virtualenv activation scripts run code from a path that contains a placeholder name

Impact

ViaTemplateActivator.instantiate_template replaces the placeholders in an activation template one key at a time, each across the whole text. When a value that it already substituted and quoted contains the name of a later placeholder, the later pass rewrites that name inside the quoted literal. The inserted value brings its own quotes, which close the outer literal, and the rest of the path runs as code when a user activates the environment.

__VIRTUAL_NAME__ holds the environment's directory name, so a destination such as A__VIRTUAL_NAME__B/x'$(touch PWNED)'y is enough. In the default activate for bash, the VIRTUAL_ENV line renders as:

VIRTUAL_ENV='/tmp/t/A'x'"'"'$(touch PWNED)'"'"'y'B/x'"'"'$(touch PWNED)'"'"'y'

Sourcing that script runs touch PWNED. The same payload in a path without the placeholder name runs nothing. activate.fish runs the command the same way, and the reporter also showed Python code execution through activate.xsh and bin/activate_this.py. In activate.csh, a directory named after a prompt display placeholder, such as __VIRTUAL_PROMPT_DISPLAY_TCSH__, breaks the script with Unmatched '''.

No change to quote() can stop this, because the rewrite happens after quote() ran; it defeats shlex.quote and repr() alike.

An attacker needs to influence the destination path or --prompt: a CI job or wrapper tool that builds the path from a branch or project name, or a directory in a cloned repository or unpacked archive.

The bug affects versions 20.26.6 through 21.14.1. Version 20.26.6 first quoted template values (#2768) and introduced the per-key loop.

Patches

Fixed in 21.14.2. Activation scripts now keep a placeholder name in the environment's path or prompt as plain text, and activating the example above runs nothing. Fix: https://github.com/pypa/virtualenv/commit/602b26623a724827ff171bf11fef4ad186d01f74

Workarounds

Do not create an environment whose path or --prompt contains a placeholder name such as __VIRTUAL_NAME__, __VIRTUAL_ENV__ or __VIRTUAL_PROMPT__, and inspect the generated activation script before you source one you did not create.

Credits

Kevin Stubbings (@Kwstubbs) of GitHub Security Lab reported this issue.