Executive brief
pam_usb is a tool that allows users to log into Linux systems using a USB flash drive instead of a password. A security flaw in how the software reads its configuration file on 32-bit systems could allow an attacker with administrative access to crash the system or potentially execute malicious code by corrupting system memory. This vulnerability is primarily a concern in environments where configuration files are automatically generated or where an attacker has already gained partial control of the system.
Technical details
A heap-based buffer overflow exists in pam_usb's configuration parser (src/conf.c) due to an integer overflow during memory allocation. The software uses libxml2 to count device nodes in the configuration file but fails to enforce an upper bound on the 'n_devices' variable. On 32-bit architectures (such as i686 or armv7l), the multiplication of 'n_devices' by the size of the device structure can wrap around size_t, resulting in a small allocation via xmalloc(). Subsequent writes to this undersized buffer lead to a heap overflow. Exploitation requires the ability to modify the root-owned pam_usb.conf file, limiting the attack to users with high privileges or through supply-chain configuration injection. The issue is fixed in version 0.9.1 by implementing a maximum device limit (PUSB_MAX_DEVICES).
Affected products
- mcdope pam_usb <= 0.9.0
Timeline
- 2021-03-01: other: Security audit initiated by maintainer
- 2026-05-19: disclosed: Issue reported on GitHub
- 2026-05-23: advisory: Project maintainer published security advisory GHSA-24mw-m2vf-36vp
- 2026-05-27: patched: Version 0.9.1 released