MultiTail: Shell injection in execute-on-match command handling

MultiTail is a utility for monitoring multiple log files in one terminal. In version 7.1.5 and earlier, the -ex and -eX execute-on-match feature appended attacker-controlled matched input to a configured command inside double quotes, then check_filter_exec() in mt.c passed the constructed command through execute_program() in exec.c to sh -c. Partial escaping left shell metacharacters such as command substitution active, allowing an attacker who could influence monitored input to execute commands as the user running MultiTail. No fixed release is available as of this review.