An unauthenticated calendar sender can place active markup in a COUNTER message's RFC From address. Selecting the message in Zimbra Classic triggers stored XSS, allowing the attacker to access mailbox data and act as the victim.
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-87102
Source: Mitre, NVD
Published: 2026-09-25
Updated: 2026-09-29
Base Score: 9.4
Vector: CVSS2#AV:N/AC:L/Au:N/C:C/I:C/A:N
Severity: High
Base Score: 9.3
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N
Severity: Critical
EPSS: 0.00229