Looking at the response, it appears to be generated by Xenforo rather than anywhere else. What's more, another Xenforo forum I use does not reject "cmd.exe" as a string in post text. However, it is using a previous version of Xenforo ('cos I get no rich text editing from iOS, like we used to here).I can't see why there should be a restriction in text in a forum post - it's not like anything will be looking for executable commands. Perhaps parties with Xenforo knowledge could look into it and see if it's a hard-coded restriction or something in the configuration for this forum in particular, or maybe imposed by the web host.
Replaced with UPD and HiDef?cmd.exe telnet.exe fixed