Summary
PHPMailer untrusted code may be run from an overridden address validator
If a function is defined that has the same name as the default built-in email address validation scheme (php), it will be called in default configuration as when no validation scheme is provided, the default scheme's callable php was being called. If an attacker is able to inject such a function into the application (a much bigger issue), it will be called whenever an email address is validated, such as when calling validateAddress().
Workarounds
Inject your own email validator function.
References
Reported by Vikrant Singh Chauhan via huntr.dev.
CVE-2021-3603
For more information
If you have any questions or comments about this advisory:
- Open an issue in the PHPMailer project
- Email us.
Impact
Low impact, exploitation requires that an attacker can already inject code into an application, but it provides a trigger pathway.
CVE-2021-3603 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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This is patched in PHPMailer 6.5.0 by denying the use of simple strings as validator function names, which is a very minor BC break.
Frequently Asked Questions
- What is CVE-2021-3603? CVE-2021-3603 is a high-severity security vulnerability in phpmailer/phpmailer (composer), affecting versions < 6.5.0. It is fixed in 6.5.0.
- How severe is CVE-2021-3603? CVE-2021-3603 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-2021-3603? phpmailer/phpmailer (composer) versions < 6.5.0 is affected.
- Is there a fix for CVE-2021-3603? Yes. CVE-2021-3603 is fixed in 6.5.0. Upgrade to this version or later.
- Is CVE-2021-3603 exploitable, and should I be worried? Whether CVE-2021-3603 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-2021-3603 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-2021-3603? Upgrade
phpmailer/phpmailerto 6.5.0 or later.