CVE-2026-62643 in Roundcube
Summary
by MITRE • 07/14/2026
In Roundcube Webmail before 1.6.17 and 1.7.x before 1.7.2, insufficient Cascading Style Sheets (CSS) sanitization in HTML e-mail messages may lead to SSRF or Information Disclosure, e.g., if stylesheet links point to local network hosts. NOTE: this issue exists because of insufficient fixes for CVE-2026-35540 and CVE-2026-48843.
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Analysis
by VulDB Data Team • 07/15/2026
This vulnerability affects Roundcube Webmail versions prior to 1.6.17 and 1.7.x before 1.7.2, where inadequate CSS sanitization mechanisms in HTML email processing create significant security risks. The flaw stems from insufficient validation of Cascading Style Sheets within email content, allowing malicious actors to craft specially formatted emails that exploit weaknesses in the application's content filtering systems. When users view affected emails, the webmail client processes embedded CSS rules that may contain references to local network resources, creating potential attack vectors for server-side request forgery and information disclosure attacks.
The vulnerability represents a critical flaw in input validation and content sanitization practices, aligning with CWE-1021 which describes insufficient restriction of XML External Entity references. Attackers can leverage this weakness by embedding CSS rules that reference internal network hosts or local resources through URL schemes such as file:// or http://localhost, enabling them to probe internal network infrastructure and potentially extract sensitive information from the server environment. This issue specifically builds upon previous vulnerabilities CVE-2026-35540 and CVE-2026-48843, indicating a recurring pattern in the application's approach to handling external resources within email content.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can enable attackers to perform server-side request forgery attacks that may allow them to access internal services or resources that would normally be restricted from external access. Network reconnaissance becomes possible through these crafted CSS references, potentially revealing internal IP addresses, service endpoints, or other sensitive infrastructure details. The attack surface includes not only direct information disclosure but also potential privilege escalation scenarios where attackers can leverage the compromised webmail client to gain insights into the underlying server architecture and network topology.
Organizations using affected Roundcube versions should prioritize immediate patching to address this vulnerability, as the security implications extend beyond simple email viewing. The mitigations should include comprehensive CSS sanitization rules that prevent processing of external resource references within email content, particularly those that could resolve to local network addresses or internal services. Security teams should implement network-level controls to block access to internal resources from webmail servers and consider deploying web application firewalls to monitor and filter suspicious CSS content patterns. Additionally, regular security assessments should verify that all CSS processing components properly validate external resource references to prevent similar vulnerabilities from emerging in future updates.
The vulnerability demonstrates the importance of maintaining robust input validation mechanisms within web applications that process untrusted content, particularly when dealing with HTML email rendering where attackers can exploit complex parsing behaviors to achieve unexpected outcomes. This issue reinforces the need for comprehensive security testing that includes evaluating how applications handle CSS rules and external resource references within email contexts, as these elements often receive less scrutiny than core application functionality but can provide significant attack vectors. Organizations should also consider implementing email filtering solutions at network boundaries to prevent potentially malicious emails from reaching user inboxes, thereby reducing exposure to such vulnerabilities regardless of patch status.