A vulnerability in virtualenv's xonsh activation script allows attacker-controlled prompt text to become executable code during environment activation if the xonsh source decoding uses legacy multibyte encodings like GBK or Big5. This occurs because the UTF-8 encoded activation script is read with a different codec, causing escape characters to be misinterpreted and enabling code injection. The issue affects POSIX activation scripts and requires an attacker to control the prompt or destination argument passed to virtualenv and a user to activate the environment with the affected encoding. A fix is available in virtualenv version 2.5.1_1.