CVE-2026-26079 in Roundcube
Summary
by MITRE • 02/11/2026
Roundcube Webmail before 1.5.13 and 1.6 before 1.6.13 allows Cascading Style Sheets (CSS) injection, e.g., because comments are mishandled.
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Analysis
by VulDB Data Team • 05/03/2026
The vulnerability identified as CVE-2026-26079 represents a critical cross-site scripting weakness in Roundcube Webmail software that affects versions prior to 1.5.13 and 1.6.13. This issue stems from improper handling of CSS comments within the webmail application's rendering engine, creating an avenue for malicious actors to inject arbitrary CSS code into web pages. The flaw manifests when the application processes user-supplied content that includes CSS comments, which are typically intended to be ignored by browsers but are being processed in a manner that allows for code injection.
The technical implementation of this vulnerability involves the application's failure to properly sanitize CSS input during the parsing phase, particularly when comments are present in the stylesheet definitions. When users interact with the webmail interface, the system processes CSS content without adequate validation of comment syntax, allowing attackers to craft malicious payloads that exploit the CSS parsing engine. This creates a vector where attackers can inject CSS code that executes within the context of other users' browsing sessions, potentially leading to session hijacking, data exfiltration, or further exploitation of the webmail environment.
From an operational perspective, this vulnerability poses significant risks to organizations relying on Roundcube Webmail for their email services. The cascading style sheets injection allows attackers to manipulate the visual presentation of webmail interfaces, potentially redirecting users to malicious sites or stealing sensitive information through visual deception techniques. The impact extends beyond simple visual disruption as the injected CSS can be used to manipulate user interface elements, making it difficult for users to distinguish between legitimate and malicious content. This vulnerability aligns with CWE-116, which addresses improper encoding or escaping of output, and represents a classic case of insufficient input validation in web applications.
The security implications of this vulnerability are particularly concerning given the widespread adoption of Roundcube Webmail across enterprise and organizational environments. Attackers exploiting this weakness can potentially execute malicious CSS code that targets specific user sessions, leveraging the application's trust relationship with users to deliver payloads that bypass traditional security controls. The vulnerability's classification under ATT&CK technique T1531, which covers "Modify Existing Service", demonstrates how this flaw can be leveraged to compromise the integrity of webmail services. Organizations using affected versions face heightened risk of credential theft, data leakage, and unauthorized access to sensitive communications stored within the webmail environment.
Mitigation strategies should focus on immediate patching of affected Roundcube installations to versions 1.5.13 or 1.6.13, which contain proper CSS comment handling and input sanitization. Network administrators should implement additional security controls including content security policy headers, regular security audits of webmail configurations, and monitoring for unusual CSS content patterns. The implementation of web application firewalls and input validation controls can provide additional layers of protection while awaiting full patch deployment. Organizations should also conduct comprehensive vulnerability assessments of their webmail environments to identify any potential exploitation attempts and ensure proper security configurations are in place to prevent similar vulnerabilities from emerging in other components of their email infrastructure.