CVE-2022-27926

What is Zimbra Collaboration Suite?

Zimbra Collaboration Suite (ZCS) is a widely deployed open-source email and collaboration platform used by governments, enterprises, and ISPs worldwide. Its web-based email client (Zimbra Web App, or ZWA) is the primary interface for users — and because it handles email from external senders, it is a high-value target for XSS attacks that can steal session tokens and access mailbox contents without requiring credentials.

Overview

CVE-2022-27926 is a reflected cross-site scripting (XSS) vulnerability (CWE-79) in Zimbra Collaboration Suite. The Zimbra webmail endpoint accepts URL parameters that are reflected back in the HTTP response without proper sanitization, allowing an attacker to inject JavaScript that executes in the victim's browser within the Zimbra application context. Exploitation requires the victim to click a malicious link. Winter Vivern (also tracked as TA473 and UAC-0114), a Russian-aligned threat actor, exploited this vulnerability in a campaign targeting NATO government officials, think tanks, and diplomats to steal Zimbra webmail session tokens and access email communications.

Affected Versions

Technical Details

The vulnerability exists in a Zimbra webmail endpoint that reflects unsanitized URL parameters into the HTML response body. An attacker crafts a URL to the Zimbra login page or another Zimbra endpoint that includes a JavaScript payload in a query parameter. When the victim visits this URL (typically via a phishing email), the script executes in their browser with full access to the Zimbra webmail session:

• Type: Reflected XSS (victim must click attacker's link)
• Payload delivery: Embedded in phishing emails sent to the target, masquerading as Zimbra notifications or organizational communications
• Session theft: Injected JavaScript reads the victim's Zimbra session cookie and exfiltrates it to an attacker-controlled server
• Post-exploitation: With the session cookie, the attacker accesses the victim's mailbox, reads emails, exfiltrates attachments, and may pivot using email content

Discovery

The XSS was patched by Zimbra in early April 2022. Proofpoint published research in February 2023 documenting TA473's use of the vulnerability in campaigns targeting European government and NATO-affiliated email accounts — roughly 11 months after the patch, indicating unpatched systems remained exploitable long after the fix was available.

Exploitation Context

Winter Vivern / TA473 is a Russian-aligned espionage actor focused on intelligence collection from European governments, NATO partners, and Ukraine-supporting organizations. In 2022–2023, the group sent phishing emails containing links to attacker-controlled websites that redirected victims to malicious Zimbra URLs triggering CVE-2022-27926. Session tokens stolen from NATO official mailboxes provided access to potentially classified diplomatic communications. The group also exploited CVE-2022-27924 (Zimbra Memcache injection) and other Zimbra vulnerabilities in related campaigns.

Remediation

1. Upgrade Zimbra ZCS to 9.0.0 Patch 20 or 8.8.15 Patch 27 or later
2. Enable Content Security Policy (CSP) headers on the Zimbra web interface to limit XSS impact
3. Configure Zimbra to use HTTPOnly and Secure flags on session cookies
4. Train users to treat unsolicited emails containing Zimbra login links with extreme suspicion
5. Monitor Zimbra access logs for sessions originating from unexpected geographic locations or IP addresses
6. Consider restricting Zimbra webmail access to corporate IPs or VPN-connected users where operationally feasible