Executive brief
pam_usb is a tool that allows users to log into Linux systems using a USB flash drive as a physical key. A flaw in the software's error handling causes it to silently skip security checks if it lacks the necessary permissions to scan for virtual input devices (often used in remote desktop attacks). This can lead administrators to believe their systems are protected against remote login bypasses when the security check is actually non-functional, potentially allowing unauthorized access in specific configurations.
Technical details
The vulnerability exists in `src/evdev.c` within the `pusb_has_virtual_input_device()` function. When attempting to open `/dev/input/event*` nodes to detect virtual input devices (a countermeasure against remote desktop-based authentication bypasses), the code silently ignores `EACCES` (Permission Denied) errors. Because these device nodes are typically restricted to root or the 'input' group, a process running with reduced privileges will fail to open any nodes and return a '0' (no virtual devices found) result. The caller in `src/local.c` cannot distinguish between a legitimate absence of virtual devices and a failed scan, causing the authentication process to proceed without the intended security enforcement. This primarily impacts administrators using `pamusb-check` for validation or deployments where the PAM module runs with restricted privileges. The issue is resolved in version 0.9.1 by tracking `EACCES` errors and returning a distinct sentinel value.
Affected products
- mcdope pam_usb < 0.9.1
Timeline
- 2021-03-01: other: Initial security audit request opened
- 2026-05-19: disclosed: Issue reported on GitHub
- 2026-05-20: advisory: GitHub Security Advisory published
- 2026-05-27: patched: CVE published and fix confirmed in 0.9.1