# CVE-2026-35540: Roundcube Webmail SSRF via insufficient CSS sanitization

Source: https://junglewise.ai/threats/cve-2026-35540-roundcube-webmail-ssrf-via-insufficient-css-sanitization  
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-2026-35540 · Severity: medium · CVSS 5.4 · Published 2026-04-03

Roundcube Webmail, a popular open-source email client, contains a security flaw in how it handles HTML emails. An attacker can send a specially crafted email that forces the webmail server to make unauthorized requests to internal network resources. This could allow an attacker to view sensitive information from within the organization's private network that is normally protected from the internet.

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-2026-35540 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-35540 being exploited in the wild.

**How severe is CVE-2026-35540?** Medium severity, with a CVSS score of 5.4, published 3 April 2026.

**Which products and versions are affected?** roundcube/roundcubemail (Packagist) ECOSYSTEM:introduced=1.7-beta,fixed=1.7-rc5; Roundcube Webmail 1.6.0 to 1.6.13.

**Is there a fix for CVE-2026-35540?** 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-2026-35540 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-35540?** 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 Server-Side Request Forgery (SSRF) vulnerability exists in Roundcube Webmail versions 1.6.0 through 1.6.13. The issue stems from insufficient sanitization of Cascading Style Sheets (CSS) within HTML email messages. Specifically, the application fails to properly validate or restrict stylesheet links, allowing an attacker to include links pointing to internal local network hosts. When the webmail server processes these emails, it may attempt to fetch these resources, leading to information disclosure or internal network scanning. The vulnerability is addressed in versions 1.6.14 and 1.7-rc5 by improving the CSS sanitization logic.

## Affected products

- Roundcube Webmail 1.6.0 to 1.6.13

## Identifiers

- CVE-2026-35540
- GHSA-vxg2-hhgr-37fx

## Timeline

- 2026-03-18: patched: Security updates 1.6.14 and 1.7-rc5 released.
- 2026-04-03: disclosed: CVE-2026-35540 published.

## References

- [https://github.com/roundcube/roundcubemail/commit/27ec6cc9cb25e1ef8b4d4ef39ce76d619caa6870](https://github.com/roundcube/roundcubemail/commit/27ec6cc9cb25e1ef8b4d4ef39ce76d619caa6870)
- [https://github.com/roundcube/roundcubemail/commit/579b68eff90650a5c782e153debd66c765648942](https://github.com/roundcube/roundcubemail/commit/579b68eff90650a5c782e153debd66c765648942)
- [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)

## Related threats

- [Roundcube Webmail improper authentication in multi-host installations](https://junglewise.ai/threats/cve-2026-114732-roundcube-webmail-before-1-6-20-and-1-7-x-before-1-7-5-is)
- [Roundcube Webmail uncontrolled resource consumption in CSS proxy](https://junglewise.ai/threats/cve-2026-114731-roundcube-webmail-before-1-6-20-and-1-7-x-before-1-7-5-is)
- [Roundcube Webmail server-side request forgery filter bypass](https://junglewise.ai/threats/cve-2026-114730-roundcube-webmail-1-6-14-before-1-6-20-and-1-7-0-before-1-7-5-is)
- [Roundcube Webmail content blocking bypass via SVG SMIL](https://junglewise.ai/threats/cve-2026-114729-roundcube-webmail-before-1-6-20-and-1-7-x-before-1-7-5-is)
- [Roundcube Webmail remote content blocking bypass in CSS FuncIRI handling](https://junglewise.ai/threats/cve-2026-114728-roundcube-webmail-before-1-6-20-and-1-7-x-before-1-7-5-is)