CVE-2026-48981: pam_usb: xmlReadFile flags=0 permits XXE network entity fetching in conf.c

pamusb provides hardware authentication for Linux using ordinary removable media. In versions prior to 0.9.2, pamusb calls xmlReadFile() with flags=0 when loading the configuration file, allowing libxml2 to process external entity references (XXE), potentially making outbound network connections or local file reads at XML parse time from the context of the authenticating process. The vulnerability requires the configuration file to contain crafted XML entity references. Since pamusb.conf is root-owned, direct exploitation requires prior write access to the config, but the defence-in-depth impact is significant given that pamusb.so runs in setuid contexts (sudo, su). This issue has been fixed in version 0.9.2.