CVE-2026-50055 in Collaboration Suite
Summary
by MITRE • 10/08/2026
A policy-enforcement flaw in Zimbra Collaboration Suite allows an authenticated user to bypass disabled mail forwarding by using a Sieve notify action to send copies of email content and headers to an arbitrary address.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified as CVE-2019-7653 represents a critical policy-enforcement flaw within the Zimbra Collaboration Suite, specifically affecting its webmail interface's handling of mail filtering rules. This issue stems from how the application processes Sieve scripts, which are used to define automated actions for incoming emails. While administrators can configure global or user-specific policies to disable certain features such as email forwarding to prevent data exfiltration or spam relay abuse, this security control is not consistently enforced at the code level when interacting with specific scripting interfaces. An authenticated attacker who has gained access to a valid Zimbra account can exploit this discrepancy by crafting and uploading a custom Sieve script that utilizes the notify action rather than the traditional redirect or forward actions. Because the system fails to validate whether forwarding capabilities are disabled for the user context before executing these scripts, it allows the transmission of email content and headers to an arbitrary external address, effectively circumventing the intended administrative restrictions.
From a technical perspective, this flaw is rooted in insufficient validation logic within the Zimbra webmail component responsible for parsing and applying Sieve rules. The vulnerability classifies under CWE-250, which denotes execution with unnecessary privileges, as well as CWE-693, protection mechanism failure, because the software relies on configuration settings that are not rigorously checked during runtime operations involving scripting engines. By leveraging the notify action in a Sieve script, an attacker can trigger external notifications or send copies of messages to addresses outside the organization's domain. This method bypasses standard forwarding controls because the security policy likely only checks for explicit forward commands rather than all mechanisms that result in data leaving the system via email protocols. The ability to extract headers and body content means sensitive information, including internal communication details, metadata, and potentially confidential business intelligence, can be exfiltrated without triggering typical alerts associated with direct forwarding attempts.
The operational impact of this vulnerability is significant for organizations relying on Zimbra for secure communications. It enables unauthorized data leakage, allowing malicious insiders or external attackers who have compromised user credentials to siphon off sensitive emails. This capability undermines the confidentiality and integrity guarantees provided by the collaboration suite's security policies. Furthermore, it can facilitate further attacks such as phishing campaigns using stolen internal communication patterns or credential harvesting if specific email types are targeted. The existence of this flaw indicates a gap in defense-in-depth strategies where application-level controls do not align with infrastructure-level restrictions, creating an attack vector that is difficult to detect through standard monitoring tools focused on explicit forwarding events rather than script-based data transfers.
To mitigate the risks associated with CVE-2019-7653, organizations must apply the official patches provided by Zimbra for their specific version of the Collaboration Suite. Immediate remediation involves upgrading to a fixed release where the Sieve engine's policy enforcement has been corrected to respect disabled forwarding settings regardless of the action type used in scripts. In addition to patching, administrators should enforce strict access controls on user accounts and monitor for unusual activity patterns such as new script uploads or unexpected outbound email traffic. Implementing network-level monitoring can also help detect anomalies related to data exfiltration attempts via SMTP connections initiated by mail servers acting under the guise of legitimate Sieve processing. Regular audits of user permissions and disabling unnecessary scripting capabilities where not required by business operations will further reduce the attack surface available to potential adversaries seeking to exploit this policy-enforcement flaw.