Loading vulnerability…
Zimbra Collaboration Suite Sieve Notification Mail Forwarding Restriction Bypass
CVE-2026-50055 is a policy-enforcement vulnerability in Zimbra Collaboration Suite that allows authenticated users to bypass disabled mail forwarding through Sieve notification rules. The forwarding-policy checks used by ModifyFilterRulesRequest and ModifyOutgoingFilterRulesRequest reject RedirectAction but permit NotifyAction and RFCCompliantNotifyAction even when zimbraFeatureMailForwardingInFiltersEnabled is FALSE. These notification actions can use variable expansion to send decoded message headers and the complete selected text body to an arbitrary external address. The vulnerability is fixed in ZCS 10.1.20.
What it means and what to do now: patch path, interim mitigations, and the assume-compromise checklist.
What an attacker gets, and what they’ve been doing with it.
An authenticated attacker can exfiltrate incoming or outgoing message text and headers despite organizational forwarding restrictions. Malicious server-side rules continue operating after session expiration or password changes and can preserve original messages in the Inbox, making ongoing disclosure less apparent. The notification mechanism does not forward attachments or raw MIME. Malicious rules are absent from the Modern client's Filters view, while the Classic overview displays only their names without the external destinations or copied fields.
If you can’t patch tonight, do this now.
Audit stored incoming and outgoing Sieve rules for unauthorized notification destinations and message-content expansion, and remove suspicious rules. Monitor outbound notification traffic for unexpected external recipients. Do not rely solely on the Modern client's Filters view, the Classic filter overview, session expiration, or password changes to detect or stop existing exfiltration.
Patch, then assume compromise.
Upgrade to Zimbra Collaboration Suite 10.1.20 or a later fixed release. The fix subjects NotifyAction and RFCCompliantNotifyAction to the same forwarding policy as RedirectAction. Inspect stored incoming and outgoing filter rules directly and remove unauthorized external notification actions: upgrading does not remove previously saved rules. Password resets alone do not eliminate this persistence.
No validated public exploits yet. Hidden: 1 rejected by Mallory's analysis as not a working exploit.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
10 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.