An issue was discovered in Roundcube Webmail 1.6.0 before 1.6.14. 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. This vulnerability has a CVSS score of 5.4, indicating a medium severity level. Organizations using affected versions should be aware of the potential risks associated with this issue.
Risk to organizations includes unauthorized access to internal resources and exposure of sensitive information through improperly handled CSS in emails. Given the nature of the vulnerability and its exploitation potential, organizations should prioritize patching immediately.
The exploitation status for this vulnerability indicates that there are no known exploits or proof-of-concept code available publicly at this time. However, the risk remains significant due to the potential for SSRF and Information Disclosure. Organizations must remain vigilant and monitor for any updates regarding this vulnerability.
To mitigate the risks associated with CVE-2026-35540, organizations should address this vulnerability in their patch management cycle and ensure that they are running the latest version of Roundcube Webmail.
Vulnerability Details
The official CVE description states that insufficient CSS sanitization in HTML emails can allow for SSRF or Information Disclosure. This vulnerability affects Roundcube Webmail versions 1.6.0 through 1.6.14, making it crucial for users to upgrade to version 1.6.14 or later. The vulnerability is classified under CWE-669, which relates to improper control of a resource through its lifetime.
The CVSS score of 5.4 suggests a medium severity, indicating that while this vulnerability requires specific conditions to be exploited, the potential impact could still be significant. The vulnerability does not require any privileges or user interaction, which further elevates the risk profile.
Technical Analysis
The root cause of this vulnerability lies in the inadequate sanitization of CSS used in HTML emails. Attackers may exploit this flaw through specially crafted email messages that include malicious stylesheet links pointing to local network hosts.
The attack vector is considered network-based, with high attack complexity. No privileges are required for exploitation, and no user interaction is necessary, making it easier for potential attackers to leverage this vulnerability.
The confidentiality and integrity impacts are rated as low, while there is no impact on availability. This means that while sensitive data may be disclosed, the overall system reliability will not be compromised.
Risk & Impact Analysis
Organizations utilizing Roundcube Webmail must recognize the potential risks associated with CVE-2026-35540. The vulnerability could facilitate unauthorized access to internal systems through SSRF, allowing attackers to probe local network resources.
As the CVSS score indicates a medium severity, organizations should address this vulnerability in their priority patch cycle. The potential blast radius for such an exploitation is considerable, especially for organizations that rely on Roundcube for critical communications.
The urgency for remediation is categorized as medium, and organizations are encouraged to schedule necessary updates to mitigate the risk associated with this vulnerability.
Exploitation Status
Signal | Status |
|---|---|
Known Exploit | No |
Public PoC | No |
Actively Exploited | No |
Ransomware Use | No |
Affected Versions
All versions of Roundcube Webmail prior to 1.6.14 are affected by this vulnerability. Users should ensure they upgrade to version 1.6.14 or later to mitigate the risks.
Mitigation & Remediation
To remediate this vulnerability, organizations should apply the latest patches provided by Roundcube. Specifically, upgrading to Roundcube Webmail version 1.6.14 or later is essential. In the absence of immediate patching, organizations may consider implementing additional security controls, such as email filtering and monitoring, to mitigate the impact of potential exploitation.
For more information on security practices, organizations can refer to the penetration testing methodology to ensure comprehensive security measures are in place.
Detection Guidance
Organizations should monitor logs for unusual email patterns and potential exploitation attempts. Behavioral anomalies in email interactions should also be identified promptly. Implementing network signatures for detecting malicious stylesheet links could further enhance security posture.
AppSecure Threat Intelligence Insight
The long-term significance of CVE-2026-35540 highlights an ongoing trend of vulnerabilities related to webmail systems and their handling of dynamic content. Security teams should learn from this incident to better secure their email systems and mitigate similar risks in the future.
Organizations are encouraged to adopt a proactive approach to security by continuously assessing their email infrastructure and ensuring robust security practices are followed. Regular vulnerability assessments and penetration testing can help in identifying and remediating weaknesses before they are exploited.
For further reading on related topics, organizations may find the following resources helpful: vulnerability management program design, penetration testing cost, and security testing best practices to enhance their overall security framework.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
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