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Known exploited vulnerability · CISA KEV

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

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.

CVE-2025-27915

Exploited in the wild. Fix this first.

CISA added it to the Known Exploited Vulnerabilities catalog on 2025-10-07: attackers are using it right now.

CISA KEV
Listed
Added 2025-10-07 · federal fix-by 2025-10-28
EPSS (30 days)
4.1%
Higher than 91% of CVEs · 2026-11-01
CVSS
No score

What CISA says

Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Got a whole scan? Scan Hub runs this check on every finding in your Nessus, Qualys, Rapid7 or OpenVAS export and ranks the report by it.

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How to check if you are exposed to CVE-2025-27915

  1. Find every Synacor Zimbra Collaboration Suite (ZCS) system you or your clients run, including ones exposed to the internet. Those come first: CISA only lists flaws attackers are already using.
  2. Compare each version with the fixed versions in the vendor advisory below. Where no fix exists yet, apply the vendor’s mitigation or take the system off the internet.
  3. Run a credentialed vulnerability scan to catch the systems you missed, then check that the next scan no longer reports CVE-2025-27915.

US federal agencies had to fix this by 2025-10-28. Everyone else should treat that deadline as a guide: if it has passed, this is overdue.

Timeline

  1. · Added to CISA's Known Exploited Vulnerabilities catalog
  2. · Fix-by date for US federal agencies, passed 369 days ago

Vendor advisories and fixes

Weakness type

More exploited Synacor vulnerabilities

CISA lists 19 exploited Synacor vulnerabilities. The closest matches:

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Sources: CISA Known Exploited Vulnerabilities catalog (version 2026.10.30); EPSS data from FIRST.org; CVSS and descriptions from the NIST National Vulnerability Database. Checked 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

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