xterm before 375 allows code execution via font ops, e.g., because an OSC 50 response may have Ctrl-g and therefore lead to command execution within the vi line-editing mode of Zsh. NOTE: font ops are not allowed in the xterm default configurations of some Linux distributions.
The default configuration of xterm on Debian GNU/Linux sid and possibly Ubuntu enables the allowWindowOps resource, which allows user-assisted attackers to execute arbitrary code or have unspecified other impact via escape sequences.