Synacor Zimbra Collaboration Suite (ZCS) — Cross-Site Scripting in Classic UI (CVE-2025-48700)

Severity Assessment

• Exploitability: 6.5/10
• Impact: 5.5/10
• Weaponization Risk: 6.0/10
• Patch Urgency: 8.0/10
• Detection Coverage: 4.5/10

Summary

CVE-2025-48700 is a cross-site scripting vulnerability in the Classic UI of Synacor Zimbra Collaboration Suite (ZCS) affecting versions 8.8.15, 9.0, 10.0, and 10.1. The vulnerability stems from insufficient sanitization of HTML content, specifically crafted tag structures and attribute values containing @import directives and related script injection vectors. An attacker sends a malicious email containing the crafted payload; when the target views the message in the Zimbra Classic UI, arbitrary JavaScript executes within the victim’s authenticated browser session. No additional user interaction beyond viewing the email is required.

Synacor published patches on 2025-06-23: 9.0.0 Patch 43, 10.0.12, 10.1.4, and 8.8.15 Patch 47. Confirmed exploitation was reported in April 2026, approximately ten months after the patch release. CISA added CVE-2025-48700 to the Known Exploited Vulnerabilities catalog on 2026-04-20 with a required remediation deadline of 2026-04-23, confirming that threat actors have actively exploited this vulnerability in the wild.

The CVSS 3.1 base score is 6.1 MEDIUM (AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N), reflecting network accessibility with no privilege requirement, changed scope, and a requirement for user interaction in the form of viewing the email.

Exploit Chain

Stage 1: Craft Malicious Email with XSS Payload

The attacker constructs an email containing HTML content with crafted tag structures and attribute values. The payload leverages @import directives and other script injection vectors that survive Zimbra’s HTML sanitization in the Classic UI. The attacker delivers the email to the target Zimbra mailbox through any available channel, including direct send or relay through a compromised mail infrastructure.

Stage 2: Victim Views Email in Classic UI

When the target opens or previews the malicious email in the Zimbra Classic UI, the browser renders the HTML body. The insufficient sanitization allows the crafted tag structures to pass through and execute the embedded JavaScript payload within the victim’s authenticated browser session. The vulnerable rendering path is specific to the Classic UI; the vulnerability does not require any additional clicks or file downloads beyond viewing the message.

Stage 3: Arbitrary JavaScript Execution in Authenticated Session

The attacker-controlled JavaScript runs with the full privileges of the victim’s Zimbra session. This enables a range of follow-on actions depending on the payload: exfiltrating session tokens or cookies to an attacker-controlled server, reading mailbox content including sensitive emails or attachments, sending emails as the victim, accessing contacts, or redirecting the session to a credential-harvesting page. The changed scope (S:C) in the CVSS vector reflects that the impact extends beyond the originating component to the user’s broader browser context.

Detection Guidance

1. Audit Zimbra deployments and identify any instances running versions prior to 9.0.0 Patch 43, 10.0.12, 10.1.4, or 8.8.15 Patch 47, treating unpatched instances as actively exploitable given confirmed KEV status.
2. Review Zimbra Classic UI access logs for sessions that made unusual outbound requests or exhibited signs of automated in-session actions (bulk email reads, contact exports, rapid navigation) immediately after a user opened an email from an external sender.
3. Inspect mail logs and message headers for emails containing encoded or obfuscated HTML payloads referencing @import directives, style attribute injection, or unusual tag nesting patterns that could represent XSS delivery.
4. Correlate Zimbra session activity with user behavior baselines: flag sessions where the session performed actions inconsistent with normal user patterns shortly after mail delivery from untrusted senders.
5. Review outbound network connections from Zimbra server hosts or client browsers in the period following the CISA KEV add date (2026-04-20) for exfiltration indicators such as unexpected POST requests carrying session token-sized payloads to external IP addresses.
6. Verify that the LC attribute zimbra_owasp_strip_alt_tags_with_handlers, introduced in earlier patches as a temporary mitigation, has been removed and replaced by the permanent fix in the applicable patch levels.

Indicators of Compromise

Behavioral indicators in Zimbra logs:

• Classic UI sessions performing automated-looking sequences (bulk reads, exports, redirects) immediately after a specific email open event
• Session tokens appearing in outbound HTTP requests to external destinations not associated with Zimbra infrastructure
• Server-side mail rendering errors or unusual HTML sanitization bypass patterns in application logs

Email content indicators:

• Inbound HTML email bodies containing @import directives within style attributes or tag structures
• Crafted attribute values using unusual character encoding, nested tags, or CSS injection vectors in HTML email parts

Disclosure Timeline

2025-06-23

Synacor published patches for CVE-2025-48700: ZCS 9.0.0 Patch 43, 10.0.12, 10.1.4, and 8.8.15 Patch 47. The National Vulnerability Database record was also published on this date. Credit for the discovery was given to lebr0nli (Alan Li).

2026-04-20

CISA added CVE-2025-48700 to the Known Exploited Vulnerabilities catalog, confirming active exploitation approximately ten months after the patch was released. CISA set a required remediation deadline of 2026-04-23 for Federal Civilian Executive Branch agencies.

Sources & References

• National Vulnerability Database: CVE-2025-48700 — National Vulnerability Database, 2025-06-23
• Cybersecurity and Infrastructure Security Agency: CVE-2025-48700 Known Exploited Vulnerabilities Entry — Cybersecurity and Infrastructure Security Agency, 2026-04-20
• Synacor: Zimbra Security Advisories — Synacor, 2025-06-23