dbcve.org
Cross-site Scripting (XSS)Weakness · CWE-79

CVE-2026-40875

HIGH · 7.0 CVSS v4.0 Published 2026-04-21
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
74/100
Remediation priority · Elevated
Remotely reachable

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 · unedited
mailcow: dockerized is an open source groupware/email suite based on docker. In versions prior to 2026-03b, the user dashboard's "Seen successful connections" (login history) renders the client IP from login logs without HTML escaping. Because the server trusts the X-Real-IP header as the source IP for logging, an attacker can inject HTML/JS into this field. This Self-XSS can be exploited by a Login CSRF to force the victim into the attacker's account, and then read emails in a previous browser tab. 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 confidence

A Self-XSS vulnerability exists in mailcow's user dashboard where the 'Seen successful connections' feature displays the client IP address from login logs without HTML escaping. Since the server trusts the X-Real-IP header for logging, an attacker can inject malicious HTML/JS code through this header. The vulnerability can be chained with a Login CSRF attack to force a victim into the attacker's account, allowing the attacker to read emails from a previously opened browser tab.

MitigationUpgrade to mailcow version 2026-03b or later, which implements proper HTML escaping on the X-Real-IP header value before rendering it in the login history view.

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
High
Privileges
Low
Authentication
X
User interaction
P
Scope
X

CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:P/VC:H/VI:N/VA:N/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 checks

Work through these to decide whether this CVE applies to you.

  1. Check installed mailcow version
    Locate the mailcow version file (typically in /mailcow/docker-compose.yml, a VERSION file, or check the mailcow container labels using 'docker ps --format "{{.Labels}}" | grep version')
    Affected if Version is earlier than 2026-03b (e.g., 2026-03a, 2026-02, or older releases)
  2. Verify login history feature accessibility
    Log into the mailcow UI as a regular user and navigate to the user dashboard to locate the 'Seen successful connections' or 'Login history' section where client IPs are displayed
    Affected if The login history page exists and displays IP addresses from login logs
  3. Confirm X-Real-IP header handling
    Examine the nginx configuration within the mailcow docker container (docker exec nginx cat /etc/nginx/conf.d/*.conf) or the mailcow configuration to check if X-Real-IP header is used for logging client IPs
    Affected if The server is configured to use X-Real-IP header for logging (trusts this header without validation)
  4. Test IP injection in X-Real-IP header
    Send a test request to the mailcow login endpoint with a crafted X-Real-IP header containing a benign test string (e.g., X-Real-IP: <script>alert(1)</script>) and check if the login history page renders this string as HTML or escapes it
    Affected if The IP address is rendered as raw HTML/JavaScript in the dashboard without sanitization

You are affected if mailcow version is before 2026-03b AND the login history feature displays IPs from the X-Real-IP header without HTML escaping.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade to mailcow version 2026-03b or later, which implements proper HTML escaping on the X-Real-IP header value before rendering it in the login history view.

Recommended fix High confidence

2026-03b

  1. Back up your current mailcow installation (docker volumes, configuration files)
  2. Stop the current mailcow containers using docker-compose
  3. Update the mailcow repository to the latest version using git pull
  4. Pull the updated Docker images
  5. Restart mailcow containers using docker-compose up -d
  6. Verify the upgrade was successful by checking the mailcow UI and logs
  7. Verify the login history now properly escapes HTML characters in displayed IPs

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-40875 in production — separate from our analysis above.

No notes yet

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What this is

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.

What belongs here
  • 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