CVE-2026-40873
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 · uneditedmailcow: dockerized is an open source groupware/email suite based on docker. In versions prior to 2026-03b, the Quarantine details modal injects attachment filenames into HTML without escaping, allowing arbitrary HTML/JS execution. An attacker can deliver an email with a crafted attachment name so that when an admin views the quarantine item, JavaScript executes in their browser, taking over their account. Version 2026-03b fixes the vulnerability.
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 confidenceThis is a stored XSS vulnerability in mailcow's Quarantine details modal where attachment filenames are rendered without HTML escaping. An attacker can craft a malicious attachment filename containing HTML/JavaScript that executes in an administrator's browser when they view the quarantined email, allowing session hijacking or further attacks.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS 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
- Authentication
- X
- User interaction
- P
- Scope
- X
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
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.
-
Verify mailcow is installedRun 'docker ps' to list running containers and look for mailcow-related containers (mailcowdockerized_*, netfilter, etc.) or check for the mailcow web UI at the expected hostnameAffected if No mailcow containers or web UI found means this CVE does not apply to your environment
-
Check mailcow versionAccess the mailcow UI and look at the footer for the version number, or run 'docker-compose ps' in the mailcow directory to see the version tagAffected if Version is earlier than 2026-03b (for example, 2026-03, 2026-02, 2025-xx)
-
Confirm Quarantine feature is enabledIn the mailcow UI, navigate to the Quarantine section, or check the configuration file (mailcow.conf) for QUARANTINE_ENABLED settingAffected if Quarantine feature is disabled - the vulnerable code path is not reachable
-
Verify admin access to QuarantineLog into the mailcow UI and check if you can access the Quarantine section (requires administrator role)Affected if Not an administrator - you cannot access the Quarantine details modal where the XSS triggers
You are affected if mailcow is installed with a version earlier than 2026-03b and you have administrator access to the enabled Quarantine feature.
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 · scopedUpgrade mailcow to version 2026-03b or later to obtain the patch that properly escapes attachment filenames in the quarantine modal.
2026-03b
- Backup your current mailcow configuration and data
- Update your mailcow installation to version 2026-03b by pulling the latest docker images
- Restart all mailcow containers to apply the update
- Verify the version upgrade was successful by checking the mailcow UI or API
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-40873 — 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-2026-40873 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
Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.
A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- 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