pacman prior to 5.1.3 allows directory traversal when installing a remote package via a specified URL "pacman -U <url>" due to an unsanitized file name received from a Content-Disposition header. pacman renames the downloaded package file to match the name given in this header. However, pacman did not sanitize this name, which may contain slashes, before calling rename(). A malicious server (or a network MitM if downloading over HTTP) can send a Content-Disposition header to make pacman place the file anywhere in the filesystem, potentially leading to arbitrary root code execution. Notably, this bypasses pacman's package signature checking. This occurs in curl_download_internal in lib/libalpm/dload.c.