mode

> Attackers can execute arbitrary scripts in victim browsers by sending malicious HTML attachments that are improperly sanitized during preview.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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:** 3 Apr 2026
- **Updated:** 3 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-35539, GHSA-x4q5-8j5g-hpjc, UBUNTU-CVE-2026-35539

## What is CVE-2026-35539?

CVE-2026-35539 is a medium-severity cross-site scripting vulnerability in Roundcube Webmail, published on 3 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

An issue was discovered in Roundcube Webmail before 1.5.14 and 1.6.14. XSS exists because of insufficient HTML attachment sanitization in preview mode. A victim must preview a text/html attachment.

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

## Which versions are affected by CVE-2026-35539?

2 open source packages ship an affected version, including roundcube/roundcubemail. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `roundcube/roundcubemail` | Packagist | From 1.7-beta | 1.7-rc5 |
| `roundcube/roundcubemail` | Packagist | From 1.7-beta | 1.7-rc5 |

Products: Roundcube Webmail.

## How do I fix CVE-2026-35539?

Upgrade roundcube/roundcubemail to 1.7-rc5 or later.

## Is CVE-2026-35539 being exploited?

There is no known exploitation of CVE-2026-35539 in the wild and no public exploit code.

## Other vulnerabilities in Roundcube Webmail

- [CVE-2024-42009](https://insights.sbomify.com/CVE-2024-42009/): RoundCube Webmail Cross-Site Scripting (critical 9.3, exploited in the wild)
- [CVE-2025-68461](https://insights.sbomify.com/CVE-2025-68461/): RoundCube Webmail Cross-site Scripting (high 7.2, exploited in the wild)
- [CVE-2008-5619](https://insights.sbomify.com/CVE-2008-5619/): PHPMailer susceptible to arbitrary code execution (critical 10.0)
- [CVE-2024-37385](https://insights.sbomify.com/CVE-2024-37385/): Command injection in n/a (critical 9.8)
- [CVE-2024-42008](https://insights.sbomify.com/CVE-2024-42008/): Cross-site scripting in n/a (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-35539 and fix it if it is affected.

What it is: CVE-2026-35539 is a medium-severity cross-site scripting vulnerability in Roundcube Webmail, published on 3 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: There is no known exploitation of CVE-2026-35539 in the wild and no public exploit code.

Affected packages and fixed versions:
- roundcube/roundcubemail (Packagist): affected from 1.7-beta; fixed in 1.7-rc5
- roundcube/roundcubemail (Packagist): affected from 1.7-beta; fixed in 1.7-rc5

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-2026-35539/ (Markdown: https://insights.sbomify.com/CVE-2026-35539/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-35539
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-35539
- OSV: https://osv.dev/vulnerability/CVE-2026-35539

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-35539/
Summaries are generated and may contain mistakes; the official description above is the record.