virtualenv is a tool for creating isolated virtual python environments. Prior to 21.13.0, the bash activate script that zsh also sources leaves the environment name unescaped; from 21.13.0 until 21.14.4, it escapes backslashes, dollar signs, and backticks only when BASH VERSION is set, so zsh receives the raw value. The script assigns that value to PS1. In zsh with PROMPT SUBST, an attacker-influenced environment directory, --prompt, or VIRTUALENV PROMPT value containing command, parameter, or arithmetic expansion syntax can therefore execute commands in the user's shell each time the prompt is drawn. A percent sign is separately interpreted as a zsh prompt escape but does not by itself execute commands. Plain zsh with PROMPT SUBST disabled and bash are not affected by this mechanism. This issue is fixed in version 21.14.4.