CVE-2026-9818: Roundcube Webmail SSRF bypass in HTML sanitization

CVE-2026-9818 · Severity: medium · CVSS 4.7 · Published 2026-05-28

Roundcube Webmail, a popular open-source webmail suite, contains a security flaw in how it handles incoming HTML emails. Even when users have disabled the loading of remote content, an attacker can send a specially crafted email that forces the victim's web browser to make requests to internal network services or the local computer. This could allow an attacker to interact with private internal systems that are otherwise protected from the public internet.

Technologies: Roundcube Webmail. Vendors: Roundcube.

Quick answers

Is CVE-2026-9818 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-9818 being exploited in the wild.
How severe is CVE-2026-9818?
Medium severity, with a CVSS score of 4.7, published 28 May 2026.
Which products and versions are affected?
Roundcube Webmail < 1.6.16, < 1.7.1.
Is there a fix for CVE-2026-9818?
A patch was published on 2026-05-24: Fixed in versions 1.6.16 and 1.7.1 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-9818 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-9818?
Check whether you run 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.

Technical details

A vulnerability exists in Roundcube's HTML sanitization logic (rcube_washtml.php) where it fails to properly block local and private network URLs. While Roundcube typically blocks remote content to protect user privacy, it incorrectly allows URLs pointing to loopback (localhost), private IP ranges (RFC1918), link-local, and Unique Local Addresses (ULA). A remote attacker can exploit this by sending an HTML email; when the victim previews or opens the message, their browser will execute requests to these internal addresses. This effectively facilitates a client-side Server-Side Request Forgery (SSRF) or 'Same-Site' request bypass, potentially allowing interaction with internal services. The issue is fixed in versions 1.6.16 and 1.7.1.

Affected products

  • Roundcube Roundcube Webmail < 1.6.16, < 1.7.1

Timeline

  • 2026-05-24: patched: Fixed in versions 1.6.16 and 1.7.1
  • 2026-05-28: disclosed: CVE published to NVD

References

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