CVE-2026-40873Public exploitNetwork attack vector

Administrator account takeover via unescaped attachment filename

Published Apr 21, 2026 · Updated Apr 21, 2026

Stored XSS in mailcow: dockerized before 2026-03b allows remote attackers to execute JavaScript when an administrator views a quarantined email. The Quarantine details modal in data/web/js/site/quarantine.js concatenates an attachment filename into an HTML string without escaping value[0]. Quarantine must be enabled, a crafted message must be retained, and an administrator must open its details; the script then runs in the mailcow origin and can take over the administrator's session.

CVSS severity8.9
High
EPSS probability0.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in mailcow: dockerized before 2026-03b allows remote attackers to execute JavaScript when an administrator views a quarantined email. The Quarantine details modal in data/web/js/site/quarantine.js concatenates an attachment filename into an HTML string without escaping value[0]. Quarantine must be enabled, a crafted message must be retained, and an administrator must open its details; the script then runs in the mailcow origin and can take over the administrator's session.

The record

CVE
CVE-2026-40873
Published
Apr 21, 2026
Updated
Apr 21, 2026
Vendor
mailcow
Product
mailcow: dockerized
Classifications
CWE-80, CWE-79, T1204
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 21, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 21, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2026-03b

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

mailcow · mailcow: dockerizedversion=< 2026-03b

Published CVSS scores

8.9GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-80Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1204User Execution

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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