CVE-2025-48700

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.

What this means for your business

It affects Synacor Zimbra Collaboration Suite (ZCS). It matters if your company, or a supplier that handles your data, runs it.
Attackers are already using it. CISA added it to its list of flaws used in real attacks on 20 April 2026.
FIRST's prediction model gives it a 2% chance of attack attempts being seen in the next 30 days, ranking above 75% of all known flaws.

What to do

1Check whether your company or your suppliers run Synacor Zimbra Collaboration Suite (ZCS).
2If you do, follow CISA's required action below. No patch reference has been published yet.
3Treat it as urgent. US federal agencies must fix it by 23 April 2026. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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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’s wording, unedited.

Scoring

CVSS: Not yet scored

Dates

Published: 2026-04-20
Added to KEV: 2026-04-20
Remediation due: 2026-04-23
Sources: CISA KEV