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

criticalknown exploitedpublic exploitCVE-2025-27915
Actively exploited. At least one CVE in this advisory is listed in the CISA Known Exploited Vulnerabilities catalog — exploitation has been observed in the wild. Treat remediation as urgent.
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.

CSIRTS triage

vendor: Synacorproduct: Zimbra Collaboration Suite (ZCS)Cross-site scripting
What
Zimbra Collaboration Suite contains a cross-site scripting vulnerability due to insufficient sanitization of HTML content in ICS files.
Who is affected
Users of the Classic Web Client in Zimbra Collaboration Suite.
Urgency
Immediate action is required due to active exploitation and critical severity.
Action
Apply the latest security updates for Zimbra Collaboration Suite.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source
CISA Known Exploited Vulnerabilities (US · database · site)
Severity
critical
Published
2025-10-07
Exploitation
Observed in the wild (CISA KEV)

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2025-27915

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Referenced CVEs

CVECSIRTS overviewExternal
CVE-2025-27915coverage & exploitation statusNVD · CVE.org

Recent advisories for Synacor Zimbra Collaboration

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

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