# CVE-2026-35539: Roundcube Webmail XSS in HTML attachment preview

Source: https://junglewise.ai/threats/cve-2026-35539-roundcube-webmail-xss-in-html-attachment-preview  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-03. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-35539 · Severity: medium · CVSS 6.1 · Published 2026-04-03

Roundcube Webmail, a widely used open-source web-based email client, is vulnerable to a security flaw that could allow an attacker to execute malicious scripts in a user's browser. This occurs when a user previews a specially crafted HTML email attachment. If exploited, an attacker could potentially steal session cookies, access sensitive email data, or perform actions on behalf of the user within the webmail interface.

Technologies: [roundcube/roundcubemail (Packagist)](https://junglewise.ai/threats/technologies/roundcube-roundcubemail), [Roundcube Webmail](https://junglewise.ai/threats/technologies/webmail). Vendors: [Packagist](https://junglewise.ai/threats/vendors/packagist), [Roundcube](https://junglewise.ai/threats/vendors/roundcube).

## Quick answers

**Is CVE-2026-35539 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-35539 being exploited in the wild.

**How severe is CVE-2026-35539?** Medium severity, with a CVSS score of 6.1, published 3 April 2026.

**Which products and versions are affected?** roundcube/roundcubemail (Packagist) ECOSYSTEM:introduced=1.7-beta,fixed=1.7-rc5; Roundcube Webmail < 1.5.14, 1.6.x < 1.6.14, 1.7-rc < 1.7-rc5.

**Is there a fix for CVE-2026-35539?** Yes. roundcube/roundcubemail (Packagist): upgrade to 1.7-rc5 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-35539 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-35539?** Check whether you run roundcube/roundcubemail (Packagist) or Roundcube Webmail at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- roundcube/roundcubemail (Packagist): upgrade to 1.7-rc5 or later

  ```sh
  composer require roundcube/roundcubemail:^1.7-rc5
  ```


## Technical details

A stored cross-site scripting (XSS) vulnerability exists in Roundcube Webmail's attachment preview functionality. The root cause is insufficient sanitization of HTML content within the `display_uploaded_file` method in `rcmail_action.php` and `rcube_uploads.php`. An attacker can send an email with a malicious `text/html` attachment; when the recipient uses the 'preview' feature, the unsanitized script executes in the context of the user's session. This can lead to session hijacking or unauthorized data access. The issue is resolved in versions 1.5.14, 1.6.14, and 1.7-rc5.

## Affected products

- Roundcube Webmail < 1.5.14, 1.6.x < 1.6.14, 1.7-rc < 1.7-rc5

## Identifiers

- CVE-2026-35539
- GHSA-x4q5-8j5g-hpjc

## Timeline

- 2026-03-18: patched: Security updates released for 1.5, 1.6, and 1.7 branches.
- 2026-04-03: disclosed: CVE published to NVD.

## References

- [https://github.com/roundcube/roundcubemail/commit/10a6d1fa8acac85c727b0a6ae4a6642bfa27bea1](https://github.com/roundcube/roundcubemail/commit/10a6d1fa8acac85c727b0a6ae4a6642bfa27bea1)
- [https://github.com/roundcube/roundcubemail/commit/1b30edf5369668c92fe91dae3d52e477c808aa4f](https://github.com/roundcube/roundcubemail/commit/1b30edf5369668c92fe91dae3d52e477c808aa4f)
- [https://github.com/roundcube/roundcubemail/commit/d742954ccbcdee7020f8f2e7c49ce0fca5a0efab](https://github.com/roundcube/roundcubemail/commit/d742954ccbcdee7020f8f2e7c49ce0fca5a0efab)
- [https://github.com/roundcube/roundcubemail/releases/tag/1.5.14](https://github.com/roundcube/roundcubemail/releases/tag/1.5.14)
- [https://github.com/roundcube/roundcubemail/releases/tag/1.6.14](https://github.com/roundcube/roundcubemail/releases/tag/1.6.14)
- [https://github.com/roundcube/roundcubemail/releases/tag/1.7-rc5](https://github.com/roundcube/roundcubemail/releases/tag/1.7-rc5)
- [https://roundcube.net/news/2026/03/18/security-updates-1.7-rc5-1.6.14-1.5.14](https://roundcube.net/news/2026/03/18/security-updates-1.7-rc5-1.6.14-1.5.14)

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