CVE-2025-48700: 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 2026-04-20, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-04-23; treat that as the outer limit, not the target.
EPSS 1.7%, percentile 77: FIRST’s model estimates a 1.7% chance of exploitation activity in the next 30 days, higher than 77% of all scored CVEs (moderate). 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) 8.8.15 and 9.0 and 10.0 and 10.1. A Cross-Site Scripting (XSS) vulnerability in the Zimbra Classic UI allows attackers to execute arbitrary JavaScript within the user's session, potentially leading to unauthorized access to sensitive information. This issue arises from insufficient sanitization of HTML content, specifically involving crafted tag structures and attribute values that include an @import directive and other script injection vectors. The vulnerability is triggered when a user views a crafted e-mail message in the Classic UI, requiring no additional user interaction.

CISA’s summary

Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that could allow attackers to execute arbitrary JavaScript within the user's session, potentially leading to unauthorized access to sensitive information.

Weakness type: CWE-79.

Product: Synacor Zimbra Collaboration Suite (ZCS) · Assigned by: mitre · Published: 2025-06-23

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

Attack vector: network (remotely reachable)
Attack complexity: low
Privileges required: none (no login)
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 2026-04-20; federal due date 2026-04-23. 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 partial.
FIRST EPSS (model of 2026-10-21): 1.71% probability of exploitation activity in the next 30 days; percentile 77.

Timeline

2025-06-23 — CVE record published by mitre
2026-04-20 — Added to CISA’s Known Exploited Vulnerabilities catalog
2026-04-21 — CVE record last updated
2026-04-23 — CISA remediation deadline for US federal agencies

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

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