CVE-2025-66376 is a high-severity vulnerability affecting Synacor's Zimbra Collaboration Suite (ZCS) versions prior to 10.0.18 and 10.1.13. This vulnerability allows for stored cross-site scripting (XSS) via Cascading Style Sheets (CSS) @import directives embedded within HTML email messages. The potential for exploitation poses a significant risk to organizations utilizing affected versions of this application.
The CVSS score of 7.2 indicates a high severity level. Risk to organizations includes the unauthorized execution of scripts within the context of an affected user's session, potentially leading to the compromise of sensitive information or the deployment of malware. Given its nature, organizations should prioritize patching immediately.
Currently, there are no known public exploits for this vulnerability, but its high CVSS score necessitates vigilance. Organizations should monitor the situation closely and implement mitigations as outlined by the vendor.
The publication of this vulnerability on January 5, 2026, highlights the criticality of timely updates and awareness within the Zimbra user community. As such, it is crucial for defenders to act promptly to secure their installations.
Vulnerability Details
The Zimbra Collaboration Suite (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 contains a stored XSS vulnerability that allows attackers to execute arbitrary scripts in the context of a user’s session. The vulnerability is classified under CWE-79, indicating it involves improper neutralization of input during web page generation. The CVSS metrics indicate a low attack complexity and no privileges are required for exploitation.
This vulnerability could be exploited remotely with no user interaction required, increasing the risk of mass exploitation across organizations utilizing vulnerable versions of ZCS.
Technical Analysis
The root cause of this vulnerability stems from the improper handling of CSS @import directives within HTML email messages. This oversight enables attackers to inject malicious scripts that can be executed in the context of the user's session, resulting in potential data theft or further exploitation.
The attack vector is network-based, allowing exploitation via crafted email messages. The attack complexity is assessed as low, as exploitation can be achieved without any required privileges or user interaction. The potential impacts on confidentiality and integrity are classified as low, while availability is unaffected.
Risk & Impact Analysis
Organizations utilizing the affected versions of Zimbra Collaboration Suite face significant risks. The stored XSS vulnerability can be leveraged to execute arbitrary scripts, leading to the compromise of user accounts and sensitive organizational data. Such exploitation could facilitate unauthorized access to systems and information, severely impacting organizational integrity.
Given the potential blast radius, organizations should assess their exposure to this vulnerability and take immediate action to mitigate risks. The urgency is underscored by its inclusion in the Known Exploited Vulnerabilities (KEV) catalog, indicating a recognized risk and priority for defenders.
Exploitation Status
Signal | Status |
|---|---|
Known Exploit | No |
Public PoC | No |
Actively Exploited | Yes |
Ransomware Use | No |
Affected Versions
The vulnerability affects Zimbra Collaboration Suite versions 10 before 10.0.18 and 10.1 before 10.1.13. Organizations running these versions should take immediate action to apply the necessary patches.
Mitigation & Remediation
Organizations should apply the latest patches provided by Synacor to remediate this vulnerability. Specifically, they should upgrade to ZCS version 10.0.18 or 10.1.13 or later. In cases where immediate patching is not feasible, consider implementing additional security measures such as input validation and sanitization for HTML content.
For further information, organizations can refer to the vendor's security advisory. Regular security testing through penetration testing can also help identify vulnerabilities and ensure the security of the application.
Detection Guidance
To detect potential exploitation of this vulnerability, organizations should monitor logs for unusual script executions and any unauthorized changes to email templates. Additionally, reviewing network traffic for anomalies and implementing behavioral analysis can assist in identifying exploitation attempts.
AppSecure Threat Intelligence Insight
The emergence of CVE-2025-66376 reflects the ongoing challenges organizations face regarding web application security. It underscores the importance of regular updates and the necessity for security teams to adopt proactive measures against XSS vulnerabilities.
Organizations should implement comprehensive security practices, including routine vulnerability assessments and vulnerability management programs to effectively mitigate risks associated with such vulnerabilities.
Additionally, organizations should remain informed about emerging threats and trends in web application vulnerabilities, ensuring they are equipped to respond effectively. Utilizing resources such as penetration testing methodologies can enhance their overall security posture.
In conclusion, the high severity of CVE-2025-66376 necessitates prompt attention and remediation efforts to protect organizational assets from potential exploitation.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
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