# Synacor Zimbra Collaboration Suite (ZCS): Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability · pwn.watch

> Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that 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. Added to CISA KEV on 2025-10-07.

Page: https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915 · generated 2026-10-22T21:08:49.695Z

## Status
- Severity: medium · status: closed · patch: patched
- Product: Synacor Zimbra Collaboration Suite (ZCS)
- Started: 2025-10-07 · closed: 2025-10-07
- CVEs: CVE-2025-27915
- Affected: See vendor advisory / CISA notes (versions not always listed in KEV feed)
- Fixed in: See vendor advisory linked in sources

## Follow this incident
- RSS (one item per update): https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915/feed.xml
- JSON Feed: https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915/feed.json
- Full data (timeline, sources, posts, agentBrief): https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915.json
- This file: https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915/llms.txt

## Updates (newest first)
- 2025-10-07: CISA added CVE-2025-27915 to the KEV catalog (due 2025-10-28).

## Fix brief
# Security brief: Synacor Zimbra Collaboration Suite (ZCS): Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

Source: https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915 · status: closed (closed Oct 7, 2025) · severity: medium · updated Oct 7, 2025

## Patch status
**PATCHED: fixed releases exist. Upgrade to a fixed version.**

## What it is
Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that 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. Added to CISA KEV on 2025-10-07.

Severity detail: CVSS 5.4 (CISA ADP, v3.1); actively exploited (CISA KEV)

- Product: Synacor Zimbra Collaboration Suite (ZCS)
- CVE / advisory ids: CVE-2025-27915

## Affected versions
- See vendor advisory / CISA notes (versions not always listed in KEV feed)

## Check whether you're affected
No generic command applies (the product is a hosted service, appliance or desktop app). Check the installed version in its admin UI / About screen and compare with the versions above.

## Fix
Upgrade to one of these fixed versions (or later on the same branch):
- See vendor advisory linked in sources

After upgrading, re-run the checks above and redeploy/restart the affected services.

## Secrets to rotate
This incident involves possible credential exposure. Rotate any credentials (API keys, tokens, env vars, SSH keys) that were present on affected systems or in affected builds while vulnerable/compromised versions were installed, and review access logs.

## Latest updates
- Oct 7, 2025: CISA added CVE-2025-27915 to the KEV catalog (due 2025-10-28).

## Advisories
- [Synacor advisory: Synacor Zimbra Collaboration Suite (ZCS)](https://wiki.zimbra.com/wiki/Security_Center) (Synacor)
- [NVD: CVE-2025-27915](https://nvd.nist.gov/vuln/detail/CVE-2025-27915) (NVD)
- [CISA Known Exploited Vulnerabilities Catalog](https://www.cisa.gov/known-exploited-vulnerabilities-catalog) (CISA)

## Caveats
Derived from CISA KEV JSON feed catalogVersion 2026.10.21. CVSS from the CVE record (cvelistV5). Exact affected/fixed version strings are not always in the KEV record; verify against the vendor advisory URL. startedAt uses dateAdded (exploitation may have begun earlier; many entries are older CVEs newly confirmed as exploited in 2025).

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Machine-readable: https://pwn.watch/i/synacor-zimbra-collaboration-suite-zcs-cve-2025-27915.json · all incidents: https://pwn.watch/incidents.json. Verify against the vendor advisory before changing production systems.