The Ubuntu-specific language helper scripts (save-to-pam-env, update-langlist) shipped with accountsservice before 23.13.9-8ubuntu7 treat the user-controlled LANGUAGE entry in ~/.pam_environment as trusted input. The value is interpolated unescaped into a GNU sed replacement expression, allowing an attacker to inject a sed 'e' flag and arbitrary shell commands that execute with the privileges of the AccountsService helper process (real UID 0) via the SetLanguage D-Bus method.
Exploitability
AV:LAC:LPR:LUI:NScope
S:UImpact
C:HI:HA:H7.8/CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:HInjection