CVE-2026-115786 in Collaboration Suite
Summary
by MITRE • 10/23/2026
A mailbox quota-accounting vulnerability in Zimbra Collaboration Suite (ZCS) allows an authenticated user to consume another user’s mailbox quota by repeatedly sharing and revoking a document. Creation of the recipient’s shared-file record charges the document size to mailbox-wide usage. Revocation deletes the record and reverses its folder counters without reversing the mailbox-wide charge. Repeated normal share notifications therefore accumulate quota usage after the visible records are gone. On accounts with a finite quota, this can prevent incoming mail and other size-increasing operations without recipient interaction. The issue affects ZCS 10.1.16 and 10.1.20.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified in Zimbra Collaboration Suite versions 10.1.16 and 10.1.20 represents a critical mailbox quota-accounting flaw that allows authenticated users to manipulate storage metrics through the manipulation of shared file records. This issue stems from an inconsistency in how the system tracks disk usage at different levels of data abstraction, specifically between folder-level counters and global mailbox-wide quotas. When a user shares a document with another recipient, Zimbra creates a shared-file record for that recipient. The accounting logic correctly charges the size of this document to the sender's mailbox-wide quota during this creation process. However, when the sharing is revoked or deleted, the system performs an operation intended to reverse the folder-level counters associated with the specific directory where the file resides. Crucially, it fails to execute a corresponding reversal for the global mailbox-wide usage counter. This asymmetry in accounting logic means that while the visible record of the shared item disappears from the recipient's view and is removed from local folder metrics, the storage cost remains permanently attributed to the sender's account quota.
The operational impact of this flaw is significant for any organization relying on strict email storage limits or service level agreements tied to mailbox size. An authenticated attacker can exploit this mechanism by repeatedly sharing a document with a target user and then immediately revoking that share. Each cycle of sharing and revoking consumes additional quota from the sender's account without leaving any trace in the shared items folder. This technique effectively allows an adversary to exhaust another user's available storage space through automated scripts or manual repetition, requiring no interaction from the victim other than being a valid recipient on the system. As the mailbox fills up due to these phantom charges, legitimate operations such as receiving new emails, saving drafts, or uploading attachments will begin to fail with quota exceeded errors. This constitutes a denial of service condition that disrupts business continuity and can be used maliciously to hinder communication within an organization without triggering typical security alerts associated with unauthorized access or data exfiltration.
From a classification perspective, this vulnerability aligns closely with CWE-754: Improper Check for Unusual or Exceptional Conditions, as the application fails to validate that quota accounting operations are symmetric and consistent across all relevant storage metrics. It also relates to CWE-829: Inclusion of Functionality from Untrusted Control Sphere if viewed through the lens of internal state manipulation by authenticated users who should not be able to alter system-wide resource allocation arbitrarily. Furthermore, this behavior can be mapped to MITRE ATT&CK technique T1496: Resource Hijacking, where an adversary uses computing resources such as memory or disk space in a way that degrades performance for other processes or users. In the context of Zimbra, while it does not hijack CPU cycles, it effectively hijacks storage capacity, rendering the mailbox unusable for its intended purpose. The lack of proper validation during the revocation phase allows this resource exhaustion to occur silently and persistently.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. For administrators currently running affected versions 10.1.16 or 10.1.20, applying the latest security patch provided by Zimbra is the primary corrective action. These updates typically include fixes to the quota accounting logic in the backend services responsible for handling share creation and deletion events. Until patches are applied, administrators can implement monitoring solutions that track rapid fluctuations in mailbox sizes or detect unusual patterns of shared item creation and deletion activity. Alerting on these anomalies may help identify active exploitation attempts before critical storage limits are reached. Additionally, reviewing user permissions to ensure that only trusted individuals have the ability to share files broadly can reduce the attack surface. Long-term architectural reviews should focus on ensuring that all resource accounting operations maintain strict consistency between local folder metrics and global mailbox quotas, preventing similar discrepancies in future software releases.