---
title: "CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability"
url: https://erroneo.us/security/cve/CVE-2025-27915/
description: "CVE-2025-27915 (Synacor Zimbra Collaboration Suite (ZCS)): Exploited in the wild (CISA KEV). An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and…"
type: vulnerability-record
section: "Security Digest"
date_published: 2025-03-12
date_modified: 2026-02-26
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2025-27915"
kev: true
cvss: 5.4
epss: 0.04064
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

**Signal:** Exploited: patch now if you run it. CISA has confirmed exploitation in the wild. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Yes, if you run Synacor Zimbra Collaboration Suite (ZCS). CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2025-10-07, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2025-10-28 ; treat that as the outer limit, not the target.
- EPSS 4.1% , percentile 91: FIRST’s model estimates a 4.1% chance of exploitation activity in the next 30 days, higher than 91% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
- Needs an account (low (any account)). Exposure depends on who can log in.

## 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 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.

## Affected products (CVE record)

- n/a n/a: see advisory

## Severity

- CISA-ADP CVSS 3.1: 5.4 `CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N`

## Exploitation

- CISA KEV: added 2025-10-07; federal due date 2025-10-28. Known ransomware use: Unknown.
- 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.
- CISA SSVC assessment: exploitation active , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 4.06% probability of exploitation activity in the next 30 days; percentile 91.

## Sources

- [Listed in CISA KEV notes (wiki.zimbra.com)](https://wiki.zimbra.com/wiki/Security_Center)
- [CVE.org record CVE-2025-27915](https://www.cve.org/CVERecord?id=CVE-2025-27915)
- [NVD entry CVE-2025-27915](https://nvd.nist.gov/vuln/detail/CVE-2025-27915)
- [CISA Known Exploited Vulnerabilities catalog entry](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-27915)
- [wiki.zimbra.com](https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.5#Security_Fixes)
- [wiki.zimbra.com](https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.13#Security_Fixes)
- [wiki.zimbra.com](https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P44#Security_Fixes)
- [strikeready.com](https://strikeready.com/blog/0day-ics-attack-in-the-wild/)

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Source page: https://erroneo.us/security/cve/CVE-2025-27915/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt