Introduction

In June 2025, cybersecurity firm Qualys disclosed two local privilege escalation (LPE) vulnerabilities affecting Linux systems, identified as CVE-2025-6018 and CVE-2025-6019. These vulnerabilities can be chained together, allowing an attacker with a non-root user account to bypass privilege controls via default system components and escalate to root.

The vulnerability chain affects a wide range of distributions, including Ubuntu, Debian, Fedora, and openSUSE. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has added them to its Known Exploited Vulnerabilities (KEV) catalog. This article provides an in-depth analysis of the vulnerabilities, including their mechanisms, exploitation process, impact, and mitigation strategies.

Vulnerability Overview

1. Identification and Basic Information

CVE IDs: CVE-2025-6018, CVE-2025-6019
Disclosure Date: June 18, 2025
Vulnerability Type: Local Privilege Escalation (LPE)
Affected Systems: Most mainstream Linux distributions (openSUSE, Ubuntu, Debian, Fedora, etc.)
Requirements: Attacker must have a non-privileged local account (e.g., via SSH login)

2. Description

The chain consists of two parts:

  • CVE-2025-6018: Abuses the Pluggable Authentication Module (PAM) system's handling of environment variables. By editing the .pam_environment file after SSH login, the attacker can inject XDG_SEAT and XDG_VTNR values to impersonate a local interactive user.
  • CVE-2025-6019: Exploits permission flaws in the udisks and libblockdev components. Once identified as a local user, the attacker can mount a crafted XFS image containing a SUID binary and gain root access by executing it.

Technical Details

1. CVE-2025-6018: Spoofing Local User Privileges

By default, Linux PAM allows users to set specific environment variables using the .pam_environment file. The attacker writes:

XDG_SEAT=seat0
XDG_VTNR=1

During login, the system mistakenly treats the user as being in a local session, granting them elevated policy permissions (allow_active). This privilege enables execution of udisks operations under polkit rules.

2. CVE-2025-6019: Mounting SUID Image for Privilege Escalation

Once allow_active is granted, the attacker uses udisks to mount an XFS image containing a SUID-root shell. Example steps:

# Create a SUID payload image
mkfs.xfs -d file,name=payload.img,size=10m
# Mount the image as a spoofed local user
udisksctl loop-setup -f payload.img
udisksctl mount -b /dev/loopX

The attacker then executes the SUID program from the mount point to obtain a root shell.

Exploitation Workflow

  1. Attacker logs in via SSH with standard user privileges.
  2. Modifies .pam_environment to spoof an interactive session.
  3. Re-logs in and is recognized by the system as a console user.
  4. Mounts a crafted SUID image using udisks.
  5. Executes the SUID binary to gain root access.

Affected Versions and Fixes

1. Vulnerable Components

  • Default PAM configuration in openSUSE / SLE15 (user environment reading enabled)
  • udisks / libblockdev as installed by default in Ubuntu, Debian, Fedora, and others

2. Vendor Response

  • openSUSE and SUSE have released patches for PAM configuration
  • Ubuntu and others have addressed the udisks permission logic
  • CISA requires federal systems to apply patches by July 15, 2025

Mitigation and Recommendations

For Enterprise Environments

  • Update systems immediately with the latest patches for PAM and udisks
  • Disable user_readenv=1 in PAM configs to prevent spoofing via .pam_environment
  • Harden polkit rules to require auth_admin for udisks operations
  • Monitor for loop devices or suspicious mounts under /tmp

For Security Teams

  • Configure SIEM to detect anomalous udisks activity and login behavior
  • Use vulnerability scanning tools to verify affected versions
  • Look for suspicious SUID binaries or rogue device nodes

Risks and Industry Implications

Local privilege escalation remains a staple in penetration testing and real-world attacks. This specific chain is notable because it starts from a remote login and exploits default configurations, without requiring any advanced exploit payloads.

This scenario highlights flaws in how Linux systems assign privilege roles and interpret local user contexts. It suggests a need for future improvements in policy binding and interprocess role separation for default services like udisks.

Conclusion

CVE-2025-6018 and CVE-2025-6019 form a critical privilege escalation chain, affecting widely used Linux distributions. They are easy to exploit and rely on typical configurations. Organizations must act quickly to patch affected systems and review privilege policies to prevent attackers from escalating to root.

References