CVE-2025-66376
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedZimbra 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceZimbra Collaboration Suite (ZCS) versions 10.x before 10.0.18 and 10.1 before 10.1.13 contains a stored XSS vulnerability in the Classic UI. Attackers can inject malicious CSS @import directives within HTML email messages, which execute when recipients view the crafted email, allowing session hijacking or data theft.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 10.0.0, < 10.0.18>= 10.1.0, < 10.1.13CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify installed ZCS versionRun the ZCS version command (zmcontrol -v or similar Zimbra administration tool) to retrieve the installed Zimbra Collaboration Suite version numberAffected if The version returned is 10.0.x before 10.0.18 OR 10.1.x before 10.1.13
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Determine if Classic UI is enabledCheck the user or global UI preference settings for the ZCS web client. This is typically viewable in the Zimbra administration console or by inspecting the client configuration for the 'Classic' UI mode being activeAffected if Classic UI (sometimes called the 'legacy' or 'HTML' client) is the active or default mail client interface
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Verify email rendering uses affected componentConfirm that incoming HTML emails are rendered using the Classic UI mail viewer rather than a sandboxed or modern UI renderer. This may be observable in mail traces or by testing receipt of a benign HTML emailAffected if Emails are rendered through the Classic UI HTML parser without CSP or sandbox restrictions applied to CSS @import directives
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Check for recent mail processing anomaliesReview mail server logs for any suspicious patterns that may indicate exploitation attempts, such as unusual CSS-related errors or unexpected script executions in mail sessionsAffected if Logs show evidence of CSS @import processing in mail context or anomalous script execution events correlated with specific email messages
A user is affected if they run a ZCS version between 10.0.0-10.0.17 or 10.1.0-10.1.12 AND have Classic UI enabled as their mail client interface.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped10.0.1810.1.13
Upgrade ZCS to version 10.0.18, 10.1.13 or later. Until patched, warn users about opening HTML emails from untrusted senders and consider disabling the Classic UI.
10.0.18 (for 10.0.x branch) or 10.1.13 (for 10.1.x branch)
- Identify your current ZCS version by running: zimbra@zimbra:~$ zmcontrol -v
- If running 10.0.x, upgrade to version 10.0.18 or later
- If running 10.1.x, upgrade to version 10.1.13 or later
- Download the appropriate fixed release from Zimbra's official download portal
- Backup your current ZCS installation and database before upgrading
- Run the standard ZCS upgrade installer: ./install.sh -s (or appropriate for your setup)
- Verify the upgrade was successful: zimbra@zimbra:~$ zmcontrol status
- Confirm the XSS vulnerability is resolved by testing HTML email rendering with CSS @import constructs
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-66376 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-66376 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
- No spam, self-promotion, credentials, or personal data