CVE-2026-9818
HTML Sanitization Bypass in Roundcube Webmail
Publication date: 2026-05-28
Last updated on: 2026-05-28
Assigner: 6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd
Description
Description
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| roundcube | roundcubemail | 1.7.1 |
| roundcube | roundcubemail | 1.6.16 |
Helpful Resources
Exploitability
| CWE ID | Description |
|---|---|
| CWE-184 | The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete. |
Attack-Flow Graph
AI Powered Q&A
How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:
The vulnerability in Roundcube Mail allowed unauthorized fetching of local or private URLs even when remote resources were restricted, potentially enabling unauthorized access to internal systems.
Such unauthorized access could lead to exposure or misuse of sensitive data, which may impact compliance with data protection standards and regulations like GDPR and HIPAA that require strict controls on data access and protection.
However, the provided information does not explicitly state the direct impact of this vulnerability on compliance with these standards.
Can you explain this vulnerability to me?
CVE-2026-9818 is a security vulnerability in Roundcube Mail's HTML sanitization process for message rendering. It allows an attacker to bypass restrictions and cause the victim's browser to make requests to local or private network URLs (such as loopback, localhost, RFC1918, link-local, and ULA addresses) even when remote content loading is disabled.
This happens because the sanitization code incorrectly permits local or private URLs, enabling a remote attacker to send a crafted HTML email that triggers these requests simply by the victim opening the message preview.
How can this vulnerability impact me? :
This vulnerability can impact you by allowing a remote attacker to make your browser send requests to internal or private network services without your consent. This can lead to unauthorized access or information disclosure from internal systems that are normally protected from external access.
While the vulnerability does not directly compromise confidentiality or availability, it can be used as a stepping stone for further attacks by interacting with internal services, potentially exposing sensitive information or enabling further exploitation.
What immediate steps should I take to mitigate this vulnerability?
To mitigate this vulnerability, you should update your Roundcube Webmail installation to a version that includes the security fix.
- Upgrade to Roundcube Webmail version 1.7.1 or later if you are using the 1.7.x stable branch.
- If you are using the Long-Term Support (LTS) 1.6 branch, upgrade to version 1.6.16 or later.
These updates address the issue where local or private URLs could be fetched even when remote resources were restricted, preventing unauthorized access to internal systems.
Additionally, it is recommended to back up your data before applying the update.