CVE-2026-26079 is a vulnerability classified under CWE-829, indicating the inclusion of functionality from an untrusted control sphere, specifically in Roundcube Webmail versions prior to 1.5.13 and 1.6 before 1.6.13. The vulnerability allows Cascading Style Sheets (CSS) injection due to improper handling of comments within the webmail interface. An attacker can craft malicious input that, when rendered by the victim's browser, injects CSS code. This CSS injection can be leveraged to perform attacks such as information leakage through CSS-based side channels, UI manipulation, or potentially exfiltrating data by exploiting browser behaviors. The vulnerability does not require authentication (PR:N) but does require user interaction (UI:R), such as opening a malicious email or clicking a crafted link. The attack vector is network-based (AV:N), meaning the attacker can exploit it remotely. The CVSS v3.1 base score is 4.7, reflecting a medium severity level with low complexity and no privileges required but limited impact on integrity and availability. No known exploits are currently reported in the wild, but the vulnerability poses a risk to confidentiality. The root cause is mishandling of comments in CSS processing within the Roundcube Webmail client, which allows untrusted CSS code to be injected and executed in the context of the user's session. This vulnerability affects a widely used open-source webmail client, which is popular among organizations for internal and external email access.

For European organizations, the primary impact of CVE-2026-26079 lies in the potential confidentiality breach. Attackers exploiting this CSS injection vulnerability can manipulate the webmail interface to leak sensitive information, such as email content or session tokens, through CSS-based side-channel attacks. While the vulnerability does not directly compromise integrity or availability, the exposure of confidential information can lead to further attacks, including phishing, account takeover, or lateral movement within networks. Organizations relying on Roundcube Webmail for critical communications may face reputational damage and compliance issues, especially under GDPR regulations that mandate protection of personal data. The requirement for user interaction reduces the likelihood of widespread automated exploitation but targeted spear-phishing campaigns could be effective. The absence of known exploits in the wild currently lowers immediate risk, but the presence of a public CVE and medium severity score means attackers may develop exploits soon. European sectors such as government, finance, and healthcare, which often use open-source solutions like Roundcube, could be particularly sensitive to this threat due to the sensitive nature of their communications.

1. Upgrade Roundcube Webmail to version 1.5.13 or later, or 1.6.13 or later, as soon as patches become available to address this vulnerability. 2. In the interim, implement strict Content Security Policies (CSP) that restrict the execution of inline styles and external CSS from untrusted sources to mitigate CSS injection risks. 3. Sanitize and validate all user inputs and email content that may be rendered in the webmail client, focusing on comment handling and CSS-related inputs. 4. Educate users about the risks of interacting with suspicious emails or links, emphasizing caution with unexpected or unsolicited messages. 5. Monitor webmail logs and network traffic for unusual patterns that may indicate attempts to exploit CSS injection or related attacks. 6. Employ web application firewalls (WAF) with custom rules to detect and block CSS injection payloads targeting Roundcube interfaces. 7. Regularly review and update security configurations and perform penetration testing focused on webmail interfaces to identify residual injection risks. 8. Coordinate with IT and security teams to ensure timely patch management and incident response readiness specific to webmail platforms.