Insufficient output escaping of attachment names in PHPMailer

Impact

CWE-116: Incorrect output escaping.

An attachment added like this (note the double quote within the attachment name, which is entirely valid):

$mail->addAttachment('/tmp/attachment.tmp', 'filename.html";.jpg');

Will result in a message containing these headers:

Content-Type: application/octet-stream; name="filename.html";.jpg" Content-Disposition: attachment; filename="filename.html";.jpg"

The attachment will be named filename.html, and the trailing ";.jpg" will be ignored. Mail filters that reject .html attachments but permit .jpg attachments may be fooled by this.

Note that the MIME type itself is obtained automatically from the *source filename* (in this case attachment.tmp, which maps to a generic application/octet-stream type), and not the *name* given to the attachment (though these are the same if a separate name is not provided), though it can be set explicitly in other parameters to attachment methods.

Patches

Patched in PHPMailer 6.1.6 by escaping double quotes within the name using a backslash, as per RFC822 section 3.4.1, resulting in correctly escaped headers like this:

Content-Type: application/octet-stream; name="filename.html\";.jpg" Content-Disposition: attachment; filename="filename.html\";.jpg"

Workarounds

Reject or filter names and filenames containing double quote (") characters before passing them to attachment functions such as addAttachment().

References

CVE-2020-13625. PHPMailer 6.1.6 release

For more information

If you have any questions or comments about this advisory:

• Open an issue in the PHPMailer repo