CVE-2026-113726: virtualenv activate runs commands from the environment name in zsh with PROMPT_SUBST

zsh users activate an environment with the bash activate script, which puts the environment name into PS1. Since 21.13.0 the script escapes backslashes, dollar signs and backticks in that name, but only when BASH_VERSION is set, so zsh gets the raw value. With the PROMPT_SUBST option set, zsh runs parameter expansion, command substitution and arithmetic expansion on PS1 each time it draws the prompt. A name that contains $(...), backticks or ${...} then runs as commands in the user’s shell, once per prompt.

The name comes from the environment directory, or from --prompt or VIRTUALENV_PROMPT. After you create an environment with:

$ virtualenv 'x$(touch PWNED)y'

source 'x$(touch PWNED)y/bin/activate' in a zsh session with PROMPT_SUBST set creates PWNED, and the prompt shows (xy). Passing that value as --prompt to an environment with a plain name has the same effect. Without PROMPT_SUBST, zsh shows the name as text and runs nothing. bash runs nothing either.