CVE-2026-115785 in Collaboration Suite
Summary
by MITRE • 10/23/2026
An SMTP command injection vulnerability in the data-source testing functionality of Zimbra Collaboration Suite (ZCS) allows an ordinary authenticated user with account options enabled to inject CR/LF into the emailAddress inserted into an SMTP envelope command by a JSON SOAP request. After a successful preliminary POP3 or IMAP login, Zimbra sends the injected commands to the selected SMTP service. If that service accepts the pipelined transaction, the attacker can submit mail with attacker-selected recipients, headers and body content without victim interaction. The vulnerable command-construction path is present in ZCS 10.1.16 and 10.1.20.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 10/23/2026
The vulnerability described constitutes a critical SMTP command injection flaw within the data-source testing functionality of Zimbra Collaboration Suite, specifically affecting versions 10.1.16 and 10.1.20. This security defect arises from insufficient input validation when handling email addresses provided by authenticated users during account configuration or verification processes. The core technical issue lies in the application's failure to sanitize carriage return (CR) and line feed (LF) characters embedded within the emailAddress field of a JSON SOAP request. By injecting these control characters, an attacker can manipulate the underlying SMTP envelope commands that Zimbra constructs to test connectivity with external mail servers. This manipulation effectively allows the injection of arbitrary SMTP commands into the transaction stream sent to the target SMTP service.
From a technical perspective, this vulnerability exploits the way the application parses and forwards user-supplied input to backend services without enforcing strict boundaries on command structure. When an authenticated user enables account options and initiates a data-source test, Zimbra attempts to establish communication with the specified mail server using POP3 or IMAP protocols for authentication before proceeding to SMTP testing. If this preliminary login is successful, the system proceeds to send email messages as part of the connectivity check. However, due to the lack of proper encoding or validation of special characters in the recipient address field, an attacker can break out of the intended command context and inject additional commands directly into the SMTP session. This technique relies on the behavior of certain SMTP servers that accept pipelined transactions, where multiple commands are sent sequentially without waiting for individual responses, thereby allowing the injected commands to be executed as part of the same transaction flow.
The operational impact of this vulnerability is severe, enabling an ordinary authenticated user with account options enabled to bypass intended restrictions and send emails with attacker-selected recipients, headers, and body content without any further interaction from a victim or administrator. This capability transforms what appears to be a simple configuration testing feature into a powerful vector for abuse. An attacker can leverage this flaw to conduct phishing campaigns by sending malicious messages that appear to originate from the compromised Zimbra account. Furthermore, the ability to manipulate headers allows for sophisticated social engineering attacks where sender information is spoofed or altered to increase the likelihood of recipient trust. The lack of victim interaction requirement means that once authentication credentials are obtained through other means such as credential stuffing or phishing, this vulnerability can be exploited immediately and autonomously by the attacker.
This flaw aligns with Common Weakness Enumeration (CWE) categories including CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component, which describes issues where special elements are not properly neutralized, leading to unintended command execution or data manipulation. Additionally, it relates to CWE-918: Server-Side Request Forgery (SSRF) if the injected commands cause the server to interact with unauthorized internal services, although in this specific case, the primary impact is direct SMTP abuse rather than network-level SSRF. In terms of the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Initial Access and Execution phases, particularly those involving email-based delivery mechanisms like T1534: Internal Spearphishing or T1078: Valid Accounts. The exploitation path demonstrates how misconfigured application logic can be leveraged to escalate privileges from a standard user account to an effective mail relay capability within the organization's infrastructure.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. The most direct solution is for organizations running Zimbra Collaboration Suite versions 10.1.16 or 10.1.20 to apply vendor-provided patches that address this specific input validation gap. Until patching is feasible, administrators should restrict access to the data-source testing functionality by limiting account options privileges to trusted administrative users only. Implementing strict output encoding for all user-supplied email addresses before they are passed to SMTP libraries can also mitigate the risk of command injection. Furthermore, network-level controls such as firewall rules that limit outbound SMTP connections from Zimbra servers to known and approved mail relays can reduce the blast radius if an attacker attempts to exploit this vulnerability. Regular security audits focusing on input validation practices in SOAP API endpoints are essential to prevent similar issues across other modules within the collaboration suite.