CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS)

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

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.

CISA’s summary

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.

Weakness type: CWE-79.

Product: Synacor Zimbra Collaboration Suite (ZCS) · Assigned by: mitre · Published: 2025-03-12

Affected and fixed versions

Vendor	Product	Affected versions (per CVE record)
n/a	n/a	see vendor advisory

Severity scores

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

Attack vector: network (remotely reachable)
Attack complexity: low
Privileges required: low (any account)
User interaction: required (a user must click/open something)
Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

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.

Timeline

2025-03-12 — CVE record published by mitre
2025-10-07 — Added to CISA’s Known Exploited Vulnerabilities catalog
2025-10-28 — CISA remediation deadline for US federal agencies
2026-02-26 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Other references from the CVE record and advisory

wiki.zimbra.com wiki.zimbra.com
wiki.zimbra.com wiki.zimbra.com
wiki.zimbra.com wiki.zimbra.com
strikeready.com strikeready.com

All exploited Synacor vulnerabilities · Today’s Security Digest