Summary
Remote Code Execution vulnerability in PHPMailer 6.4.1 running on Windows
PHPMailer 6.4.1 contains a possible remote code execution vulnerability through the $lang_path parameter of the setLanguage() method. If the $lang_path parameter is passed unfiltered from user input, it can be set to a UNC path, and if an attacker is also able to create a remote mount on the server that the UNC path points to, a script file under their control may be executed.
Workarounds
Any of:
- Ensure that calling code does not pass unfiltered user-supplied data to the
$lang_pathparameter of thesetLanguage()method. - Block or filter the use of unknown UNC paths in this parameter (or altogether).
- Ensure that unauthorised users do not have the ability to read from unknown remote servers via UNC paths.
- Run on an OS that does not support UNC paths
References
Reported by listensec.com via Tidelift.
For more information
If you have any questions or comments about this advisory:
- Open an issue in PHPMailer
- Email the maintainers
Impact
Arbitrary code may be run by a remote attacker under the web server or PHP process running on Window hosts.
Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.
CVE-2021-34551 has a CVSS score of 8.1 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.5.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Mitigated in PHPMailer 6.5.0 by no longer treating translation files as PHP code, but by parsing their text content directly.
This approach avoids the possibility of executing unknown code while retaining backward compatibility. This isn't ideal, so the current translation format is deprecated and will be replaced in the next major release.
Frequently Asked Questions
- What is CVE-2021-34551? CVE-2021-34551 is a high-severity code injection vulnerability in phpmailer/phpmailer (composer), affecting versions < 6.5.0. It is fixed in 6.5.0. Untrusted input is evaluated as executable code within the application's runtime environment.
- How severe is CVE-2021-34551? CVE-2021-34551 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of phpmailer/phpmailer are affected by CVE-2021-34551? phpmailer/phpmailer (composer) versions < 6.5.0 is affected.
- Is there a fix for CVE-2021-34551? Yes. CVE-2021-34551 is fixed in 6.5.0. Upgrade to this version or later.
- Is CVE-2021-34551 exploitable, and should I be worried? Whether CVE-2021-34551 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-34551 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-34551? Upgrade
phpmailer/phpmailerto 6.5.0 or later.