CVE-2025-48700: Zimbra Collaboration Suite XSS Vulnerability

CVE-2025-48700 Overview

CVE-2025-48700 is a Cross-Site Scripting (XSS) vulnerability affecting the Zimbra Collaboration Suite (ZCS) Classic UI. The flaw exists in versions 8.8.15, 9.0, 10.0, and 10.1. Attackers exploit the vulnerability by sending a crafted email that executes arbitrary JavaScript in the victim's browser session when viewed in the Classic UI. The issue stems from insufficient sanitization of HTML content, specifically crafted tag structures and attribute values containing an @import directive and other script injection vectors. CISA has added this vulnerability to the Known Exploited Vulnerabilities (KEV) Catalog, confirming active exploitation.

Critical Impact
Attackers can hijack Zimbra webmail sessions, exfiltrate email content, and steal authentication tokens by delivering a single crafted email message. No user interaction beyond opening the email is required.

Affected Products

Zimbra Collaboration Suite 8.8.15 (including patches p1 through p46)
Zimbra Collaboration Suite 9.0.0 (including patches p1 through p42)
Zimbra Collaboration Suite 10.0 and 10.1

Discovery Timeline

2025-06-23 - CVE-2025-48700 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database
Listed - Added to CISA Known Exploited Vulnerabilities Catalog

Technical Details for CVE-2025-48700

Vulnerability Analysis

The vulnerability is a stored/reflected Cross-Site Scripting flaw [CWE-79] in the Zimbra Classic UI mail rendering pipeline. When the Classic UI parses inbound HTML email, it fails to strip or neutralize certain hostile constructs. Attackers embed script payloads inside crafted tag structures and attribute values, including CSS @import directives that pull attacker-controlled stylesheets or scripts. When the recipient views the message, the payload executes inside the origin of the Zimbra webmail application. This grants the attacker access to the authenticated session, allowing mailbox reads, message sending, contact enumeration, and theft of session identifiers. The EPSS probability sits in the upper quartile of published CVEs, and the CISA KEV listing confirms observed in-the-wild abuse.

Root Cause

The root cause is incomplete HTML sanitization in the Classic UI rendering path. The sanitizer allowlist does not fully account for CSS @import directives and specific attribute constructions that permit JavaScript execution. HTML mail bodies pass through the filter with dangerous constructs intact, and the browser interprets them at render time.

Attack Vector

Attack execution is straightforward. An adversary crafts an email containing malicious HTML with an embedded @import directive or script injection vector inside an attribute value. The email is delivered to a Zimbra mailbox. The victim opens the message in the Classic UI, and the browser executes the attacker's JavaScript inside the Zimbra origin. The script can issue authenticated SOAP requests to the Zimbra backend, exfiltrate content over HTTPS to attacker infrastructure, or modify mailbox rules for persistence.

No verified proof-of-concept code is publicly available.
See the Zimbra Security Advisories archive for technical details:
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories

Detection Methods for CVE-2025-48700

Indicators of Compromise

Inbound emails containing HTML <style> blocks with @import directives pointing to external URLs
Outbound HTTP/HTTPS requests from Zimbra webmail sessions to unrecognized domains shortly after a user opens an email
Unexpected mailbox filter rules, forwarders, or delegated access grants created on Zimbra accounts
Access to /service/soap endpoints from user-agent strings and IPs inconsistent with normal user behavior

Detection Strategies

Inspect inbound mail at the gateway for HTML bodies containing @import, expression(, javascript: URIs, and event-handler attributes in unusual tag positions
Correlate Zimbra mailbox.log entries with proxy egress logs to surface anomalous outbound requests originating from webmail sessions
Baseline SOAP API usage per user account and alert on deviations such as bulk message reads or new filter rule creation

Monitoring Recommendations

Forward Zimbra access.log, audit.log, and mailbox.log into a centralized analytics platform such as the Singularity AI SIEM and Data Lake for cross-source correlation
Deploy endpoint telemetry on Zimbra server hosts to identify follow-on activity if the XSS is chained with server-side flaws
Monitor for creation of Zimbra data source, filter, and forwarding configurations outside change windows

How to Mitigate CVE-2025-48700

Immediate Actions Required

Apply the latest Zimbra security patch for your version branch as documented in the Zimbra Security Advisories archive
Direct users to the Modern UI until patching is complete, as the vulnerability is specific to the Classic UI rendering path
Review mailbox filter rules, forwarders, and delegated access grants for all users, prioritizing high-value accounts
Rotate authentication tokens and force reauthentication after patching

Patch Information

Synacor has published fixes through the standard Zimbra patch channel. Refer to the Zimbra Security Center and the Zimbra Security Advisories page for exact patch version numbers per branch. Because the vulnerability affects all supported branches (8.8.15, 9.0.0, 10.0, 10.1), administrators must confirm the installed patch level matches or exceeds the advisory's fixed release.

Workarounds

Disable the Classic UI globally via zmprov and require the Modern UI for all users until patches are applied
Configure inbound mail filtering at the gateway to strip <style> blocks containing @import directives and known XSS attribute patterns
Restrict Zimbra webmail access to VPN or trusted networks to limit exposure while patches are staged

bash

# Disable the Classic UI for a specific account or COS
zmprov modifyCos default zimbraFeatureClassicUIEnabled FALSE
zmprov modifyAccount user@example.com zimbraFeatureClassicUIEnabled FALSE

# Verify current setting
zmprov getCos default | grep -i ClassicUI