CVE-2023-43770 is a cross-site scripting (XSS) vulnerability identified in the Roundcube webmail client, specifically affecting versions before 1.4.14, 1.5.x before 1.5.4, and 1.6.x before 1.6.3. The vulnerability stems from the way the rcube_string_replacer.php script processes text/plain email messages containing crafted links. This improper sanitization allows an attacker to inject malicious scripts that execute in the context of the victim's browser when they open a specially crafted email. The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), indicating that user-supplied input is not correctly sanitized before being rendered. The CVSS v3.1 base score is 6.1, with the vector indicating network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), user interaction required (UI:R), scope changed (S:C), and impacts on confidentiality and integrity (C:L/I:L) but no impact on availability (A:N). This means an attacker can exploit the vulnerability remotely without authentication but requires the user to open the malicious email. Successful exploitation can lead to theft of session cookies, unauthorized actions within the webmail session, or phishing attacks leveraging the victim's session context. No public exploits or active exploitation campaigns have been reported yet. The vulnerability affects multiple branches of Roundcube, a widely used open-source webmail client, often deployed by ISPs, hosting providers, and enterprises for email access.

For European organizations, this vulnerability poses a moderate risk primarily to confidentiality and integrity of email communications and user sessions. Exploitation could enable attackers to hijack user sessions, steal sensitive email content, or perform actions on behalf of the user within the webmail interface. This could lead to data leakage, unauthorized access to internal communications, and potential lateral movement within networks if email credentials or session tokens are compromised. Organizations relying on Roundcube for email access, especially in sectors like government, education, and SMEs, may face increased risk of targeted phishing campaigns leveraging this vulnerability. The lack of known exploits reduces immediate risk, but the ease of exploitation and widespread deployment of Roundcube necessitate prompt remediation. Additionally, the vulnerability could be leveraged as part of multi-stage attacks to gain footholds in corporate environments. The scope change in the CVSS vector indicates that exploitation can affect resources beyond the vulnerable component, potentially impacting broader systems if session tokens are reused or if webmail integrates with other internal services.

The primary mitigation is to upgrade Roundcube installations to versions 1.4.14, 1.5.4, or 1.6.3 or later, where the vulnerability has been addressed. Organizations should verify their current Roundcube version and apply patches promptly. In parallel, implement strict email content filtering to detect and quarantine suspicious emails containing crafted links or unusual text/plain content. Employ Content Security Policy (CSP) headers on the webmail server to restrict script execution and reduce the impact of XSS attacks. Educate users to be cautious when opening unexpected or suspicious emails, especially those containing links. Consider sandboxing or isolating webmail access environments to limit session token exposure. Monitor webmail logs for unusual activity indicative of exploitation attempts. If upgrading is delayed, disable or restrict the rendering of text/plain emails or links within them as a temporary workaround. Regularly review and update web application firewall (WAF) rules to detect and block XSS payloads targeting Roundcube. Finally, ensure session management uses secure, HttpOnly cookies to reduce the risk of session hijacking.