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,…