Remote Code Execution vulnerability in PHPMailer 6.4.1 running on Windows

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.1 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS

• CVE-2021-34551: 2.80% estimated exploitation activity in the next 30 days; percentile 86.50 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

PHPMailer 6.4.1 contains a possible remote code execution vulnerability through the $langpath parameter of the setLanguage() method. If the $langpath 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 $langpath parameter of the setLanguage() method. Block or…

Published by the source

22 Jun 2021, 15:17 UTC

Last modified by the source

7 Feb 2024, 18:15 UTC

Source last checked

26 Oct 2026, 14:15 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-7q44-r25x-wm4q

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# Patch Next advisory brief: CVE-2021-34551

Advisory: Remote Code Execution vulnerability in PHPMailer 6.4.1 running on Windows
IDs: CVE-2021-34551, GHSA-7q44-r25x-wm4q
Record: https://patchnext.com/advisories/cve-2021-34551 (revision 1, source checked 26 Oct 2026, 14:15 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: phpmailer/phpmailer 6.5.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist phpmailer/phpmailer: affected < 6.5.0; fixed in 6.5.0

## Suggested inspection (read-only)
- Packagist phpmailer/phpmailer: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on phpmailer/phpmailer.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-7q44-r25x-wm4q (GitHub-reviewed advisory GHSA-7q44-r25x-wm4q lists affected ranges: Packagist phpmailer/phpmailer >=0 <6.5.0)

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