The Custom Content Shortcode WordPress plugin through 4.0.2 does not validate one of its shortcode attribute, which could allow users with a contributor role and above to include arbitrary files via a traversal attack. This could also allow them to read non PHP files and retrieve their content. RCE could also be achieved if the attacker manage to upload a malicious image containing PHP code, and then include it via the affected attribute, on a default WP install, authors could easily achieve that given that they have the upload_file capability.
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