Node Version Manager (nvm) is a POSIX-compliant shell function for managing multiple node.js versions. In versions 0.31.2 through 0.40.5, nvmls filters installed versions with a sed program that embeds the requested version pattern in a \#...# address, escaping . (and, since 0.40.4, #) but not backslash. A version string containing # (or \#) therefore closes the address early, and the rest of it is parsed as sed commands: with GNU sed, the e command runs a shell command as the user; with BSD sed, the w and r commands write and read files. This is reachable whenever an attacker-influenced version string is passed to nvm use, nvm which, nvm ls, nvm version, nvm run, or nvm exec, or to nvm-exec via NODEVERSION, and at least one version is installed. nvm's own .nvmrc parser strips comments, so a bare nvm use is not affected; however, the bash cdnvm and zsh load-nvmrc auto-switch hooks documented in the README read .nvmrc as-is and pass it to nvm ls and nvm version, so a user with one of those hooks installed runs the attacker's command just by changing into a directory under a malicious .nvmrc; scripts that run nvm use "$(cat .nvmrc)" are affected the same way. Version 0.40.6 passes any version pattern containing # or a newline through the .nvmrc content parser first, so no # reaches the sed program.