Starship vulnerable to shell injection via undocumented, unpredictable shell expansion in custom commands

Description

Starship is a cross-shell prompt. Starting in version 1.0.0 and prior to version 1.20.0, undocumented and unpredictable shell expansion and/or quoting rules make it easily to accidentally cause shell injection when using custom commands with starship in bash. Version 1.20.0 fixes the vulnerability.

PoC

Have some custom command which prints out information from a potentially untrusted/unverified source.

[custom.git_commit_name]
command = 'git show -s --format="%<(25,mtrunc)%s"'
style = "italic"
when = true

Impact

This issue only affects users with custom commands, so the scope is limited, and without knowledge of others' commands, it could be hard to successfully target someone.