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Actively exploited

CVE-2016-10033 PHPMailer Command Injection Vulnerability

PHPMailer contains a command injection vulnerability because it fails to sanitize user-supplied input. Specifically, this issue affects the 'mail()' function of 'class.phpmailer.php' script. An attacker can exploit this issue to execute arbitrary code within the context of the application. Failed exploit attempts will result in a denial-of-service condition.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 7 Jul 2025.
  • Rated Critical severity (CVSS 9.8, scored by CISA).
  • It has been in the catalogue for 487 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

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.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for PHPMailer. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 28 Jul 2025 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

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CVE-2019-11043 · RansomwarePHP FastCGI Process Manager (FPM) Buffer Overflow VulnerabilityAdded 25 Mar 2022CVE-2012-1823PHP-CGI Query String Parameter VulnerabilityAdded 25 Mar 2022

All 3 exploited PHP vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2016-10033
  3. CVE record: CVE-2016-10033
  4. FIRST EPSS
  5. github.com/advisories/GHSA-5f37-gxvh-23v6
  6. packetstormsecurity.com/files/140350/PHPMailer-Sendmail-Argument-Injection.html
  7. drupal.org/psa-2016-004
  8. exploit-db.com/exploits/42221/
  9. exploit-db.com/exploits/40969/
  10. exploit-db.com/exploits/41962/
  11. exploit-db.com/exploits/40968/
  12. legalhackers.com/advisories/PHPMailer-Exploit-Remote-Code-Exec-CVE-2016-10033-Vuln.html