CVE-2026-48986: pam_usb Infinite Loop DoS in Process Parent ID Lookup

Technical summary

The vulnerability exists in the usb_get_process_parent_id() function, which retrieves parent process IDs by reading /proc/<pid>/stat. The function fails to initialize the output parameter (*ppid) when the stat file cannot be read—for example, if a parent process exits during authentication. In pusb_local_login(), this same variable is reused in a while loop that traverses the process tree. When initialization fails and the PID is not updated, the loop condition never becomes false, causing an infinite loop. The vulnerability is classified as CWE-835 (Loop with Unreachable Exit Condition). Exploitation requires local access and the ability to trigger process termination during active authentication, which can occur naturally in multi-process authentication flows or intentionally by an unprivileged user.

Business impact

Organizations relying on pam_usb for two-factor authentication face potential service disruption. A successful attack hangs critical authentication processes (login, sudo, sshd), preventing legitimate users from accessing systems until the hung processes are manually killed or the system is rebooted. In environments where pam_usb is deployed as a mandatory authentication layer—such as secure workstations or servers requiring hardware token verification—this DoS can effectively lock users out of their systems. The attack requires no elevated privileges and can be triggered by any local user with the ability to interact with the authentication system, making it a significant availability risk in multi-user or shared computing environments.

Affected systems

pam_usb version 0.9.1 and earlier are affected. Any Linux system using pam_usb as part of its authentication configuration (typically PAM stack configuration in /etc/pam.d/) is vulnerable. The vulnerability manifests during login, sudo execution, or any other PAM-authenticated operation. Systems using pam_usb exclusively for specific roles (e.g., restricting authentication to certain user accounts or SSH sessions) have a narrower attack surface but remain vulnerable to denial of service within that scope.

Exploitability

Exploitability is practical but requires specific conditions. An attacker must have local access to the target system and must trigger or coincide with a process termination during active pam_usb authentication. This can happen naturally when a parent process exits unexpectedly, or an attacker can intentionally terminate a parent process if they have sufficient permissions. The attack does not require special privileges—an unprivileged user can potentially cause the condition to occur. However, reliably triggering the infinite loop requires understanding the authentication flow and timing, which moderately elevates the skill level needed. Once triggered, the impact is immediate: the authenticating process hangs.

Remediation

Upgrade pam_usb to version 0.9.2 or later. This version includes a fix that properly initializes the ppid variable on error, ensuring the while loop can terminate even if parent processes exit during traversal. Administrators should apply this patch across all systems using pam_usb, prioritizing systems where pam_usb is a mandatory part of the authentication stack or where service availability is critical. Until patched, mitigations include restricting local access to the system and monitoring process behavior during authentication operations, though these do not eliminate the underlying vulnerability.

Patch guidance

Version 0.9.2 addresses this vulnerability by ensuring the output parameter is properly initialized when usb_get_process_parent_id() fails, allowing the process-tree traversal loop to exit cleanly. Verify the patched version against the official pam_usb release notes and repository (typically available on GitHub or the project's official distribution channels). Test the patch in a non-production environment first, particularly in authentication-critical systems, to ensure PAM stack functionality remains intact. After upgrading, verify that normal login and sudo authentication workflows operate without delay or hanging.

Detection guidance

Monitor system logs for hung processes during authentication attempts, particularly those involving pam_usb. Look for authentication timeout events or forcibly-terminated login/sudo/sshd processes that align with authentication failures. In environments where pam_usb is active, correlate process exits with subsequent authentication hangs. On systems with pam_usb installed, check /var/log/auth.log or equivalent for repeated authentication timeouts or unusual process terminations. Additionally, check the installed pam_usb version using package managers (e.g., dpkg -l pam-usb on Debian/Ubuntu systems) to identify vulnerable installations. Real-time process monitoring tools may help detect and alert on infinite loops or long-running pam processes.

Why prioritize this

Although assigned a MEDIUM CVSS score (4.7), this vulnerability deserves prompt attention because (1) it causes direct service disruption to authentication systems, which are security-critical, (2) exploitation requires only local access and no special privileges, making it accessible to any system user, (3) the impact is immediate and fully relieves the availability of affected services, and (4) the fix is straightforward and available. Organizations using pam_usb should prioritize patching, especially if pam_usb is enforced as a mandatory authentication requirement. However, the requirement for local access and specific timing conditions prevents this from being rated CRITICAL.

Risk score, explained

The CVSS 3.1 score of 4.7 MEDIUM reflects: Attack Vector Local (requires system access), Attack Complexity High (requires process termination timing or natural coincidence), Privileges Required Low (an unprivileged user can trigger it), User Interaction Not Required, Scope Unchanged, and impact limited to Availability (High). The scoring correctly penalizes the attack vector (local-only) and complexity (timing-dependent) while acknowledging the severity of availability impact. Organizations with strict authentication policies or critical service availability requirements may justify treating this as higher priority than the base score suggests.

Frequently asked questions