CVE-2026-3184: util-linux Auth Bypass Vulnerability

CVE-2026-3184 Overview

A flaw was found in util-linux. Improper hostname canonicalization in the login(1) utility, when invoked with the -h option, can modify the supplied remote hostname before setting PAM_RHOST. A remote attacker could exploit this by providing a specially crafted hostname, potentially bypassing host-based Pluggable Authentication Modules (PAM) access control rules that rely on fully qualified domain names. This could lead to unauthorized access.

Critical Impact
This vulnerability allows attackers to bypass PAM-based host access controls by exploiting improper hostname validation, potentially enabling unauthorized access to protected systems.

Affected Products

util-linux (affected versions not specified)

Discovery Timeline

2026-04-03 - CVE CVE-2026-3184 published to NVD
2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-3184

Vulnerability Analysis

This vulnerability (CWE-289: Authentication Bypass by Alternate Name) stems from improper hostname canonicalization in the util-linux login(1) utility. When an administrator configures PAM modules to restrict access based on hostnames (typically using pam_access or similar modules), the system expects to receive the authentic fully qualified domain name (FQDN) of the connecting host.

However, when login is invoked with the -h option to specify a remote hostname, the utility performs canonicalization operations that can alter the hostname before it is passed to PAM via the PAM_RHOST item. This creates a mismatch between what PAM expects and what it receives, allowing an attacker to craft hostnames that bypass access control lists while still appearing legitimate.

The attack requires specific network conditions and relies on the target system using hostname-based PAM access controls, which limits the attack surface. However, in environments where such controls are the primary authentication mechanism, this vulnerability could enable complete bypass of host-based restrictions.

Root Cause

The root cause lies in the hostname processing logic within the login(1) utility. When processing the -h flag argument, the utility performs canonicalization that may truncate, modify, or otherwise transform the provided hostname. This transformation occurs before the hostname is set as PAM_RHOST, meaning PAM modules receive an altered version of the original hostname. If PAM access rules are configured to allow or deny specific FQDNs, an attacker can craft input that canonicalizes to an allowed hostname while originating from a different source.

Attack Vector

The attack vector is network-based with high complexity requirements. An attacker must:

Identify a target system using hostname-based PAM access controls
Determine the allowed hostnames in the PAM configuration
Craft a specially formatted hostname that will canonicalize to match an allowed entry
Connect to the target system through a service that invokes login -h with attacker-controlled hostname data

The vulnerability exploits the mismatch between the attacker-supplied hostname and the canonicalized version that PAM evaluates. For example, services like telnetd or rlogind that use login -h to record remote hostnames could be potential attack surfaces. The attacker provides a crafted hostname string that, after canonicalization, matches an entry in the PAM access control list, thereby gaining unauthorized access.

Detection Methods for CVE-2026-3184

Indicators of Compromise

Login attempts with unusually formatted or malformed hostnames in authentication logs
Discrepancies between DNS-resolved hostnames and PAM_RHOST values recorded in logs
Successful authentications from hosts that should be blocked by PAM access rules
Authentication entries with truncated or altered hostname strings compared to source IP reverse DNS

Detection Strategies

Monitor /var/log/secure and /var/log/auth.log for login events with suspicious hostname patterns
Implement correlation rules to compare PAM_RHOST values against known allowed hostnames and reverse DNS lookups
Configure PAM auditing to log detailed authentication context including raw and processed hostname values
Deploy network intrusion detection rules to identify crafted hostname patterns in authentication protocols

Monitoring Recommendations

Enable verbose PAM logging to capture full authentication context including hostname processing
Configure SIEM rules to alert on hostname mismatches between expected and actual values
Implement baseline monitoring for remote login patterns to detect anomalous access sources
Review PAM access control configurations to identify rules that rely solely on hostname-based restrictions

How to Mitigate CVE-2026-3184

Immediate Actions Required

Review PAM configurations for modules using hostname-based access controls (pam_access, etc.)
Consider supplementing hostname-based restrictions with IP-based controls where possible
Enable enhanced logging for authentication events to detect potential exploitation attempts
Restrict or disable remote login services that invoke login -h until patches are applied

Patch Information

Security updates are expected from util-linux maintainers and downstream Linux distributions. Organizations should monitor the Red Hat CVE-2026-3184 Advisory and Red Hat Bug Report #2442570 for patch availability and apply updates as soon as they become available through official distribution channels.

Workarounds

Migrate from hostname-based PAM access controls to IP-based restrictions where feasible
Implement additional authentication layers (multi-factor authentication) that do not rely on hostname validation
Use firewall rules at the network level to enforce host restrictions independently of PAM
Consider disabling the -h option processing in login if not required in your environment

bash

# Example: Supplement hostname-based PAM rules with IP restrictions in /etc/security/access.conf
# Instead of relying solely on hostnames:
# + : admin : trusted.example.com
# Use IP-based restrictions:
+ : admin : 192.168.1.0/24
- : ALL : ALL