PHPMailer Command Injection (CVE-2016-10033)

Historical catalog analysis: CISA added this entry on July 07, 2025. The entry reflects catalog information retrieved on August 31, 2026, not a snapshot archived on the inclusion date. Check current vendor guidance before acting.

What the vulnerability is

CVE-2016-10033 is a command injection vulnerability located within the mail() function of the class.phpmailer.php script in PHPMailer. The flaw stems from a failure to properly sanitize user-supplied input, which could allow an attacker to execute arbitrary code within the context of the application. In instances where exploit attempts fail, the result may be a denial-of-service (DoS) condition.

Exposure and applicability

This vulnerability affects systems utilizing the PHPMailer library where the mail() function is exposed to untrusted user input. Because PHPMailer is an open-source component frequently integrated into various third-party libraries, proprietary applications, and diverse software products, exposure is not limited to a single application but extends to any environment deploying the affected version of this script.

Remediation priorities

Our analysis suggests prioritizing remediation based on the accessibility of the PHPMailer component to external inputs. We recommend the following actions:

1. Component Identification: Infrastructure owners should audit their codebase and third-party dependencies to identify all instances of class.phpmailer.php currently in use.
2. Vendor Mitigation Application: Apply corrective updates as specified in the vendor’s official instructions (e.g., referencing GitHub releases/advisories).
3. Service Evaluation: For cloud services, follow applicable BOD 22-01 guidance to ensure the vulnerability is addressed across managed environments.
4. Decommissioning: If a viable mitigation or patch cannot be applied to a specific legacy implementation, the component should be discontinued to eliminate the attack surface.

How to validate remediation

To verify that exposure has been reduced, defenders should move beyond simple version checks. Validation should include:

• Configuration Audit: Confirming that the mail() function is no longer processing unsanitized user input.
• Dependency Verification: Ensuring that the deployed library matches the vendor’s remediated release through cryptographic hash verification of the script files.
• Functional Testing: Verifying that the application continues to send mail correctly while rejecting inputs containing command injection characters (e.g., shell metacharacters) in fields passed to the mail() function.

Limits and open questions

Applying a patch or updating the library reduces the likelihood of exploitation but does not guarantee total immunity if other architectural flaws exist. Residual risk remains if the application employs additional custom wrappers around PHPMailer that introduce new sanitization failures. It remains unknown how many proprietary products have bundled this specific version of the library without providing an independent update mechanism for their end users.

Source and editorial note

CVE-2016-10033: PHPMailer Command Injection Vulnerability · Source date: July 07, 2025 · Retrieved August 31, 2026.

Material facts are tied to the cited primary source. Recommendations are independent defensive analysis unless attributed to the source. Confirm consequential decisions against current authoritative guidance.

Archive date: July 10, 2025. The displayed post date is assigned three days after the source date to organize this retrospective archive; it does not mean this site published the analysis then. First published by this site: October 09, 2026 at 00:44 UTC.

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