CVE-2023-37580
Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability
Description
CVE-2023-37580 is a cross-site scripting (XSS) vulnerability in Zimbra Collaboration Suite (ZCS) version 8 before 8.8.15 Patch 41, specifically affecting the Zimbra Classic Web Client. The flaw allows an attacker to inject malicious scripts into the application, which execute in the context of a victim's browser session when they interact with crafted content. Successful exploitation can lead to session hijacking, credential theft, and unauthorized actions performed on behalf of the victim. CISA has confirmed active exploitation of CVE-2023-37580 in the wild, and the EPSS score of 93.9% (99th percentile) indicates this Zimbra XSS vulnerability is being widely targeted across the internet.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:NOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| synacor | zimbra collaboration suite | >= 8.8.0, < 8.8.15; 8.8.15 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
References
- http://www.openwall.com/lists/oss-security/2023/11/17/2(Mailing List, Patch)
- https://wiki.zimbra.com/wiki/Security_Center(Release Notes)
- https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy(Not Applicable)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-37580(US Government Resource)
Weakness Type
CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
In the context of CVE-2023-37580, the cross-site scripting weakness in Zimbra Collaboration Suite's Classic Web Client allows attackers to inject malicious scripts that are rendered and executed within the browsers of other users. The application fails to properly neutralize user-supplied input before incorporating it into generated web pages, enabling reflected or stored script injection that can compromise user sessions and sensitive data.
Learn more: CWE-79 — Improper Neutralization of Input During Web Page Generation
Impact Analysis
CVE-2023-37580 carries a CVSS 3.1 base score of 6.1 (Medium severity), reflecting the nature of this cross-site scripting vulnerability in Zimbra Collaboration Suite. Attack Vector (Network): The vulnerability is remotely exploitable, allowing attackers to target victims over the internet without physical access. Attack Complexity (Low): Exploitation requires no special conditions — crafting a malicious URL or injecting script content is straightforward. Privileges Required (None): No authentication is needed for the attacker, making this accessible to any external threat actor. User Interaction (Required): A victim must interact with the malicious content, such as clicking a crafted link, which is typically achieved through phishing or social engineering. Scope (Changed): The vulnerability can affect resources beyond the Zimbra application itself, as injected scripts execute within the user's browser context and may access cookies, tokens, or other data from the same origin. Confidentiality and Integrity (Low): While the direct data exposure per exploitation instance is limited, the ability to steal session tokens, read email content, and perform actions as the victim makes the cumulative impact significant. The EPSS score of 93.9% (99th percentile) indicates this is one of the most actively targeted vulnerabilities in the current threat landscape.
Exploit Maturity
CVE-2023-37580 is confirmed as actively exploited in the wild, with CISA adding this Zimbra Collaboration Suite XSS vulnerability to the Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of August 17, 2023. The EPSS score of 93.9% (99th percentile) indicates near-certain exploitation activity, making this one of the most targeted XSS vulnerabilities currently tracked. Multiple threat actors have been observed leveraging this flaw in campaigns targeting government and enterprise email systems. While the ransomware association is listed as unknown, the high exploitation probability and confirmed KEV status demand immediate remediation.
Remediation
- Apply the vendor patch immediately — Upgrade Zimbra Collaboration Suite 8 to version 8.8.15 Patch 41 or later, which addresses the XSS vulnerability in the Classic Web Client. If patching is not feasible, discontinue use of the product as recommended by CISA's KEV required action.
- Migrate away from the Classic Web Client — If possible, transition users to the modern Zimbra web client, which may not be affected by this specific XSS vector. This reduces the attack surface for Classic Web Client-specific vulnerabilities.
- Implement web application firewall (WAF) rules — Deploy WAF rules to detect and block common XSS payloads targeting the Zimbra Classic Web Client. Configure Content Security Policy (CSP) headers to restrict inline script execution and limit the impact of successful injection.
- Review session logs and reset credentials — Inspect Zimbra access logs for suspicious login activity or unusual session patterns that may indicate exploitation. Reset passwords and invalidate sessions for any accounts that may have been compromised, particularly administrative accounts.
- Deploy input validation and output encoding — As a long-term hardening measure, ensure all user-supplied input is validated and sanitized before rendering in HTML contexts. Implement output encoding consistently across the application to prevent future XSS vulnerabilities.
Technical Details
CVE-2023-37580 exploits a cross-site scripting weakness (CWE-79) in the Zimbra Collaboration Suite's Classic Web Client, where user-supplied input is incorporated into rendered HTML pages without proper sanitization or encoding. Vulnerability mechanism: The application fails to neutralize special characters in user input before including it in the web page output, allowing an attacker to inject JavaScript code that executes in the victim's browser session. The CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N) reveals that while user interaction is required (clicking a crafted link), no privileges are needed to launch the attack, and the scope is changed — meaning the injected script can affect resources beyond the vulnerable Zimbra component, such as accessing browser cookies, session tokens, and email content. Exploitation impact: Once the malicious script executes in the victim's browser, the attacker can hijack the authenticated Zimbra session, read and exfiltrate email messages, send emails on behalf of the victim, and potentially pivot to other systems accessible from the victim's browser context.
Frequently Asked Questions
Is CVE-2023-37580 being actively exploited?
Yes, CVE-2023-37580 is confirmed as actively exploited in the wild. CISA added this Zimbra Collaboration Suite XSS vulnerability to the Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of August 17, 2023. The EPSS score of 93.9% (99th percentile) confirms widespread targeting by threat actors.
What products are affected by CVE-2023-37580?
CVE-2023-37580 affects Zimbra Collaboration Suite (ZCS) version 8 before 8.8.15 Patch 41. Specifically, the vulnerability is in the Zimbra Classic Web Client component. Organizations running ZCS 8 without Patch 41 applied should treat this as an urgent priority.
How do I fix CVE-2023-37580?
Upgrade Zimbra Collaboration Suite 8 to version 8.8.15 Patch 41 or later. Additionally, consider migrating users from the Classic Web Client to the modern web interface and deploying WAF rules with Content Security Policy headers to mitigate XSS risks. Review session logs for signs of compromise.
How severe is CVE-2023-37580?
CVE-2023-37580 has a CVSS 3.1 base score of 6.1 (Medium severity). Despite the medium CVSS score, the EPSS score of 93.9% (99th percentile) indicates extremely high exploitation probability. The real-world impact is amplified by the ability to steal email credentials and session tokens from enterprise email systems.
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