CVE-2025-66376
CVE-2025-66376 is a security flaw in Synacor Zimbra Collaboration Suite (ZCS), email and calendar software companies run on their own servers. Attackers can run code in the browser of someone using Synacor Zimbra Collaboration Suite (ZCS) and act as them. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Mar 2026.[1]
Synacor Zimbra Collaboration Suite (ZCS) is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 10.0.18 and 10.1.13 of Collaboration.
Description
CISA calls it a cross-site scripting in Synacor Zimbra Collaboration Suite (ZCS). In its words: “Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability in the Classic UI where attackers could abuse Cascading Style Sheets (CSS) @import directives in email HTML.”[1]
The underlying weakness is cross-site scripting (CWE-79), in MITRE’s list of common software weaknesses.[2]
It scores 6.1 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Collaboration | 10.0.18 |
| Collaboration | 10.1.13 |
Most families won’t have Synacor Zimbra Collaboration Suite (ZCS); it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 18 Mar 2026, 72 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 1 Apr 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 20%. That’s in the top 3% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Synacor has published a fix or advice.[3] CISA’s 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.”[1]
- Check whether you use it.Synacor Zimbra Collaboration Suite (ZCS) is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 10.0.18 and 10.1.13 of Collaboration), or not in use. CISA gave US government agencies until 1 Apr 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 6.1.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 20%.[4] |
See also
Other Synacor Zimbra Collaboration Suite (ZCS) flaws
- CVE-2026-73570: OS command injection ·
- CVE-2025-48700: cross-site scripting ·
- CVE-2025-68645: PHP remote file inclusion ·
- CVE-2025-27915: cross-site scripting ·
- CVE-2019-9621: server-side request forgery (SSRF) ·
- CVE-2024-27443: cross-site scripting (XSS) ·
Other Synacor flaws
- CVE-2020-7796, Synacor Zimbra Collaboration Suite: (ZCS) server-side request forgery ·
- CVE-2022-24682, Synacor Zimbra Collaborate Suite (ZCS): cross-site scripting ·
Other cross-site scripting flaws
- CVE-2026-42897, Microsoft: exchange server cross-site scripting ·
- CVE-2025-68461, Roundcube Webmail: cross-site scripting ·
- CVE-2021-26829, OpenPLC ScadaBR: cross-site scripting ·
- CVE-2024-42009, Roundcube Webmail: cross-site scripting ·
- CVE-2024-11182, MDaemon Email Server: cross-site scripting (XSS) ·
Added the same day
- CVE-2026-20963, Microsoft SharePoint: deserialization of untrusted data ·
Every Synacor flaw on CISA’s list · Everything added in March 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability”. Known Exploited Vulnerabilities Catalog, added 18 Mar 2026.
- ^ National Institute of Standards and Technology. “CVE-2025-66376 Detail”. National Vulnerability Database, published 5 Jan 2026.
- ^ Synacor. Advisories and fixes: wiki.zimbra.com, wiki.zimbra.com, wiki.zimbra.com, wiki.zimbra.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.202, percentile 97.5, 31 Oct 2026.
- CVE Program. “CVE-2025-66376”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.