virtualenv is a tool for creating isolated virtual python environments. From 21.14.9 until 21.14.13, the bash bin/activate script can copy an attacker-controlled --prompt value into PS1 without escaping dollar signs or backticks when both promptvars and POSIX mode are disabled during activation. Re-enabling promptvars, running set -o posix, or starting an interactive child bash that inherits the exported PS1 can then expand command substitution in the stored VIRTUAL_ENV_PROMPT value and execute commands as the user who activates the environment. This issue is fixed in version 21.14.13.
Affected: PyPI: virtualenv
## Summary
The bash activator puts the caller-supplied prompt value into `PS1` unescaped whenever `promptvars` is off at activation time, and assumes prompt expansion can never run afterwards. `promptvars` and posix mode are runtime shell state, not a property of the activation moment: re-enabling prompt expansion in the same shell (`shopt -s promptvars`, `set -o posix`) or starting an interactive child bash that inherits the exported `PS1` makes bash expand the stored value and run the command substitution in it. An attacker who controls the prompt value gets command execution as the user who sources `bin/activate`.
This is a regression. 21.14.8 escaped `$` and the backtick unconditionally and stays inert under the same sequence. The 21.14.9 fix for GHSA-6w3r-xr62-4xcp (#3418) rewrote that block and left bash with a path that stores the raw value whenever `promptvars` is off.
## Affected area
Activation scripts (activate, activate.bat, activate.csh, activate.fish, activate.nu, Activate.ps1, activate.xsh) — specifically the bash `bin/activate`.
## Attacker-controlled input
The `--prompt` value, which the security policy already lists as a value that may come from project metadata the user did not write. It reaches `virtualenv` as `--prompt` on the command line or as the `prompt` argument of the API, is written into `bin/activate` as the literal `VIRTUAL_ENV_PROMPT` assignment (correctly quoted, the value arrives there as data), and the script then copies it into `PS1`…