CVE-2026-47272: pam_usb: OTP pad authentication bypass via missing system pad check and uninitialized RNG buffer

pamusb provides hardware authentication for Linux using ordinary removable media. Prior to 0.9.0, the pusbpadcompare() function in src/pad.c only verified that the user-side pad (~/.pamusb/device.pad) could be read, but did not enforce that the system-side pad (the pad file on the USB device) was also present and readable. If the user-side pad was deleted or unreadable, the function returned a failure that was treated as non-fatal in certain code paths, allowing authentication to succeed without the USB device being verified. A local user can delete their own ~/.pamusb/device.pad to remove the USB device requirement and authenticate without the physical device. This vulnerability is fixed in 0.9.0.