CVE-2021-3603: PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such co

Q: How do I fix CVE-2021-3603?

phpmailer: upgrade to newer than 6.4.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-3603 being exploited?

CVE-2021-3603 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-3603 affect?

CVE-2021-3603 affects phpmailer, fedora. Severity is high (CVSS 8.1). PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such code is injected into the host project's scope by other means). If the $patternselect parameter to validateAddress() is set to 'php'

Q: How urgent is CVE-2021-3603?

Priority 54/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 8.1REMOTEUPGRADEMITIGATE

CVE-2021-3603 is a high-severity flaw (CVSS 8.1) affecting phpmailer, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such code is injected into the host project's scope by other means). If the $patternselect parameter to validateAddress() is set to 'php' (the default, defined by PHPMailer::$validator), and the global namespace contains a function called php, it will be called in preference to the built-in validator of the same name. Mitigated in PHPMailer 6.5.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	8.1 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	2.3% (top 17%)
CISA exploited list	No
Weakness	CWE-829
Published	2021-06-17
Last modified	2026-06-17

Publisher (CNA): huntr.dev · Vendor: fedoraproject · phpmailer project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-06-17 12:15 UTCPublished CVE record published
2026-06-17 04:05 UTCUpdated Record last modified

phpmailer project phpmailer UPGRADE

Affected: <= 6.4.1

Fixed in: newer than 6.4.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy phpmailer || rpm -q phpmailer || phpmailer --version

2. UPGRADE to newer than 6.4.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade phpmailer

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade phpmailer

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

fedoraproject fedora MITIGATE

Affected: 33 · 34

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3603?

phpmailer: upgrade to newer than 6.4.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-3603 being exploited?

CVE-2021-3603 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3603 affect?

CVE-2021-3603 affects phpmailer, fedora. Severity is high (CVSS 8.1). PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such code is injected into the host project's scope by other means). If the $patternselect parameter to validateAddress() is set to 'php'

How urgent is CVE-2021-3603?

Priority 54/100. High severity; Remotely exploitable, no login, no user action

Software pages

PHP vulnerabilities

Related CVEs

CVE-2021-34551 HIGH 8.1 UPGRADE MITIGATE PHPMailer before 6.5.0 on Windows allows remote code execution if lang_path is untrusted data and has a UNC pa
CVE-2020-36326 CRITICAL 9.8 UPGRADE UPGRADE PHPMailer 6.1.8 through 6.4.0 allows object injection through Phar Deserialization via addAttachment with a UN
CVE-2020-13625 HIGH 7.5 UPGRADE MITIGATE PHPMailer before 6.1.6 contains an output escaping bug when the name of a file attachment contains a double qu
CVE-2018-19296 HIGH 8.8 UPGRADE MITIGATE PHPMailer before 5.2.27 and 6.x before 6.0.6 is vulnerable to an object injection attack.
CVE-2017-5223 MEDIUM 5.5 UPGRADE An issue was discovered in PHPMailer before 5.2.22

References

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