virtualenv activate runs commands from the environment name in zsh with PROMPT_SUBST

Impact

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.

A plain zsh starts with PROMPT_SUBST off. oh-my-zsh users have it on: oh-my-zsh loads lib/theme-and-appearance.zsh at startup, and that file sets it. An attacker needs to influence the environment path or prompt: a CI job or wrapper tool that passes a branch, project or job name through, or a directory in a cloned repository or unpacked archive. --prompt . takes the current directory's name.

zsh also reads % in PS1 as a prompt escape, so a name with %n showed the user name in place of those characters, though without running commands.

The bug is in releases up to and including 21.14.3. The payload runs on 20.26.6 and 21.14.3, and the reporter confirmed 21.9.1.

Patches

Fixed in 21.14.4. Under zsh, activate now escapes the name the way it does under bash and doubles %, so the example above runs nothing and the prompt shows the name as given. zsh gets the escaping even when PROMPT_SUBST is off, because a user can set the option after activation; a name with a backslash, dollar sign or backtick then shows the added backslashes. Fix: https://github.com/pypa/virtualenv/commit/5b915367da82bd024b2a0d28d2f98007034b4aa2