CVE-2025-66376: Synacor exploited

Advisory generated from the CVE record, CISA, Exploit Observer and VEDAS: what it is, whether it is exploited, who is affected, and how to fix and detect it.

What it is

Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.

Description quoted from the CVE record (assigned by mitre, published 2026-01-05).

Weakness: CWE-79 Improper Neutralization of Input During Web Page Generation (XSS or ‘Cross-site Scripting’)

How it can be exploited

Exploitable over the network with low attack complexity needing no privileges and no user interaction. Impact: confidentiality low, integrity low, availability none. CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

CISA assessment (SSVC): CISA assesses exploitation as active. Automatable: no. Technical impact: partial.

On CISA KEV since: 2026-03-18
US federal deadline: 2026-04-01
Ransomware use: Not known

Public exploit evidence (Exploit Observer)

Also known as: BDU:2026-01010, CNNVD-202601-876, EUVD-2026-0850, GHSA-h7wg-85fj-3c6g

Exploit Observer has no public exploit code that names this CVE or one of its aliases.

Vulnerability databases and vendor advisories (5)

Catalogue entries, not exploit evidence.

Vulnerability database or vendor advisory: https://euvd.enisa.europa.eu/enisa/EUVD-2026-0850
Vulnerability database or vendor advisory: https://github.com/advisories/GHSA-h7wg-85fj-3c6g
Vulnerability database or vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2025-66376
Vulnerability database or vendor advisory: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376
Vulnerability database or vendor advisory: https://www.cnnvd.org.cn/home/globalSearch?keyword=CNNVD-202601-876

7 more references returned by Exploit Observer did not name this CVE or an alias, so they are left out.

VEDAS data as of 2026-11-01T17:12 UTC; fetched 2026-11-02.

Live scores

EPSS: 0.202
likely to be attacked soon
VEDAS: 0.557
some exploit evidence
On CISA KEV: Yes, since 2026-03-18
Detection rule: None yet

EPSS (chance of attack)

VEDAS (exploit evidence)

Thin horizontal line: the “likely” (EPSS 0.10) or “strong” (VEDAS 0.90) threshold. Shaded “no data” areas are periods when cve-scores did not publish. The vertical line is the day CISA added it to its exploited list.

Who is affected

Product	Affected	Not affected
Zimbra Collaboration	from 10.0 before 10.0.18 from 10.1 before 10.1.13	Not stated

Version ranges exactly as the CVE record states them. “Before X” means X is the first fixed version.

Exploit Observer also maps it to: synacor zimbra_collaboration_suite, zimbra_inc. zimbra_collaboration_suite.

How to fix it

Fixed versions stated in the CVE record: 10.0.18, 10.1.13.

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.

Vendor guidance:

How to detect it

There is no open VEDAS detection rule for this bug yet. It is listed on the detection gaps page if CISA has confirmed attacks.

Advisories on this CVE

Joint advisory AA26-204A: LAUNDRY BEAR zero-click Zimbra campaign (23 Jul 2026)

References

Government sources

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376 (government-resource)

Other references

Other databases

Where this comes from

This advisory is generated automatically by VEDAS from: CVE.org record, CISA KEV, cve-scores (EPSS and VEDAS), Exploit Observer. Text in quotes is reproduced from the source; everything else is a fixed template filled with source fields. Nothing here is written by a language model. Check the vendor advisory before acting, and read how VEDAS and Exploit Observer work, and their limits.

CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability