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

Vulnerability Description

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.

References

• https://github.com/asterisk/asterisk/security/advisories/GHSA-c4cg-9275-6w44
• https://github.com/asterisk/asterisk/commit/42a2f4ccfa2c7062a15063e765916b3332e34cc4
• https://github.com/asterisk/asterisk/commit/7a0090325bfa9d778a39ae5f7d0a98109e4651c8
• https://github.com/asterisk/asterisk/commit/b4063bf756272254b160b6d1bd6e9a3f8e16cc71
• https://github.com/asterisk/asterisk/commit/bbe68db10ab8a80c29db383e4dfe14f6eafaf993
• https://github.com/asterisk/asterisk/commit/faddd99f2b9408b524e5eb8a01589fe1fa282df2
• https://github.com/asterisk/asterisk/blob/14367caaf7241df1eceea7c45c5b261989c2c6db/main/manager.c#L6426
• https://github.com/asterisk/asterisk/blob/7d28165cb1b2d02d66e8693bd3fe23ee72fc55d8/main/manager.c#L6426