CVE-2023-23943 in Mail
Summary
by MITRE • 02/06/2023
Nextcloud mail is an email app for the nextcloud home server platform. In affected versions the SMTP, IMAP and Sieve host fields allowed to scan for internal services and servers reachable from within the local network of the Nextcloud Server. It is recommended that the Nextcloud Maill app is upgraded to 1.15.0 or 2.2.2. The only known workaround for this issue is to completely disable the nextcloud mail app.
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Analysis
by VulDB Data Team • 03/06/2023
The vulnerability identified as CVE-2023-23943 affects the Nextcloud mail application, which serves as an email client within the Nextcloud home server platform. This issue represents a significant security flaw that enables unauthorized network discovery and reconnaissance capabilities through the mail application's configuration fields. The vulnerability specifically impacts the SMTP, IMAP, and Sieve host fields that are used to configure email server connections, creating an attack vector that allows malicious actors to perform network scanning operations against internal network resources.
The technical flaw stems from insufficient input validation and sanitization within the Nextcloud mail application's configuration handling mechanisms. When users configure email accounts through the application interface, the host fields for SMTP, IMAP, and Sieve servers do not properly restrict or validate the input values, allowing arbitrary hostnames or IP addresses to be specified. This weakness enables attackers to probe internal network services by attempting to connect to various hostnames or IP addresses, effectively turning the mail application into a network reconnaissance tool that can scan for accessible services within the local network of the Nextcloud server.
The operational impact of this vulnerability is substantial as it provides attackers with the ability to discover internal network infrastructure and services without requiring direct access to the Nextcloud server itself. This reconnaissance capability can be leveraged to identify running services, open ports, and potentially vulnerable systems within the local network. The vulnerability essentially creates a covert channel for network mapping activities, allowing attackers to gather intelligence about the internal network topology and service availability, which can then be used to plan more sophisticated attacks. The issue is particularly concerning in environments where Nextcloud servers are deployed in corporate or organizational networks where internal network segmentation is expected but may be bypassed through this vulnerability.
According to industry standards and frameworks, this vulnerability can be classified as a variant of CWE-20: Improper Input Validation, which encompasses weaknesses related to insufficient validation of input data. The attack pattern aligns with techniques described in the MITRE ATT&CK framework under the T1046: Network Service Scanning and T1083: File and Directory Discovery tactics, as the vulnerability enables network reconnaissance activities that can reveal internal system information. The vulnerability also demonstrates characteristics of insecure configuration management practices that can lead to information disclosure and reconnaissance activities.
The recommended remediation involves upgrading the Nextcloud mail application to version 1.15.0 or 2.2.2, which contain patches that address the input validation issues in the host configuration fields. These updates implement proper input sanitization and validation mechanisms that prevent arbitrary hostnames from being processed without proper restrictions. Organizations that cannot immediately upgrade may consider disabling the Nextcloud mail application entirely as a temporary workaround, though this approach significantly impacts functionality. The vulnerability highlights the importance of proper input validation in web applications and demonstrates how seemingly innocuous configuration fields can become attack vectors when not properly secured against malicious input.