CVE-2016-10033: Detection and Response Guide for 2025

Almost a decade after its discovery, the critical remote code execution vulnerability known as CVE-2016-10033 continues to pose a significant threat to web applications worldwide.

In this post, we explain why it's so dangerous and the essential steps to protect your systems from this critical exposure in 2025.

What is CVE-2016-10033?

CVE-2016-10033 is a critical remote code execution vulnerability discovered in PHPMailer, an immensely popular code library for sending emails from PHP scripts. The issue stems from how PHPMailer handles the Sender field when using the mail() transport. Attackers can inject extra parameters into the sendmail command, allowing them to execute arbitrary code on the server..

The vulnerability specifically affects PHPMailer versions prior to 5.2.18. While a fix was issued in version 5.2.18, it was later found to be incomplete. This led to a second vulnerability, CVE-2016-10045, because the initial patch did not properly sanitize the sender's email address before passing it to the mail() function.

The core problem is that special characters meant for the shell were not being escaped correctly, allowing an attacker to craft a malicious address that could bypass the first patch. A complete fix addressing both vulnerabilities wasn’t implemented until version 5.2.20.

What makes CVE-2016-10033 so dangerous?

PHPMailer is embedded in numerous content management systems (CMS) and custom applications. As a result, a vulnerability in PHPMailer can surface in any number of downstream apps, even if developers don’t realize they’re using it.

With a CVSS score of 9.8 (Critical), CVE-2016-10033 poses a severe damage potential.

An attacker can exploit this flaw through common, publicly accessible website features:

• Contact and feedback forms
• User registration forms
• Password reset functionalities

By submitting a specially crafted email address to one of these forms, an attacker can write a malicious file (a "backdoor" or "shell") to the web root and then execute it. This gives the attacker control over the web server, allowing them to steal data, deface the website, or use the server for further malicious activities. 

Here’s why CVE-2016-10033 remains a serious risk in 2025:

• Widespread use: Found in WordPress, Joomla!, Drupal, SugarCRM, and many custom PHP applications.
• Unauthenticated remote code execution: Attackers don’t need login access. A single malicious contact form submission can compromise the server.
• Still exploited today: As of July 2025, over 13,500 internet-facing systems were confirmed to be running vulnerable versions.

How to detect if you're exposed

Start by checking your codebase and infrastructure. You’ll need to determine if you or any third parties you rely on are using outdated versions of PHPMailer.

If you use a Content Management System (CMS) like WordPress, Joomla, or Drupal, ensure the core platform and all third-party plugins/extensions are fully updated. These systems often bundle PHPMailer, and an outdated plugin could expose your entire site.

Ask your vendors

Ask your third-party service providers and vendors if their software uses PHPMailer and if they have applied the necessary patches. This can be done through a custom security questionnaire.

Watch this video to learn how UpGuard streamlines vendor collaboration during such time-critical scenarios.

How to fix exposures to CVE-2016-10033

If you find you're running a vulnerable version (or aren’t sure), take these actions immediately:

1. Upgrade PHPMailer

• The safest option is to move to the latest 6.x version.
• At a minimum, upgrade to 5.2.20, which addresses both CVE-2016-10033 and the subsequent bypass (CVE-2016-10045).

2. Patch via your operating system

• For Debian-based systems, update PHPMailer using:

sudo apt-get update
sudo apt-get install --only-upgrade libphp-phpmailer

• Confirm vendors have also applied these patches or upgraded their software.

3. Harden mail handling

• Switch to SMTP: Avoid using isMail() (which invokes sendmail) and instead configure PHPMailer to use authenticated SMTP with isSMTP().
• Validate email inputs: Sanitize and reject suspicious From or Sender values that include quotes or control characters.
• Apply WAF rules: Temporarily block web form payloads containing dangerous strings like -X or -OQueueDirectory.

4. Verify and monitor

• Re-scan affected systems after patching.
• Monitor CISA’s Known Exploited Vulnerabilities (KEV) catalog and distribution security trackers for any future advisories.