The vulnerability in virtualenv occurs because the activate scripts for bash, zsh, and fish place values already escaped by shlex.quote inside an additional quoted context. In bash and zsh, a crafted virtual environment path can reach the __VIRTUAL_ENV__ variable when a relocated environment's recorded directory is missing. In fish scripts, crafted Tcl or Tk library paths can reach __TCL_LIBRARY__ or __TK_LIBRARY__. The surplus quotes terminate the intended quoted string, allowing shell metacharacters to be parsed as commands during script sourcing, leading to OS command injection. This flaw is resolved in virtualenv version 21.7.13.

An attacker who can influence the virtual environment path or Tcl/Tk library paths can craft values that, when the activation script is sourced, execute arbitrary commands with the privileges of the user running the script. This can lead to full compromise of the user's environment. The CVSS 3.1 score is 7.8 (high), reflecting local attack vector, low complexity, no privileges required, user interaction required, and high impact on confidentiality, integrity, and availability.

Upgrade virtualenv to version 21.7.13 or later, where this vulnerability is fixed. There is no indication of any alternative mitigations or temporary workarounds. Users should avoid sourcing activation scripts from untrusted virtual environments until patched.