CVE-2026-50055: Zimbra Collaboration Mail Forwarding Bypass

CVE-2026-50055 Overview

CVE-2026-50055 is a policy-enforcement flaw in Zimbra Collaboration Suite. An authenticated user can bypass disabled mail forwarding by abusing a Sieve notify action. The action sends copies of email content and headers to an arbitrary external address, defeating administrator controls that prohibit outbound forwarding.

The weakness is classified as [CWE-863] Incorrect Authorization. It allows exfiltration of mailbox contents by any user with Sieve filter privileges, even when the forwarding policy is explicitly disabled.

Critical Impact

Authenticated Zimbra users can exfiltrate full email bodies and headers to attacker-controlled addresses, bypassing administrator-enforced no-forwarding policies.

Affected Products

• Zimbra Collaboration Suite (versions subject to vendor advisory confirmation)
• Deployments using Sieve filtering with the notify action enabled
• Environments where mail forwarding is administratively disabled but Sieve scripting remains available to users

Discovery Timeline

• 2026-10-08 - CVE-2026-50055 published to the National Vulnerability Database
• 2026-10-08 - Last updated in NVD database

Technical Details for CVE-2026-50055

Vulnerability Analysis

The flaw lives in how Zimbra enforces mail forwarding policy. Administrators can disable the forwarding feature on a per-account or domain basis. This control blocks the standard redirect Sieve action and the webmail forwarding settings. However, the enforcement check does not extend to the Sieve notify action.

The notify action, defined in RFC 5435, lets Sieve scripts send out-of-band messages when mail arrives. In Zimbra, the action supports including the triggering message body and headers in the notification payload. An authenticated user can author a filter that fires on every inbound message and routes a full copy to any external address.

The result is a silent forwarding channel. The behavior violates the confidentiality guarantee the forwarding policy is meant to enforce.

Root Cause

The root cause is incomplete authorization logic. The mail forwarding policy check guards only specific code paths. The Sieve notify handler executes under a separate code path that does not consult the same policy, enabling the bypass.

Attack Vector

Exploitation requires authenticated access to a Zimbra mailbox over the network with permission to create or modify Sieve filters. No user interaction is needed after the filter is installed. Confidentiality is fully compromised; integrity and availability are not affected.

The attacker logs into the webmail or ZCS administration interface, creates a Sieve script invoking notify with the message content as the payload, and targets an external mail address. Each incoming message triggers the script and leaks the contents.

See the Zimbra Security Advisories for additional technical references.

Detection Methods for CVE-2026-50055

Indicators of Compromise

• Sieve filter scripts containing the notify action with external recipient addresses or URIs
• Outbound SMTP traffic from the Zimbra MTA to unknown domains sourced from notification messages
• Unexpected modifications to user .sieve script files on the Zimbra server
• Mailbox audit log entries showing Sieve script changes outside normal administrative windows

Detection Strategies

• Enumerate all user Sieve scripts and flag any containing the notify action, especially those referencing external domains
• Correlate Zimbra mailbox.log entries for zimbraPrefMailForwardingAddress policy state with observed outbound notification traffic
• Monitor the ZCS admin API for Sieve filter creation and modification events per user

Monitoring Recommendations

• Ingest Zimbra MTA and mailbox logs into a centralized SIEM for query and retention
• Alert on any Sieve script that includes notify, :method, or mailto: directives pointing outside the organization
• Track baseline outbound mail volume per account to detect sudden spikes consistent with per-message notifications

How to Mitigate CVE-2026-50055

Immediate Actions Required

• Audit all existing user Sieve scripts for the notify action and remove or quarantine filters routing content externally
• Review the Zimbra Security Advisories page for the patched version and apply it as soon as it is available for your release train
• Restrict Sieve script authoring permissions to trusted administrators where feasible

Patch Information

Zimbra publishes fixes through its security advisory wiki. Administrators should consult the Zimbra Security Advisories page for the specific patched build addressing CVE-2026-50055 and follow the standard ZCS upgrade procedure.

Workarounds

• Disable the Sieve notify extension at the server level if it is not required for business use
• Enforce egress filtering on the Zimbra MTA to allow outbound mail only to approved domains
• Set zimbraSieveFeatureVariablesEnabled and related notification capability flags to FALSE until the patch is applied

# Disable Sieve notify capability globally (verify flag names against your ZCS version)
zmprov mcf zimbraSieveNotifyActionRHSAddressEnabled FALSE
zmprov mcf zimbraMtaSmtpdRecipientRestrictions "check_recipient_access regexp:/opt/zimbra/conf/approved_domains, reject"
zmcontrol restart