CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
Synacor · Zimbra Collaboration Suite (ZCS) · NVD published 2025-03-12 · posted 2025-10-07 · last modified 2026-06-17
What it is
An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0 and 10.1. A stored cross-site scripting (XSS) vulnerability exists in the Classic Web Client due to insufficient sanitization of HTML content in ICS files. When a user views an e-mail message containing a malicious ICS entry, its embedded JavaScript executes via an ontoggle event inside a <details> tag. This allows an attacker to run arbitrary JavaScript within the victim's session, potentially leading to unauthorized actions such as setting e-mail filters to redirect messages to an attacker-controlled address. As a result, an attacker can perform unauthorized actions on the victim's account, including e-mail redirection and data exfiltration.
| Weakness | CWE-79 |
|---|---|
| CVSS vector | CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N (source: 134c704f-9b21-4f2e-91b3-4a467353bcc0) |
| CISA due date | 2025-10-28 (for US federal agencies under BOD; a good deadline for everyone) |
| ATT&CK tactics | Initial Access → Execution |
Exploitation evidence
First exploit evidence seen: 2025-10-07 (earliest date across the sources below).
Exploited in the wild
- CISA KEV: listed since 2025-10-07. Source
Sources: CISA KEV.
Defend the Chain
The attack chain for CVE-2025-27915 laid across the MITRE ATT&CK Enterprise tactics. Solid cards come from the MITRE CTID KEV mappings; dashed cards are inferred by CVERadar from the CWE or description. Correct defences are real ATT&CK mitigations for that technique.
Pick a red technique card on the board, then choose the defence that stops it.
Defend against T1189 Drive-by Compromise Inferred
Inferred from CWE CWE-79
How to fix it
CISA required action
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. (source: CISA KEV)
Fixed versions
- synacor zimbra collaboration suite: upgrade to 10.0.13 or later (affected from 10.0.0)
- synacor zimbra collaboration suite: upgrade to 10.1.5 or later (affected from 10.1.0)
Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.
Vendor advisories and patches
- https://wiki.zimbra.com/wiki/Security_Center (CISA KEV notes)
- https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.13#Security_Fixes (Release Notes)
- https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.5#Security_Fixes (Release Notes)
- https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P44#Security_Fixes (Release Notes)
MITRE ATT&CK mitigations for this chain
- M1050 Exploit Protection (for T1189) : Security applications that look for behavior used during exploitation such as Windows Defender Exploit Guard (WDEG) and the Enhanced Mitigation Experience Toolkit (EMET) can be used to mitigate some exploitation behavior. Control flow integrity checking is another way to potentially identify and stop a software exploit
- M1051 Update Software (for T1189) : Ensuring that all browsers and plugins are kept updated can help prevent the exploit phase of this technique. Use modern browsers with security features turned on.
- M1048 Application Isolation and Sandboxing (for T1189) : Browser sandboxes can be used to mitigate some of the impact of exploitation, but sandbox escapes may still exist. Other types of virtualization and application microsegmentation may also mitigate the impact of client-side exploitation. The risks of additional exploits and weaknesses in implementation may still exist
- M1021 Restrict Web-Based Content (for T1189, T1059.007) : Adblockers can help prevent malicious code served through ads from executing in the first place. Script blocking extensions can also help to prevent the execution of JavaScript. Consider disabling browser push notifications from certain applications and browsers.
- M1017 User Training (for T1189) : Train users to be aware of access or manipulation attempts by an adversary to reduce the risk of successful spearphishing, social engineering, and other techniques that involve user interaction.
- M1040 Behavior Prevention on Endpoint (for T1059.007) : On Windows 10, enable Attack Surface Reduction (ASR) rules to prevent [JavaScript](https://attack.mitre.org/techniques/T1059/007) scripts from executing potentially malicious downloaded content .
- M1038 Execution Prevention (for T1059.007) : Denylist scripting where appropriate.
- M1042 Disable or Remove Feature or Program (for T1059.007) : Turn off or restrict access to unneeded scripting components.
Detection Generic guidance
- Initial Access: Alert on unusual requests to the affected service (odd paths, oversized payloads, exploit strings) in WAF, reverse-proxy and application logs.
- Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.
Response checklist Generic guidance
- Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
- Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
- Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
- Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.