What’s Vulnerable › Vulnerabilities

CVE-2026-104083: SmarterMail < Build 9777 Stored Mutation XSS via MathML Foreign Content

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-09 · updated 2026-10-27
• Weakness: CWE-79

Description

SmarterMail before build 9777 contains a stored mutation cross-site scripting vulnerability that allows remote attackers to inject executable script by placing payloads inside a <style> element nested within MathML foreign content (<math><mtext><mglyph>), which the custom HTML sanitizer treats as inert CDATA text but browsers reparse as live markup. Attackers can deliver a crafted calendar (iCal) message containing an <img src=x onerror=...> payload that executes automatically in the recipient's webmail session at /interface/message-iframe when the message is opened, enabling script execution and data exfiltration unconstrained by the interface's permissive Content-Security-Policy.

Affected products

• SmarterTools SmarterMail: before Build 9777 — fixed in Build 9777

How to fix

Upgrade to a fixed version: SmarterTools SmarterMail Build 9777. Check the vendor advisory for mitigations.

References

• https://blog.evan.lat/posts/sm/
• https://www.smartertools.com/smartermail/release-notes/current#/9526:~:text=Build%209777%20(Oct%208%2C%202026)
• https://www.vulncheck.com/advisories/smartermail-build-9777-stored-mutation-xss-via-mathml-foreign-content

Get alerted when CVE-2026-104083 or the next vulnerability affects your stack