TypeError: PHPMailer\DKIMValidator\DKIM::canonicalizeBody(): Argument #2 ($style) must be of type string, null given

[prilly-dev]

Steps to reproduce

Describe the bug
A TypeError occurs during DKIM validation in Nextcloud Mail. The PHPMailer DKIMValidator crashes because it receives a null value for the canonicalization $style argument, which is expected to be a string. This issue is particularly prevalent in environments running PHP 8.2+ where type hinting is strictly enforced.
Steps to reproduce

Use Nextcloud Mail in an environment running PHP 8.2 or 8.3 (e.g., Debian 13 based Docker containers).
Receive or view an email that triggers the internal DKIM validation logic.
The app fails to render the mail or throws a 500 error, and the following entry appears in the Nextcloud logs.

Nextcloud version: (32.0.3)
Mail app version: 5.6.5
PHP version: 8.2 / 8.3 (Debian 13 Docker environment) Nextcloud-fpm image
Database: MySQL/MariaDB
MTA: Postfix + Rspamd (External validation confirms DKIM signatures are valid)

Expected behavior

Expected behavior
The DKIM validator should gracefully handle cases where the canonicalization style cannot be determined, or provide a default string value (e.g., 'simple' or 'relaxed') to avoid a fatal TypeError.
Actual behavior
The application logs a fatal error:

PHPMailer\DKIMValidator\DKIM::canonicalizeBody(): Argument #2 ($style) must be of type string, null given, called in /var/www/html/custom_apps/mail/vendor/phpmailer/dkimvalidator/src/Validator.php on line 102

Environment information

Actual behavior

TypeError: PHPMailer\DKIMValidator\DKIM::canonicalizeBody(): Argument #2 ($style) must be of type string, null given, called in /var/www/html/custom_apps/mail/vendor/phpmailer/dkimvalidator/src/Validator.php on line 102 in file '/var/www/html/custom_apps/mail/vendor/phpmailer/dkimvalidator/src/DKIM.php' line 127

Mail app version

5.6.5

Nextcloud version

32.0.3

Mailserver or service

postfix + dovecot + rspamd

Operating system

nextcloud-fpm image

PHP engine version

PHP 8.3

Nextcloud memory caching

apcu redis

Web server

Apache (supported)

Database

MariaDB

Additional info

Additional context
External testers (like Mail-Tester/Mecasa) confirm the DKIM signatures on the processed emails are technically correct and valid. The issue seems isolated to the internal PHP-based parsing of the message body during the validation phase within the Mail app. Disabling dkim-validation via OCC resolves the crash, confirming the source of the error.

Update: I've also identified an Undefined array key 1 error at Validator.php#87. This happens when the DKIM c= tag contains only a single value (e.g., c=relaxed instead of c=relaxed/relaxed). The code assumes a split will always yield two elements, leading to a null value being passed to canonicalizeBody(), which triggers the subsequent TypeError.

[joshtrichards]

https://github.com/PHPMailer/DKIMValidator/issues?q=is%3Aissue%20state%3Aopen%20style

[midiakiasat]

This looks like a DKIM parsing invariant violation rather than a PHP 8.x issue.

Per RFC 6376, the c= tag may legally be a single value (e.g. c=relaxed), which applies to both header and body canonicalization. The current code path appears to assume c= always contains two parts (header/body), leading to an undefined index and a null body style.

Invariant to enforce:

• If c= contains one value, apply it to both header and body.
• If it contains two values, map explicitly.
• Never pass null into canonicalization functions.

Failing closed (explicit default or validation error) is preferable to implicit null propagation.

[DerDreschner]

Thank you very much for your bug report! We were able to confirm the issue. This is an upstream issue, which is being reported in the following two tickets:

PHPMailer/DKIMValidator#10
PHPMailer/DKIMValidator#11

I'll leave the ticket open here as well that we can have a look into it in the future.

[prilly-dev]

Thank you