How severe is CVE-2021-3603?

CVE-2021-3603 has a CVSS score of 8.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-3603?

CVE-2021-3603 affects the following packages: phpmailer/phpmailer (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-3603?

Update phpmailer/phpmailer to 6.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3603 is resolved across your whole dependency graph.

How do I detect CVE-2021-3603 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for phpmailer/phpmailer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-3603 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-3603 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-3603 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-3603?

CVE-2021-3603 has an EPSS (Exploit Prediction Scoring System) score of 2.3%, placing it in the 83th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-3603?

CVE-2021-3603 is classified as CWE-829 (CWE-829). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-3603 published, and has it been updated?

CVE-2021-3603 was published on June 17, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

HIGH severity

CVE-2021-3603 — phpmailer

HIGHFix: PHPMailer/PHPMailer@45f3c18

CVE-2021-3603 is a high-severity (CVSS 8.1) CWE-829 vulnerability in phpmailer/phpmailer. A fix is available for phpmailer/phpmailer — see the affected versions and patch details below.

PHPMailer untrusted code may be run from an overridden address validator

Also known asBIT-phpmailer-2021-3603GHSA-77mr-wc79-m8j3

Published

Jun 17, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 21, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs83th percentile — riskier than 83% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-3603 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 395,249 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐘phpmailer/phpmailer

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

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().

Impact

Low impact – exploitation requires that an attacker can already inject code into an application, but it provides a trigger pathway.

Patches

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.

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`phpmailer/phpmailer`	all versions	6.5.0`composer require phpmailer/phpmailer:^6.5.0`

Affected Products

2 products · 3 configurations

fedorafedoraproject

2 versions

3334

Application

phpmailerphpmailer_project

≤ 6.4.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for phpmailer/phpmailer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update phpmailer/phpmailer to 6.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3603 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

UbuntuHIGH

USN-5956-1Ubuntu

Frequently Asked Questions

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()`. ### Impact Low impact – exploitation requires that an attacker can already inject code into an application, but it provides a trigger pathway. ### Patche