CVE-2016-10033 in PHPMailer
Summary
by MITRE
The mailSend function in the isMail transport in PHPMailer before 5.2.18 might allow remote attackers to pass extra parameters to the mail command and consequently execute arbitrary code via a \" (backslash double quote) in a crafted Sender property.
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Analysis
by VulDB Data Team • 07/07/2025
The vulnerability identified as CVE-2016-10033 represents a critical command injection flaw within the PHPMailer library that affects versions prior to 5.2.18. This issue specifically targets the isMail transport implementation and exploits a weakness in how the mailSend function processes the Sender property. The vulnerability arises from insufficient input validation and sanitization mechanisms that fail to properly escape or filter special characters in user-supplied data. Attackers can leverage this weakness by crafting a malicious Sender property containing a backslash double quote sequence which gets interpreted by the underlying mail command execution mechanism, potentially leading to arbitrary code execution on the target system. The flaw exists at the intersection of improper input handling and command execution, creating a pathway for remote attackers to compromise systems that utilize vulnerable PHPMailer versions.
The technical exploitation of this vulnerability relies on the improper handling of the mail command parameters within the isMail transport method. When PHPMailer processes email sending operations through the mail transport, it directly passes user-provided data to the system's mail command without adequate sanitization. The backslash double quote character sequence creates a situation where the mail command interprets additional parameters that were not intended by the application logic. This behavior stems from the fact that the mail command accepts additional parameters through the command line interface, and when these parameters are not properly escaped or quoted, they can be interpreted as command arguments rather than literal data. The vulnerability is classified as a command injection attack pattern, specifically related to CWE-77 which encompasses improper neutralization of special elements used in commands.
The operational impact of CVE-2016-10033 extends beyond simple code execution to potentially enable full system compromise. An attacker who successfully exploits this vulnerability can execute arbitrary commands with the privileges of the web server process, which typically runs with limited system permissions but may still provide access to sensitive data or system resources. This vulnerability is particularly dangerous in web applications that process user input through email functionality, as it allows attackers to bypass traditional input validation measures. The attack vector is straightforward and requires minimal technical expertise to execute, making it attractive to both skilled and less experienced threat actors. Additionally, the vulnerability can be exploited in various contexts including web applications, content management systems, and any platform that relies on PHPMailer for email functionality, potentially affecting thousands of systems across different organizations.
Mitigation strategies for CVE-2016-10033 primarily focus on immediate remediation through version updates and input validation enhancements. Organizations should prioritize upgrading to PHPMailer version 5.2.18 or later, which includes proper input sanitization and parameter handling mechanisms that prevent the injection of malicious command parameters. The fix implemented in the patched version involves rigorous escaping and validation of all user-supplied data before it is passed to the mail command execution. Additional defensive measures include implementing proper input validation at multiple layers of the application, using secure coding practices that prevent direct command injection, and employing web application firewalls that can detect and block suspicious parameter sequences. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving command injection and privilege escalation, and should be addressed through both defensive and detection measures to prevent exploitation. Organizations should also conduct comprehensive vulnerability assessments to identify other potential command injection points in their applications and implement proper security monitoring to detect exploitation attempts.