---
title: "CVE-2024-27443: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability"
url: https://erroneo.us/security/cve/CVE-2024-27443/
description: "CVE-2024-27443 (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: 2024-08-12
date_modified: 2025-10-21
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2024-27443"
kev: true
cvss: 6.1
epss: 0.24606
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2024-27443: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) 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 2025-05-19, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2025-06-09 ; treat that as the outer limit, not the target.
- EPSS 24.6% , percentile 98: FIRST’s model estimates a 24.6% chance of exploitation activity in the next 30 days, higher than 98% of all scored CVEs (very high). EPSS is a probability, not a verdict.
- Needs a user to click or open something. Phishing makes that realistic, but it is not wormable.

## What it is

An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0. A Cross-Site Scripting (XSS) vulnerability exists in the CalendarInvite feature of the Zimbra webmail classic user interface, because of improper input validation in the handling of the calendar header. An attacker can exploit this via an email message containing a crafted calendar header with an embedded XSS payload. When a victim views this message in the Zimbra webmail classic interface, the payload is executed in the context of the victim's session, potentially leading to execution of arbitrary JavaScript code.

## Affected products (CVE record)

- n/a n/a: see advisory

## Severity

- CISA-ADP CVSS 3.1: 6.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`

## Exploitation

- CISA KEV: added 2025-05-19; federal due date 2025-06-09. 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): 24.61% probability of exploitation activity in the next 30 days; percentile 98.

## Sources

- [Listed in CISA KEV notes (wiki.zimbra.com)](https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P46#Security_Fixes)
- [Listed in CISA KEV notes (wiki.zimbra.com)](https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P39#Security_Fixes)
- [Listed in CISA KEV notes (wiki.zimbra.com)](https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.7#Security_Fixes)
- [CVE.org record CVE-2024-27443](https://www.cve.org/CVERecord?id=CVE-2024-27443)
- [NVD entry CVE-2024-27443](https://nvd.nist.gov/vuln/detail/CVE-2024-27443)
- [CISA Known Exploited Vulnerabilities catalog entry](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2024-27443)
- [welivesecurity.com](https://www.welivesecurity.com/en/eset-research/operation-roundpress/)

---
Source page: https://erroneo.us/security/cve/CVE-2024-27443/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt