Asterisk allows `Write=originate` as sufficient permissions for code execution / `System()` dialplan

Asterisk is an open source private branch exchange (PBX) and telephony toolkit. Prior to asterisk versions 18.24.2, 20.9.2, and 21.4.2 and certified-asterisk versions 18.9-cert11 and 20.7-cert2, an AMI user with write=originate may change all configuration files in the /etc/asterisk/ directory. This occurs because they are able to curl remote files and write them to disk, but are also able to append to existing files using the FILE function inside the SET application. This issue may result in privilege escalation, remote code execution and/or blind server-side request forgery with arbitrary protocol. Asterisk versions 18.24.2, 20.9.2, and 21.4.2 and certified-asterisk versions 18.9-cert11 and 20.7-cert2 contain a fix for this issue.