Zimbra Collaboration Suite Calendar Processing Authentication Bypass via Forged iCalendar REPLY Attendee Response

Description

An attendee authentication vulnerability in the email calendar processing of Zimbra Collaboration Suite (ZCS) allows an unauthenticated sender to forge another attendee's response to an existing meeting. Zimbra passes a null sender identity when processing iCalendar REPLY components, causing the attendee authorization filter to be skipped. An attacker who knows the meeting UID, organizer address and attendee address can send a non-outdated reply that changes the attendee's participation status in the organizer's calendar. The change occurs automatically during delivery without organizer or attendee interaction. ZCS 10.1.16 and 10.1.21 are affected.

Scores

EPSS (exploitation in 30 days)

0.27% · top 81% of all CVEs (6 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: yes · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 5.3.

Weakness

• CWE-290 Authentication bypass by spoofing (opens in a new tab)All about CWE-290 and its CVEs

Affected products

• Zimbra · Zimbra Collaboration Suite

  • affected ≤ 10.1.21

References (2)

• https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories (opens in a new tab)vendor-advisory
• https://www.rapid7.com/blog/post/ve-breaking-zimbra-identity-boundaries-part-1/ (opens in a new tab)related

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-93628](https://trackcve.app/badge/CVE-2026-93628.svg)](https://trackcve.app/cve/CVE-2026-93628)

Record last updated 28 Oct 2026, 20:02 UTC.