GHSA-cx6f-vjhg-g3cc

virtualenv: Bash activator leaves the --prompt value unescaped in PS1 when promptvars is off, so prompt expansion later executes it (regression in 21.14.9)

## 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 `$` and the backtick unconditionally and stays inert under the same sequence. The 21.14.9 fix for GHSA-6w3r-xr62-4xcp (#3418) rewrote that block and left bash with a path that stores the raw value whenever `promptvars` is off. ## Affected area Activation scripts (activate, activate.bat, activate.csh, activate.fish, activate.nu, Activate.ps1, activate.xsh) — specifically the bash `bin/activate`. ## Attacker-controlled input The `--prompt` value, which the security policy already lists as a value that may come from project metadata the user did not write. It reaches `virtualenv` as `--prompt` on the command line or as the `prompt` argument of the API, is written into `bin/activate` as the literal `VIRTUAL_ENV_PROMPT` assignment (correctly quoted, the value arrives there as data), and the script then copies it into `PS1` without escaping `$` or the backtick. Example: a repository or tool config supplies a prompt such as `x$(id > /tmp/ve-pwn)y`; the victim creates and later activates the environment. ## Details Source of the latest release: [`src/virtualenv/activation/bash/activate.sh` lines 140-160](https://github.com/pypa/virtualenv/blob/21.14.12/src/virtualenv/activation/bash/activate.sh#L140-L160) ```sh if [ -z "${VIRTUAL_ENV_DISABLE_PROMPT-}" ] && { [ -n "${BASH_VERSION-}" ] || [ -n "${ZSH_VERSION-}" ]; }; then _OLD_VIRTUAL_PS1="${PS1-}" # bash expands PS1 after decoding its prompt escapes when promptvars is on or in posix mode, so the prompt holds a # reference to the variable and its value never reaches that expansion; otherwise only the decode runs and # backslashes need doubling. ... if [ -n "${BASH_VERSION-}" ] && { shopt -q promptvars || shopt -qo posix; }; then PS1='(${VIRTUAL_ENV_PROMPT-}) '"${PS1-}" else _VIRTUAL_PROMPT="${VIRTUAL_ENV_PROMPT//\\/\\\\}" if [ -n "${ZSH_VERSION-}" ]; then _VIRTUAL_PROMPT="${_VIRTUAL_PROMPT//\$/\\\$}" _VIRTUAL_PROMPT="${_VIRTUAL_PROMPT//\`/\\\`}" _VIRTUAL_PROMPT="${_VIRTUAL_PROMPT//\%/%%}" fi PS1="(${_VIRTUAL_PROMPT}) ${PS1-}" unset _VIRTUAL_PROMPT fi fi ``` Two separate mistakes combine: 1. A runtime shell option is treated as a constant. `shopt -u promptvars` at the moment of `source` does not bind the shell forever: `shopt -s promptvars` and `set -o posix` both re-enable PS1 expansion in the same shell. The code itself tests `shopt -qo posix` because posix mode expands the prompt regardless of `promptvars`, which shows the expansion state is not fixed by the activation. 2. The escaping that would keep the value inert in that mode is missing for bash. GHSA-5vjq-rrrf-7h2q ("activate runs commands from the environment name in zsh with PROMPT_SUBST") fixed exactly this shape for zsh, reasoning that "a user can set it after activation", by escaping `$` and the backtick. Bash lost the equivalent escaping in the same commit that added the zsh branch. The rendered prompt shows the regression directly: ``` 21.14.8 PS1 = (x\\$(id > /tmp/ve-pwn)y) <- escaped, inert 21.14.9 PS1 = (x$(id > /tmp/ve-pwn)y) <- raw value 21.14.12 PS1 = (x$(id > /tmp/ve-pwn)y) <- raw value ``` Relevant commit: [`5f48d711` "fix(bash): preserve literal prompts in supported shells (#3418)"](https://github.com/pypa/virtualenv/commit/5f48d711731311428aebfae61d46957762155b22), which removed the unconditional `$`/backtick escaping from the bash path. Advisory for that change: [GHSA-6w3r-xr62-4xcp](https://github.com/pypa/virtualenv/security/advisories/GHSA-6w3r-xr62-4xcp). ## Proof of concept Non-destructive payload: it runs `id` and writes the output to a file. ```bash python -m virtualenv --no-seed --no-periodic-update --activators bash \ --prompt 'x$(id > /tmp/ve-pwn)y' /tmp/venv-demo cat > /tmp/rc <<'EOF' shopt -u promptvars . /tmp/venv-demo/bin/activate shopt -s promptvars EOF # the prompt needs a tty; script(1) provides one printf 'exit\n' | script -qec 'bash --noprofile --rcfile /tmp/rc -i' /dev/null cat /tmp/ve-pwn ``` Observed on 21.14.12: ``` $ cat /tmp/ve-pwn uid=1000(asrun) gid=1000(asrun) groups=1000(asrun),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),999(ollama) ``` The prompt printed as `(xy)`. The same script against 21.14.8 creates no file; against 21.14.9 and 21.14.12 it does. Triggers, all run in a pty, activation itself never executing anything; execution happens at the next prompt redraw after prompt expansion is enabled: | After `. bin/activate` (activated with `shopt -u promptvars`) | 21.14.8 | 21.14.9 | 21.14.12 | | --- | --- | --- | --- | | `shopt -s promptvars` | inert | executes | executes | | `set -o posix` | inert | executes | executes | | `export PS1`, then start an interactive child `bash` (its `promptvars` defaults to on) | inert | executes | executes | | control: activation with default options | inert | inert | inert | | control: activation with `set -o posix` already on | inert | inert | inert | The third trigger needs no explicit option change: the poisoned `PS1` is inherited and the child bash expands it on its first prompt. `shopt -u promptvars` is a documented way to make an expensive bash prompt cheap and appears in `~/.bashrc` files for that reason; it is also the state left by some prompt wrappers. Whoever sources such an environment gets the unescaped value in `PS1`. ## Impact Command execution as the user who sources `bin/activate`, from a value the user may not have written (project metadata or tool config reaching `--prompt`). The value is also persisted in `pyvenv.cfg` and in the generated script, so it survives the activation that created it and fires later, in a different shell session, on the first prompt redraw with expansion enabled. No privilege elevation beyond the activating user. Severity: high, matching GHSA-5vjq-rrrf-7h2q for the same class of prompt-expansion command injection reached by toggling a shell option after activation. If you consider the `promptvars` state a caller-controlled condition rather than a shell condition, this is medium; the fix is the same either way. ## Versions tested virtualenv 21.14.12 and 21.14.9 (affected), virtualenv 21.14.8 (unaffected control); CPython 3.14.7; Linux 6.x (WSL2); bash 5.1.16(1)-release. `--prompt` handling is platform independent; zsh/fish/csh/nu/PowerShell activators were not part of this report. ## Known mitigation For a user: activate with `promptvars` left at its default, or set `VIRTUAL_ENV_DISABLE_PROMPT=1` before activating, which skips the whole prompt block. Do not pass an untrusted value to `--prompt`. Fix in the activator: escape `$` and the backtick for bash as well, in both branches, with a single backslash (`$` -> `\$`, backtick -> ``\` ``, `\` -> `\\`). Verified on bash 5.1.16 that a `PS1` built that way is inert with `promptvars` on, with `promptvars` off, and with `promptvars` off followed by posix mode, while still rendering the literal value on screen. That is better than both current shapes: 21.14.8's four-level escaping left stray backslashes on screen, which is why the block was rewritten. ```sh else _VIRTUAL_PROMPT="${VIRTUAL_ENV_PROMPT//\\/\\\\}" _VIRTUAL_PROMPT="${_VIRTUAL_PROMPT//\$/\\\$}" _VIRTUAL_PROMPT="${_VIRTUAL_PROMPT//\`/\\\`}" if [ -n "${ZSH_VERSION-}" ]; then _VIRTUAL_PROMPT="${_VIRTUAL_PROMPT//\%/%%}" fi PS1="(${_VIRTUAL_PROMPT}) ${PS1-}" ``` For the tests, `tests/unit/activation/test_bash.py::test_bash_prompt_options` already parametrises `("before", "after", "expected")` and covers `before="shopt -u promptvars"` with `after=""`. The missing axis is the transition: `("shopt -u promptvars", "shopt -s promptvars", ...)` and `("shopt -u promptvars", "set -o posix", ...)`, with the existing `PWNED` check asserting no file was created. ## Latest release - [x] The problem reproduces on the latest virtualenv release.