CVE-2025-48700
Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
Description
CVE-2025-48700 is a HIGH severity cross-site scripting (XSS) vulnerability in Synacor Zimbra Collaboration Suite (ZCS), one of the most widely deployed open-source enterprise email and collaboration platforms used by government agencies, universities, healthcare organizations, and corporations worldwide. The vulnerability allows an attacker to inject and execute arbitrary JavaScript within the context of a victim user's authenticated Zimbra session, enabling unauthorized access to sensitive email content, calendar data, contacts, and session credentials. XSS in email platforms is particularly dangerous because the attack surface is the email inbox itself — a trusted application context that users interact with constantly and rarely treat with suspicion. CISA added CVE-2025-48700 to the Known Exploited Vulnerabilities (KEV) catalog on April 20, 2026, with a remediation due date of April 23, 2026 — a three-day Emergency Directive-level window that reflects confirmed, active, time-sensitive exploitation. Zimbra XSS vulnerabilities have a well-documented history of being used in targeted spear-phishing and account takeover campaigns by nation-state threat actors, including APT groups documented by Google TAG and ESET. The EPSS score is 0.22405, placing CVE-2025-48700 at the 95.9th percentile — in the top 5% of all CVEs by predicted exploitation probability, an exceptionally high ranking for an XSS vulnerability.
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 | >= 10.0.0, < 10.0.12; >= 10.1.0, < 10.1.4; 8.8.15; 9.0.0 |
References
- https://wiki.zimbra.com/wiki/Security_Center(Release Notes)
- https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy(Product)
- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories(Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-48700(US Government Resource)
Weakness Type
CWE-79: Improper Neutralization of Input During Web Page Generation (Cross-site Scripting)
CWE-79, cross-site scripting (XSS), occurs when a web application incorporates user-controllable or externally supplied data into web page output without properly escaping or sanitizing it, allowing an attacker to inject client-side scripts (typically JavaScript) that execute in the browser's security context of the target web application. Reflected XSS executes when the victim is induced to follow a crafted link; stored (persistent) XSS executes when malicious content is stored in the application's database and rendered for any user who views the affected page; DOM-based XSS occurs entirely in the browser without the malicious payload touching the server. In email platforms, stored XSS is the most common and impactful variant because a malicious email can deliver the XSS payload to any recipient who opens it in the webmail interface.
In CVE-2025-48700, the XSS vulnerability in Zimbra Collaboration Suite enables attackers to execute arbitrary JavaScript within the context of a victim's authenticated Zimbra session. Zimbra's webmail interface operates under the same origin as the full collaboration suite, meaning injected JavaScript has access to the victim's email, contacts, calendar, authentication tokens, and any Zimbra REST API operations the victim's session is authorized to perform. Historically, Zimbra XSS vulnerabilities have been exploited to steal authentication tokens, extract email content at scale, add persistent backdoors (by modifying Zimbra filters or delegated access settings), and propagate the attack to additional victims by sending XSS-laden emails from the compromised account. Learn more: CWE-79 — Improper Neutralization of Input During Web Page Generation
Impact Analysis
CVE-2025-48700 is rated HIGH severity. The following analysis is based on the vulnerability's technical characteristics, exploitation history of analogous Zimbra XSS vulnerabilities, and the 95.9th percentile EPSS score.
CVSS 3.1 Contextual Vector: AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N — Score: approximately 8.2 (HIGH)
Attack Vector (Network): Zimbra is a web-delivered email platform; the attack is delivered via email or another network channel to the victim's browser.
Attack Complexity (Low): XSS delivery via email in a webmail platform is technically straightforward. The attacker composes a message containing the XSS payload and sends it to the victim; no race conditions or complex pre-conditions are required.
Privileges Required (None): The attacker does not need any credentials on the Zimbra server. Any external party who can send email to a Zimbra user can deliver the XSS payload.
User Interaction (Required): The victim must open the malicious email in the Zimbra webmail interface for the XSS to execute. This is the primary barrier — however, email is a trusted communication channel and phishing emails routinely achieve high open rates, particularly in targeted attacks.
Scope (Changed): The injected script executes in the victim's browser within the Zimbra web application's origin, but its effects extend beyond a single resource — the compromised session grants access to the entire Zimbra collaboration suite.
Confidentiality (High): Successful exploitation gives the attacker full read access to the victim's email inbox, sent items, contacts, calendar events, and any attachments — including confidential communications, credentials shared via email, and sensitive organizational data. Session token theft enables persistent access even after the XSS payload has executed.
Integrity (Low to Medium): The attacker can send emails as the victim, modify calendar entries, alter Zimbra settings (e.g., add email forwarding rules for persistent exfiltration), and delegate access to the attacker's own account.
Prioritization Guidance: The 95.9th percentile EPSS, three-day KEV due date, confirmed nation-state exploitation of analogous Zimbra XSS vulnerabilities, and the zero-credential delivery mechanism via email make this a critical priority for all Zimbra deployments.
Exploit Maturity
Active Exploitation Confirmed: CISA added CVE-2025-48700 to the KEV catalog on April 20, 2026, with a three-day due date of April 23, 2026. This is CISA's most urgent response tier, confirming active exploitation against real targets. The short deadline reflects both confirmed exploitation and the assessed risk level to critical infrastructure.
EPSS — 95.9th Percentile: The EPSS score of 0.22405 (95.9th percentile) is exceptionally high for an XSS vulnerability, which as a class tends to score lower than memory corruption or authentication bypass flaws. This elevated percentile reflects the well-documented attacker interest in Zimbra as an exploitation target, the relatively low barrier to exploitation (deliver via email, no credentials needed), and the high value of email account compromise as an initial access vector.
Nation-State Exploitation History: Zimbra XSS vulnerabilities have been documented as attack tools in targeted campaigns attributed to multiple nation-state threat actors. Google TAG (Threat Analysis Group) and ESET have published research on APT groups exploiting Zimbra XSS zero-days in targeted campaigns against government, military, and defense sector organizations in Europe and Central Asia. These campaigns use XSS to steal authentication cookies from Zimbra sessions, enabling persistent email access without requiring the victim's password. CVE-2025-48700 fits this established exploitation pattern.
Ransomware: No ransomware campaigns have been specifically attributed to CVE-2025-48700. Zimbra XSS exploitation has historically been used for espionage, credential theft, and persistent mailbox access rather than ransomware deployment. However, compromised email accounts are frequently used as pivot points for business email compromise (BEC) and supply-chain phishing.
Public PoC: No confirmed public proof-of-concept exploit was available as of April 24, 2026. Nation-state actors operating in this space typically do not publish PoC code, preferring to maintain operational advantage.
Remediation
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Comply with the CISA KEV three-day deadline. CISA listed CVE-2025-48700 with a remediation due date of April 23, 2026 — the same urgent timeline applied to Emergency Directives. Federal agencies must remediate by this date. All organizations running Zimbra Collaboration Suite should treat this as an emergency and prioritize immediate patching.
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Apply the Zimbra security patch. Install the Zimbra hotfix or upgrade to the patched ZCS version specified in the Zimbra Security Advisory for CVE-2025-48700. Zimbra security patches for XSS vulnerabilities are typically distributed as hotfix JARs applicable without a full upgrade. Follow the official Zimbra upgrade and hotfix documentation at wiki.zimbra.com.
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Enable Zimbra proxy and restrict direct access. Ensure all Zimbra access occurs via Zimbra Proxy (nginx-based reverse proxy) rather than direct connections to the mailbox server. Zimbra Proxy can enforce additional input validation and provides a security layer between clients and the mailbox application.
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Review Zimbra account settings for persistence indicators. XSS in Zimbra has historically been used to add email forwarding rules, delegated access grants, and filters that persist after the XSS session ends. Audit all user accounts for unexpected forwarding rules (Preferences > Mail > Filters and Accounts > External Accounts), delegated access settings, and recently added trusted devices or app-specific passwords.
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Monitor for malicious email delivery and anomalous webmail activity. Inspect email gateway logs for messages containing suspicious HTML or JavaScript in the body — particularly messages with obfuscated script tags, event handler injections (onerror, onload attributes), or SVG/IMG tags with embedded script. Review Zimbra access logs for unusual patterns such as bulk email access, API calls to export mailbox content, or changes to account settings performed during a session where normal user activity would not be expected.
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Harden Zimbra's Content Security Policy and input sanitization. Verify that Zimbra's built-in HTML sanitization for email rendering is active and up to date. For organizations that can enforce it, configuring a restrictive Content-Security-Policy header via Zimbra Proxy reduces the impact of XSS by blocking inline script execution and external script loads. Apply Zimbra's security hardening guide to limit the JavaScript APIs accessible within the webmail context.
Technical Details
CVE-2025-48700 is a cross-site scripting (XSS) vulnerability (CWE-79) in Synacor Zimbra Collaboration Suite (ZCS), a widely deployed enterprise email, calendar, and collaboration platform.
Zimbra Architecture Context: Zimbra ZCS is a Java-based collaboration server with a rich Ajax web client (Zimbra Web Client, ZWC) delivered as a single-page application. Email content is rendered within the ZWC's origin, subject to an HTML sanitization layer designed to strip or neutralize potentially dangerous HTML tags and attributes from email bodies before rendering them in the browser. The XSS vulnerability in CVE-2025-48700 indicates a bypass or gap in this sanitization layer — a specific HTML construct, attribute combination, or encoding technique that the sanitizer fails to neutralize, allowing injected script to execute in the ZWC's browser context.
XSS in Zimbra — Historical Pattern: Zimbra has been affected by multiple XSS vulnerabilities over the years, several of which have been exploited by nation-state threat actors. The attack pattern is consistent: the attacker sends a crafted email to the victim's Zimbra address; when the victim opens the email in the Zimbra webmail interface, the unsanitized content renders in the browser and executes the attacker's JavaScript. The JavaScript can: (1) extract the victim's auth token from cookies or local storage, (2) make same-origin Zimbra REST API calls to export email content, (3) add persistent email forwarding rules that survive the XSS session, (4) use fetch() or XMLHttpRequest to exfiltrate data to an attacker-controlled server, or (5) send further XSS-laden emails from the victim's account to propagate the attack.
Specific Mechanism: The exact sanitization bypass technique for CVE-2025-48700 is described in the Zimbra Security Advisory. Common bypass classes in email HTML sanitizers include: malformed HTML that browsers repair into executable form after sanitization, MIME-type-based content sniffing vulnerabilities, SVG namespace injections (SVG elements can contain script elements that some sanitizers miss), CSS-based script execution (expression() in IE-compatible parsers), and use of HTML entities or encoding to obfuscate script payloads past pattern-matching sanitizers.
CVSS Context: Based on the attack pattern (network delivery, no privileges, user interaction required, changed scope, high confidentiality impact), a CVSS 3.1 score in the 8.0–8.5 HIGH range is consistent with Zimbra's own severity classification and with analogous XSS CVEs in prior Zimbra versions. Refer to the official Zimbra Security Advisory for the authoritative score and affected version list.
Frequently Asked Questions
Is CVE-2025-48700 being actively exploited?
Yes. CISA listed CVE-2025-48700 in the KEV catalog on April 20, 2026, with a three-day remediation deadline, confirming active exploitation. Zimbra XSS vulnerabilities have a well-documented history of exploitation by nation-state threat actors for email account compromise and espionage, and CVE-2025-48700 fits this established attack pattern.
What products are affected?
Synacor Zimbra Collaboration Suite (ZCS) is affected. Consult the Zimbra Security Advisory for the specific ZCS versions that contain the vulnerability. Both the open-source Zimbra community edition and commercial Zimbra Network Edition may be affected depending on version.
How do I fix it?
Apply the Zimbra hotfix or upgrade to the patched ZCS version specified in the official Zimbra Security Advisory. As immediate interim steps: review all user accounts for unexpected email forwarding rules or delegated access (common post-exploitation persistence mechanisms), monitor email gateway logs for XSS payload delivery patterns, and restrict Zimbra access to trusted IP ranges where possible.
How severe is it?
CVE-2025-48700 is rated HIGH severity. The 95.9th percentile EPSS score, three-day CISA KEV deadline, zero-credential delivery mechanism via email, and the history of nation-state exploitation of Zimbra XSS make this a critical operational risk. Email account compromise via XSS gives attackers persistent read access to all mailbox content, the ability to send email as the victim, and a platform for further phishing and espionage activity.
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