A vulnerability in virtualenv's xonsh activation script allows attacker-controlled prompt text to become executable syntax when the activation script is decoded using legacy multibyte encodings such as GBK or Big5. This occurs because the script is written in UTF-8 but read with a different codec, causing escape sequences to be misinterpreted and enabling code execution during environment activation. The issue affects POSIX activation scripts and requires an attacker to control the prompt or destination data passed to virtualenv and for the user to activate the environment with the affected encoding. A fix is available in version 2026.10.20.