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, "nvm_ls" 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 "NODE_VERSION", 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.