How severe is CVE-2021-34551?

CVE-2021-34551 has a CVSS score of 8.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-34551?

CVE-2021-34551 affects the following packages: phpmailer/phpmailer (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-34551?

Update phpmailer/phpmailer to 6.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-34551 is resolved across your whole dependency graph.

How do I detect CVE-2021-34551 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-2021-34551 if there is no patch (or I can't update yet)?

Treat uploaded files as untrusted until proven otherwise: validate the actual content type rather than the supplied extension, store uploads outside the web root on a volume mounted without execute permission, and rename them to server-generated identifiers so an attacker cannot choose the path a request will later resolve.

Is CVE-2021-34551 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-34551 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-2021-34551?

CVE-2021-34551 has an EPSS (Exploit Prediction Scoring System) score of 2.8%, placing it in the 86th 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-2021-34551?

CVE-2021-34551 is classified as Unrestricted File Upload (CWE-434). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-34551 published, and has it been updated?

CVE-2021-34551 was published on June 16, 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-2021-34551 — phpmailer

HIGHFix: PHPMailer/PHPMailer@acd264b

CVE-2021-34551 is a high-severity (CVSS 8.1) Unrestricted File Upload vulnerability in phpmailer/phpmailer. A fix is available for phpmailer/phpmailer — see the affected versions and patch details below.

Remote Code Execution vulnerability in PHPMailer 6.4.1 running on Windows

Also known asBIT-phpmailer-2021-34551GHSA-7q44-r25x-wm4q

Published

Jun 16, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 24, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs86th percentile — riskier than 86% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-34551 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

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.

Impact

Arbitrary code may be run by a remote attacker under the web server or PHP process running on Window hosts.

Patches

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.

Workarounds

Any of:

Ensure that calling code does not pass unfiltered user-supplied data to the $lang_path parameter of the setLanguage() 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

CVE-2021-34551.

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

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

< 6.5.0

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-2021-34551 is resolved across your whole dependency graph.
Workarounds
Treat uploaded files as untrusted until proven otherwise: validate the actual content type rather than the supplied extension, store uploads outside the web root on a volume mounted without execute permission, and rename them to server-generated identifiers so an attacker cannot choose the path a request will later resolve.

Frequently Asked Questions

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](https://docs.microsoft.com/en-us/dotnet/standard/io/file-path-formats#unc-paths), 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. ### Impact Arbitrary code may be run by a remote attacker under the web server or PHP process running on Window hosts. ### Patch