CVE-2026-93629 in Collaboration Suite
Summary
by MITRE • 10/23/2026
An organizer authentication vulnerability in Zimbra Collaboration Suite (ZCS) allows an unauthenticated email sender to insert calendar invitations attributed to another user. The email delivery path permits external invitations but does not verify that the message sender is authorized to use the iCalendar ORGANIZER identity. When automatic invitation insertion is enabled, a crafted REQUEST component creates an event containing attacker-selected meeting details and reminders under a forged organizer without recipient interaction. No corresponding event is created in the claimed organizer's calendar. ZCS 10.1.16 and 10.1.21 are affected.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified in Zimbra Collaboration Suite represents a critical failure in identity verification within the email processing pipeline, specifically concerning iCalendar event handling. This flaw allows an unauthenticated external sender to forge calendar invitations that appear to originate from any valid user account within the organization. The core technical deficiency lies in the application's inability to validate whether the entity sending the email is actually authorized to act as the ORGANIZER specified in the iCalendar metadata. In standard collaborative environments, the organizer field should be strictly bound to the authenticated sender or require explicit delegation proof; however, this implementation accepts the identity declared in the message headers and body without cross-referencing it against the actual authentication credentials of the SMTP session. This architectural oversight creates a significant trust boundary violation where external actors can manipulate internal calendar data structures by injecting maliciously crafted REQUEST components that bypass standard authorization checks.
From an operational perspective, this vulnerability enables sophisticated social engineering attacks with minimal effort from the attacker. By leveraging automatic invitation insertion features often enabled for convenience in corporate environments, attackers can force-calendar events onto victim calendars without any interaction required from the target user. The injected event contains details selected by the attacker, such as meeting titles, times, and reminders, which are displayed prominently on the recipient's calendar interface. This capability facilitates a range of malicious activities including scheduling conflicts to disrupt business operations, spreading misinformation through fake meetings that appear legitimate due to their association with trusted internal identities, or serving as a vector for further phishing campaigns where victims may be lured into clicking links within the forged event details. The absence of an entry in the claimed organizer's calendar confirms that this is purely a client-side manipulation via email injection rather than true account compromise, yet it effectively undermines user trust and operational integrity.
This flaw aligns with CWE-287 Improper Authentication as well as CWE-345 Insufficient Verification of Data Authenticity, reflecting the system's failure to ensure that the claimed identity matches the actual source of the data. In terms of offensive security frameworks, this technique maps closely to MITRE ATT&CK T1098.002 SSH Authorized Keys Manipulation in concept but applies here to email-based calendar injection, resembling aspects of T1566.001 Spearphishing Attachment where malicious content is disguised as legitimate communication. The impact extends beyond simple spam, affecting the reliability of scheduling systems and potentially exposing sensitive meeting information if the forged events contain confidential details intended for specific groups.
Mitigation strategies must focus on enforcing strict identity binding between email senders and calendar organizer fields. Administrators should configure Zimbra to reject or flag any iCalendar REQUEST components where the ORGANIZER field does not match the authenticated SMTP sender's address unless explicit delegation permissions are verified through standard protocols like IMAP ACLs or delegated access lists. Additionally, disabling automatic invitation insertion for external domains can prevent silent injection of forged events into user calendars. Patching to a version that addresses this validation logic is essential, and organizations should implement email authentication standards such as SPF, DKIM, and DMARC to reduce the likelihood of spoofed emails reaching the mail processing pipeline in the first place. Regular auditing of calendar access logs can also help detect anomalous patterns indicative of exploitation attempts.