A vulnerability in virtualenv's xonsh activation script allows attacker-controlled prompt text to become executable code during environment activation if the xonsh source encoding is a legacy multibyte encoding such as GBK or Big5. This occurs because the activation script is written in UTF-8 but read with a different codec, causing misinterpretation of escape sequences and enabling code injection. The issue affects POSIX activation scripts and requires an attacker to control the prompt or destination argument passed to virtualenv, with the user subsequently activating the environment. The vulnerability is fixed in virtualenv version 1.18.1_2.