CVE-2023-5631 is a stored cross-site scripting vulnerability identified in the Roundcube Webmail client, affecting versions before 1.4.15, 1.5.5, and 1.6.4. The root cause lies in the improper neutralization of input during web page generation, specifically in the handling of HTML emails containing crafted SVG documents. The vulnerable component is program/lib/Roundcube/rcube_washtml.php, which fails to adequately sanitize SVG content embedded within emails. This flaw allows an attacker to embed malicious JavaScript code inside an email message that, when opened by a user in the vulnerable Roundcube client, executes in the context of the victim's browser session. The attack vector is remote and does not require prior authentication, but it does require the user to open or preview the crafted email, thus involving user interaction. The vulnerability impacts confidentiality and integrity by enabling session hijacking, cookie theft, or unauthorized actions performed with the victim's privileges. The CVSS 3.1 score is 6.1, reflecting medium severity with network attack vector, low attack complexity, no privileges required, user interaction required, and a scope change due to the potential to affect other components or data. No known exploits are currently reported in the wild, but the presence of this vulnerability in widely deployed open-source webmail software poses a significant risk if left unpatched. The vulnerability underscores the importance of robust input validation and output encoding in web applications handling complex content types like SVG within emails.

For European organizations, the exploitation of CVE-2023-5631 could lead to unauthorized access to sensitive email content, session hijacking, and potential lateral movement within internal networks if attackers leverage stolen credentials or session tokens. Given that Roundcube is a popular open-source webmail client used by many institutions, including governmental, educational, and private sectors across Europe, the risk of data leakage and compromise of confidential communications is significant. Attackers could use this vulnerability to implant persistent malicious scripts, potentially leading to phishing campaigns, credential theft, or further exploitation of internal systems. The impact is heightened in environments where webmail is a critical communication tool and where users may not be trained to recognize malicious emails. Additionally, the scope change indicated in the CVSS vector suggests that the vulnerability could affect other components or services integrated with Roundcube, amplifying the potential damage. Disruption of email services or loss of trust in communication channels could also have operational and reputational consequences for affected organizations.

1. Immediately upgrade Roundcube installations to versions 1.4.15, 1.5.5, or 1.6.4 or later, where the vulnerability has been patched. 2. If upgrading is not immediately feasible, disable SVG rendering in emails by configuring Roundcube to strip or block SVG content or by disabling HTML email rendering temporarily. 3. Implement strict Content Security Policy (CSP) headers to restrict the execution of inline scripts and limit the domains from which scripts can be loaded. 4. Educate users to be cautious when opening emails from unknown or untrusted sources, especially those containing HTML or SVG content. 5. Monitor webmail logs for unusual activity that could indicate exploitation attempts, such as repeated access to crafted emails or anomalous session behaviors. 6. Employ web application firewalls (WAFs) with rules targeting XSS payloads in HTTP requests and responses related to Roundcube. 7. Regularly audit and sanitize email content at the mail server level to detect and block malicious payloads before delivery to end users. 8. Review and harden the overall email infrastructure, including spam filtering and anti-malware scanning, to reduce the likelihood of malicious emails reaching users.