CVE-2016-9920 in RoundCube
Summary
by MITRE
steps/mail/sendmail.inc in Roundcube before 1.1.7 and 1.2.x before 1.2.3, when no SMTP server is configured and the sendmail program is enabled, does not properly restrict the use of custom envelope-from addresses on the sendmail command line, which allows remote authenticated users to execute arbitrary code via a modified HTTP request that sends a crafted e-mail message.
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Analysis
by VulDB Data Team • 09/15/2024
The vulnerability identified as CVE-2016-9920 affects Roundcube webmail applications versions prior to 1.1.7 and 1.2.x before 1.2.3, representing a critical security flaw in the email sending functionality that enables remote code execution through improper input validation. This issue specifically manifests when the application is configured to use the sendmail program instead of an SMTP server, creating a dangerous attack vector for authenticated users who can manipulate email headers to execute arbitrary commands on the underlying system. The vulnerability resides in the steps/mail/sendmail.inc file where the application fails to properly sanitize or validate envelope-from addresses that are passed directly to the sendmail command line without adequate security controls. This misconfiguration allows attackers to inject malicious command-line parameters that bypass normal security boundaries and execute system commands with the privileges of the web application user.
The technical implementation of this vulnerability stems from a lack of proper input sanitization and command execution validation within the email sending mechanism. When Roundcube processes email messages using sendmail, it constructs command-line arguments by incorporating user-provided envelope-from addresses directly into the sendmail command without proper escaping or validation. This creates a classic command injection vulnerability where attackers can append additional command-line parameters to the sendmail invocation, effectively allowing them to execute arbitrary system commands. The flaw is categorized under CWE-78 as a failure to properly escape shell metacharacters, and it aligns with ATT&CK technique T1059.001 for command and scripting interpreter execution. The vulnerability requires authentication to exploit, meaning that only users with valid email accounts can leverage this flaw, but once exploited, the attacker gains the ability to execute commands as the web application user, potentially leading to full system compromise depending on the application's privileges.
The operational impact of CVE-2016-9920 extends beyond simple code execution, as it can lead to complete system compromise when combined with other vulnerabilities or when the web application runs with elevated privileges. Attackers can use this vulnerability to establish persistent access, escalate privileges, or move laterally within a network by executing commands such as creating backdoor accounts, installing malware, or exfiltrating data. The vulnerability affects organizations using Roundcube webmail applications in environments where sendmail is the preferred email delivery method and SMTP configuration is disabled or unavailable, which is common in many enterprise and educational settings. Security professionals should note that this vulnerability was particularly dangerous because it could be exploited by authenticated users without requiring special privileges beyond normal email account access, making it a significant concern for organizations with less strict access controls. The attack surface is further expanded when considering that many organizations deploy Roundcube without proper security hardening, leaving these applications vulnerable to exploitation.
Mitigation strategies for CVE-2016-9920 should focus on immediate patching of affected Roundcube installations to versions 1.1.7 or 1.2.3, which contain the necessary security fixes. Organizations should also implement proper input validation and sanitization for all user-provided data that gets incorporated into system commands, particularly when using sendmail or other external programs. The recommended approach includes disabling the sendmail functionality when SMTP servers are available, as this removes the attack vector entirely. Additionally, implementing proper privilege separation where web applications run with minimal required permissions can limit the damage from successful exploitation. Network segmentation and monitoring of unusual command execution patterns can help detect exploitation attempts. Security teams should also review and harden their Roundcube configurations to ensure that only necessary email functionality is enabled, and that proper access controls are maintained to limit the impact of authenticated attacks. The vulnerability demonstrates the importance of validating and sanitizing all inputs that are passed to system commands, aligning with security best practices outlined in NIST SP 800-160 and ISO/IEC 27001 standards for secure application development and deployment.