Abstract

The Pluggable Authentication Module (PAM) framework represents a cornerstone of the security architecture in Unix-like operating systems, providing a highly modular and extensible approach to user authentication, authorization, session management, and password control. A significant recent disclosure, identified as CVE-2025-6018, has brought to light a critical vulnerability residing within the pam_env module. This module is specifically tasked with managing user environment variables during the authentication and session setup phases. This comprehensive report undertakes an in-depth technical analysis of CVE-2025-6018, meticulously detailing its underlying mechanisms, the potential ramifications for system security, and the broader implications it holds for the fundamental design principles and recommended configuration practices of PAM. Furthermore, this document provides an extensive review of historical vulnerabilities affecting the PAM framework, proposes a robust set of best practices for its secure configuration, and outlines systematic strategies for auditing PAM setups to proactively prevent privilege escalation attacks and maintain system integrity.

1. Introduction

The robustness and integrity of authentication mechanisms are unequivocally paramount in safeguarding modern computing systems from unauthorized access and malicious exploitation. In the intricate landscape of Unix-like operating environments, the Pluggable Authentication Module (PAM) framework stands as a pivotal component, offering an unparalleled flexible and extensible methodology for integrating a diverse array of authentication technologies. This modularity, while providing immense adaptability for system administrators to tailor security policies, inherently introduces a layer of complexity. If not meticulously configured and diligently managed, this complexity can inadvertently lead to significant security vulnerabilities.

CVE-2025-6018 serves as a profoundly pertinent and timely case study, vividly illustrating the critical necessity for scrupulous configuration, continuous vigilance, and regular, systematic auditing of PAM modules and their associated service policies. This vulnerability underscores the delicate balance between flexibility and security within highly configurable systems, highlighting how a seemingly innocuous default setting can become a critical vector for privilege escalation when combined with other system components. This report aims to dissect this specific vulnerability, drawing connections to past issues and outlining a forward-looking approach to PAM security.

2. Overview of PAM Architecture

PAM functions as an indispensable intermediary layer, bridging the gap between system services (such as login, sshd, su, sudo, cron) and various underlying authentication modules. This architectural design enables the seamless integration of disparate authentication methods, ranging from traditional password-based authentication to more advanced mechanisms like Kerberos, LDAP, biometrics, or hardware tokens, without requiring modifications to the core service applications themselves. The framework’s strength lies in its ability to centralize authentication policy management, allowing administrators to define how users are authenticated, authorized, and managed across different services.

Its architecture is composed of several key, interconnected components:

2.1. PAM Modules

PAM modules are dynamically loadable shared libraries (typically .so files located in /lib/security/ or /usr/lib/security/) that implement specific authentication, authorization, and session management tasks. They are categorized into four primary management groups, each addressing a distinct aspect of the user’s interaction with the system:

• Authentication Modules (auth): These modules verify the user’s identity. This often involves prompting for a password, checking a cryptographic key, or interacting with an external authentication service. Examples include pam_unix.so (for traditional UNIX password authentication), pam_ldap.so (for LDAP-based authentication), pam_krb5.so (for Kerberos), and pam_ssh_agent_auth.so (for SSH agent authentication). The pam_env.so module, central to CVE-2025-6018, primarily functions within the session management context but can be implicitly involved in authentication flows due to its impact on the session environment.

• Account Modules (account): These modules check if the authentication is currently valid based on non-authentication credentials. This includes checking password expiry, account expiry, time-of-day restrictions, and whether the user is allowed to log in from a particular host. pam_nologin.so (checks for /etc/nologin file), pam_time.so (limits login times), and pam_unix.so (checks account validity) are common examples.

• Session Modules (session): These modules manage the environment for a user’s session, which typically involves actions performed before and after authentication. This includes tasks like mounting home directories, establishing Kerberos tickets, and setting up environment variables. pam_limits.so (enforces resource limits), pam_mkhomedir.so (creates home directory if it doesn’t exist), pam_systemd.so (integrates with systemd login sessions), and crucially, pam_env.so (sets environment variables) fall into this category.

• Password Modules (password): These modules are responsible for updating a user’s authentication token (e.g., changing a password). They handle password complexity policies, history checks, and actual password updates in the system’s credential store. pam_unix.so (for changing UNIX passwords) and pam_cracklib.so (for strong password enforcement) are frequently used.

2.2. PAM Service Modules (Configuration Files)

These are plain text configuration files, typically located in the /etc/pam.d/ directory, though some older systems might use /etc/pam.conf. Each file corresponds to a specific system service (e.g., sshd, login, sudo, cron, su). These files define the precise authentication policies for that service, specifying which PAM modules to invoke for each of the four management groups (auth, account, session, password), in what sequence, and with what control flags.

Each line in a PAM configuration file generally follows this format:

<module-type> <control-flag> <module-path> <module-arguments>

• module-type: Specifies one of the four management groups: auth, account, session, or password.
• control-flag: Dictates how the success or failure of a module affects the overall authentication process. The most common control flags are:
  • requisite: If this module fails, the entire stack fails immediately, and no further modules are processed for this type. Success is not required for the stack to continue.
  • required: If this module fails, the entire stack will eventually fail, but all subsequent modules in the stack are still processed (to prevent information leakage to an attacker).
  • sufficient: If this module succeeds, and no required module has failed so far, then the entire stack succeeds immediately, and no further modules are processed for this type.
  • optional: The success or failure of this module generally does not determine the overall success or failure of the stack. It’s often used for modules that provide additional features but aren’t strictly necessary for authentication.
  • include: Allows including other PAM configuration files, promoting modularity and reducing redundancy.
  • substack: Similar to include, but used for including a sub-stack of modules. The success or failure of the sub-stack is then evaluated based on specific rules.
• module-path: The path to the PAM module shared library (e.g., pam_unix.so).
• module-arguments: Optional arguments passed to the module to modify its behavior (e.g., nullok for pam_unix.so to allow empty passwords, user_readenv for pam_env.so).

The modularity of PAM, coupled with the granular control offered by these configuration files, empowers system administrators to finely tune authentication mechanisms to meet highly specific security requirements. However, this inherent flexibility necessitates a comprehensive and deep understanding of each module’s functionality, its default behavior, and its potential security implications when configured in various combinations.

2.3. PAM Management Functions (PAM API)

The PAM management functions constitute the Application Programming Interface (API) that system services utilize to interact with the PAM library. These functions handle the flow of authentication requests, mediate communication between the service and the PAM modules, and manage the overall authentication state. The typical flow of an authentication attempt through the PAM API involves several key functions:

1. pam_start(): Initializes the PAM transaction for a specific service and user.
2. pam_authenticate(): Calls the modules in the auth stack to verify the user’s identity.
3. pam_acct_mgmt(): Calls the modules in the account stack to check account validity.
4. pam_setcred(): Calls the modules in the auth stack to establish or destroy user credentials.
5. pam_open_session(): Calls the modules in the session stack to prepare the user’s environment for a session.
6. pam_close_session(): Calls the modules in the session stack to clean up the user’s environment after a session.
7. pam_end(): Cleans up the PAM transaction and frees resources.

This robust API allows services to delegate complex authentication logic to PAM, ensuring consistency and ease of integration. However, the correct implementation and configuration of this interaction are critical to avoid security loopholes.

3. The pam_env Module and CVE-2025-6018

The pam_env module is a core PAM component specifically designed to set up user environment variables during the authentication and session initialization process. Its primary function is to populate the user’s shell environment with values defined in system-wide and user-specific configuration files. This functionality is crucial for tailoring user sessions, ensuring that necessary paths, locale settings, and other application-specific variables are correctly defined.

3.1. Functionality of pam_env

The pam_env module typically reads environment variable definitions from two primary locations:

1. /etc/environment: A system-wide file that defines environment variables for all users. This file is generally managed by the system administrator.
2. ~/.pam_environment: A user-specific file located in the user’s home directory. This file allows individual users to define or override environment variables for their own sessions, offering a high degree of personalization. This is where the core of CVE-2025-6018 resides.

Additionally, pam_env supports various options that can be passed as arguments in the PAM configuration files:

• conffile=/path/to/file: Specifies an alternative system-wide configuration file instead of /etc/environment.
• env_file=/path/to/file: Specifies an alternative user-specific configuration file instead of ~/.pam_environment.
• user_readenv: This critical option, when enabled (or implicitly true by default on certain distributions), instructs pam_env to read the user’s .pam_environment file. This option is central to the vulnerability.
• debug: Enables verbose logging for troubleshooting.
• allow_unset: Allows environment variables to be unset if specified in the configuration files.

While this feature offers significant convenience and flexibility for users and administrators alike, enabling personalized environment configurations, it also presents a non-trivial vector for exploitation if not properly secured and validated. Environment variables, especially those controlled by users, can profoundly influence how programs execute and interact with the system. Variables like PATH, LD_PRELOAD, IFS, or various XDG-related variables are frequently abused in privilege escalation attempts.

3.2. Technical Deep Dive into CVE-2025-6018

CVE-2025-6018 arises from a critical default configuration choice on certain Linux distributions, notably openSUSE and SUSE Linux Enterprise. In these specific distributions, the user_readenv option within the pam_env module is enabled by default in key PAM service configuration files, such as /etc/pam.d/sshd and /etc/pam.d/login. This default setting permits the pam_env module to read and process the ~/.pam_environment file from the user’s home directory, even for unprivileged users attempting to establish a session.

The core of the vulnerability lies in the ability of an unprivileged user to inject arbitrary environment variables into their session context during the authentication process. Specifically, by carefully crafting a ~/.pam_environment file with particular XDG_ variables, an attacker can manipulate the system into treating their session as a ‘local, active user session’ — even if they are connecting remotely (e.g., via SSH) or through a less privileged channel.

The key environment variables leveraged in this attack are:

• XDG_SEAT=seat0
• XDG_VTNR=1

The XDG Base Directory Specification, managed by the FreeDesktop.org project, defines standards for user-specific data and configuration files. XDG_SEAT and XDG_VTNR are variables typically set by display managers (like GDM, SDDM, LightDM) to identify the user’s physical console session (seat0 refers to the primary console) and the virtual terminal number (VT1 is usually the graphical login screen). When these variables are present in a user’s session environment, higher-level components of the desktop environment and system services, particularly PolicyKit (Polkit), interpret the session as a legitimate, locally active, graphical user session.

PolicyKit (Polkit), a framework for controlling system-wide privileges, often grants elevated permissions to processes running within an ‘active console session’. Polkit rules can define that certain administrative actions (e.g., shutting down the system, suspending, mounting devices, managing users) are allowed without further password prompts for users deemed ‘active’ on ‘seat0’. This is a common design choice aimed at enhancing user convenience on personal workstations, where the physically present user is implicitly trusted.

By injecting XDG_SEAT=seat0 and XDG_VTNR=1 through the ~/.pam_environment file, an unprivileged attacker, upon successful authentication (even via SSH), can deceive Polkit into believing their session is an ‘active local user session’. This misrepresentation grants the attacker certain elevated privileges ordinarily reserved for physically present users. For instance, the attacker could then perform actions typically restricted to privileged users or those on the local console, such as initiating system shutdowns or suspensions, even without supplying administrative credentials, directly bypassing Polkit’s intended authorization checks for remote or non-console users.

This seemingly minor environmental manipulation becomes a critical stepping stone in a broader attack chain, illustrating how environmental context can dramatically alter the security posture of a session. The vulnerability highlights a fundamental disconnect between how session context is established and how privileges are subsequently evaluated by higher-level policy enforcement frameworks like Polkit.

4. Historical Context and Similar Vulnerabilities

The pam_env module, and the broader PAM framework itself, has a documented history of vulnerabilities stemming from the handling of user-controlled environment variables and the complex interplay of modular components. These historical precedents underscore a recurring pattern of risk associated with flexible configuration and the often-underestimated power of environment variable manipulation.

4.1. Previous pam_env Vulnerabilities

• CVE-2010-4708: Environment Variable Injection and PATH Manipulation: This vulnerability in pam_env highlighted the risks associated with the module reading the .pam_environment file without sufficient sanitization or validation of the variables being set. Specifically, it allowed local users to manipulate their PATH environment variable in a way that could lead to unintended program execution or privilege escalation. An attacker could define a PATH variable in ~/.pam_environment that pointed to a malicious directory containing a specially crafted executable with the same name as a legitimate system utility. When a SUID (Set User ID) program or another privileged service that relied on the PATH variable was subsequently executed, it might inadvertently execute the attacker’s malicious binary instead of the legitimate one, potentially allowing local users to execute arbitrary code with elevated privileges. This underscored the danger of allowing arbitrary environment variable control without robust validation and the subtle ways environment variables affect program execution.

• CVE-2011-3149: Environment Variable Expansion and Resource Exhaustion: This vulnerability addressed issues related to environment variable expansion within pam_env. Improper handling of excessively long or recursively defined environment variables (e.g., VAR1=$VAR2, VAR2=$VAR1) could lead to denial-of-service (DoS) conditions. An attacker could craft a ~/.pam_environment file that, when parsed by pam_env, would cause the module to consume excessive amounts of memory or CPU cycles due to infinite recursion or excessively large variable expansions. This could lead to a system crash or render the authentication service unresponsive, effectively denying legitimate users access to the system resources. This vulnerability emphasized the importance of input validation and resource management even for seemingly innocuous configuration files.

4.2. Broader PAM Vulnerabilities and Lessons Learned

Beyond pam_env, the PAM framework has faced other classes of vulnerabilities, often related to misconfigurations, race conditions, or logic flaws in specific modules or their interactions:

• pam_unix Race Conditions: Historically, pam_unix.so (the module handling traditional Unix authentication) has been susceptible to race conditions, particularly when dealing with password changes or shadow file access. For instance, bugs allowing time-of-check-to-time-of-use (TOCTOU) race conditions could potentially enable an attacker to manipulate the password file during a brief window, leading to authentication bypass or privilege escalation.

• Misconfiguration Leading to Bypass: A common theme in PAM vulnerabilities is not a flaw in the code itself, but rather a misconfiguration of the PAM service files in /etc/pam.d/. For example, incorrect control flags (e.g., using optional instead of required for a critical module) or the omission of necessary modules can inadvertently create authentication bypasses. A system administrator might intend for users to authenticate via two factors, but if one factor’s module is set to optional, a user might bypass it by simply failing that module while passing another.

• Interactions with External Services: PAM modules often interact with external services (e.g., LDAP, Kerberos, RADIUS). Vulnerabilities can arise from insecure communication channels, improper handling of network errors, or reliance on unvalidated data from these external sources.

These historical vulnerabilities collectively underscore several critical points:

1. Environment Variables as an Attack Surface: User-controlled environment variables are a potent attack vector, capable of influencing program execution paths, resource consumption, and the perceived context of a user session. Modules that process these variables must employ robust sanitization, validation, and resource limits.
2. The Complexity of Modularity: While PAM’s modularity offers immense flexibility, it also introduces complexity. The interaction of multiple modules, their specific arguments, and control flags can lead to unexpected security implications if not thoroughly understood. A seemingly minor change in one module’s configuration can have cascading effects across the entire authentication stack.
3. Default Configurations Matter: As demonstrated by CVE-2025-6018, the default behavior of modules and their inclusion in default PAM service files are critical. Secure-by-default principles should always be prioritized, meaning that potentially dangerous features (like user_readenv) should be opt-in rather than opt-out.
4. Continuous Auditing and Patching: The recurring nature of PAM-related vulnerabilities necessitates continuous vigilance. Regular audits of PAM configurations and prompt application of security updates are essential to mitigate known and newly discovered risks.

5. Exploitation Chain: From CVE-2025-6018 to Privilege Escalation

The true severity of CVE-2025-6018 is amplified when it is chained with other vulnerabilities or misconfigurations, forming a multi-stage attack path leading to full system compromise. The primary companion vulnerability in this specific exploitation chain is CVE-2025-6019, which affects the udisks service. This section details how an attacker can leverage these two vulnerabilities in sequence to achieve root privileges.

5.1. Understanding the udisks Service and CVE-2025-6019

udisks (specifically udisks2) is a D-Bus system service that provides an interface to manage storage devices, including hard drives, removable media (USB drives, CDs/DVDs), and loop devices. Desktop environments and various applications rely on udisks to perform operations like mounting, unmounting, formatting, and creating disk images without requiring direct root privileges from the user. It integrates deeply with PolicyKit to enforce authorization rules for these sensitive operations.

CVE-2025-6019 pertains to a vulnerability within the udisks service itself, or more accurately, its PolicyKit integration, where certain operations (such as mounting arbitrary file system images or loop devices) are granted excessive permissions when an ‘active local session’ is detected. The vulnerability exploited here is that udisks (and its Polkit rules) relax security checks for users believed to be actively interacting with the local console.

5.2. The Chained Attack Steps

The exploitation of CVE-2025-6018, combined with CVE-2025-6019, typically follows these steps:

Step 1: Initial Access and Environment Setup (Leveraging CVE-2025-6018)

• Initial Foothold: An attacker first needs an unprivileged user account on the target Linux system. This could be gained through various means, such as compromised credentials, a weak service, or a vulnerable web application providing shell access (e.g., SSH, a web shell).
• Crafting ~/.pam_environment: The attacker, as the unprivileged user, creates or modifies their ~/.pam_environment file in their home directory. This file is crafted to contain the specific environment variables that trick PolicyKit:
  XDG_SEAT=seat0
XDG_VTNR=1
• Session Initialization: The attacker then initiates a new session that triggers the PAM stack configured to use pam_env with user_readenv enabled. This commonly occurs upon login via SSH (/etc/pam.d/sshd) or local console (/etc/pam.d/login). When pam_env processes ~/.pam_environment, it sets XDG_SEAT and XDG_VTNR in the user’s session environment.
• PolicyKit Deception: With these variables set, PolicyKit now perceives the attacker’s session as an ‘active local user session’ on the primary console (seat0, virtual terminal 1). This is the crucial deception that relaxes Polkit’s authorization rules for certain actions.

Step 2: Leveraging PolicyKit Relaxation for udisks (Leading to CVE-2025-6019)

• Privilege Relaxation: Many default Polkit rules are configured to grant actions like mounting devices (org.freedesktop.udisks2.filesystem-mount, org.freedesktop.udisks2.loop-setup) with allow_active=yes or auth_active. This means if a user is deemed ‘active’ on ‘seat0’, these actions can be performed without requiring a password or further administrative authentication.
• udisks Exploitation: Since the attacker’s session is now considered ‘active’, they can interact with the udisks service via its D-Bus interface. Specifically, the attacker can now instruct udisks to perform privileged operations that would normally require administrative authentication or be restricted to the console.
• Mounting a Malicious Disk Image: The most common exploitation path involves mounting a specially crafted disk image. The attacker prepares a disk image file (e.g., a .iso or .img file) containing a malicious binary. Crucially, this binary is configured with the SUID (Set User ID) root bit set (-rwsr-xr-x). This means when the binary is executed, it will run with the privileges of its owner, which in this case is root.
  • Example: An attacker creates a small ext4 filesystem image:
    bash
dd if=/dev/zero of=malicious.img bs=1M count=10
mkfs.ext4 malicious.img
mkdir /mnt/tmp_loop
sudo mount malicious.img /mnt/tmp_loop
sudo cp /bin/bash /mnt/tmp_loop/rootshell
sudo chmod 4755 /mnt/tmp_loop/rootshell
sudo umount /mnt/tmp_loop
rmdir /mnt/tmp_loop
    This malicious.img now contains a rootshell binary that, when executed, will run as root.
• udisks D-Bus Call: The attacker, from their unprivileged session (now perceived as ‘active’), uses a tool like dbus-send or udisksctl to instruct the udisks service to mount their malicious.img file. Because of the relaxed Polkit rules, this mount operation is permitted without administrative password prompts. udisks will typically mount this image to a path like /media/user/mountpoint.

Step 3: Root Privilege Escalation

• Executing the SUID Binary: Once the malicious.img is mounted, the attacker simply navigates to the mount point (e.g., /media/user/mountpoint) and executes the SUID root binary (./rootshell or ./bash).
• Root Shell: Upon execution, the SUID binary runs with root privileges, granting the attacker a fully functional root shell, thus achieving complete system compromise.

This intricate exploitation chain demonstrates how seemingly minor vulnerabilities, when combined with logical flaws in privilege management frameworks and the contextual understanding of a session, can lead to devastating consequences. The key takeaway is that security defenses must consider the entire attack surface, including environmental context and cross-component interactions.

6. Mitigation Strategies

Preventing the exploitation of CVE-2025-6018 and similar vulnerabilities requires a multi-layered approach, addressing not only the specific module configuration but also the broader PAM and PolicyKit ecosystem.

6.1. Disable user_readenv in PAM Configurations

This is the most direct and effective mitigation for CVE-2025-6018. By disabling the user_readenv option, you prevent the pam_env module from reading the ~/.pam_environment file, thereby eliminating the attacker’s ability to inject arbitrary environment variables into their session. This should be applied to all PAM service files that might be used for remote or unprivileged access.

• Locate Relevant PAM Files: The most critical files to check are typically associated with login services:

  • /etc/pam.d/sshd (for SSH connections)
  • /etc/pam.d/login (for local console logins)
  • /etc/pam.d/su
  • /etc/pam.d/sudo
  • /etc/pam.d/system-auth (if it includes pam_env or is symlinked to by other services)
  • /etc/pam.d/gdm-password or similar for graphical login managers.

• Modify the pam_env Line: Find the line referring to pam_env.so within the session type and ensure that user_readenv is either explicitly set to 0 or completely removed if it defaults to 1 on your distribution. A common modification would look like this:

  • Original (vulnerable) example:
    session optional pam_env.so
    (This implicitly allows user_readenv on vulnerable distributions)
  • Mitigated example:
    session optional pam_env.so user_readenv=0

• Consider Impact: Disabling user_readenv means that legitimate users will no longer be able to set session-specific environment variables via ~/.pam_environment. While this enhances security, it might slightly reduce user convenience for those who rely on this feature. Communicate this change to users if implemented widely.

6.2. Update PAM Modules and System Components

Always ensure that all PAM modules, the PAM library itself, and related system components (like PolicyKit and udisks) are updated to the latest stable versions provided by your operating system vendor. Vendors typically release patches that address known vulnerabilities. For CVE-2025-6018, distributions like SUSE have released updates (e.g., SUSE-SU-2025:02013-1) that change the default behavior of pam_env.so to not read the user’s ~/.pam_environment file by default, or to include stricter checks. Applying these updates is crucial for receiving official fixes.

• Regularly check for security advisories from your distribution (e.g., Ubuntu Security Notices, Red Hat Security Advisories, SUSE Security Announcements).
• Implement a robust patch management policy to ensure timely application of security updates.

6.3. Audit PAM Configurations Regularly

Systematic and periodic auditing of PAM configurations is essential for identifying and rectifying potential misconfigurations, detecting unauthorized changes, and ensuring ongoing compliance with security policies. This practice helps maintain a secure authentication environment and significantly reduces the risk of privilege escalation.

• Manual Review: Regularly review files in /etc/pam.d/ and /etc/pam.conf. Pay close attention to:
  • The control-flag for each module, ensuring it aligns with the intended security posture (e.g., required for critical authentication steps).
  • The arguments passed to modules, especially those that enable potentially dangerous features (like user_readenv for pam_env).
  • The presence of any unexpected or unfamiliar modules.
  • Ensure that any include or substack directives point to valid and securely configured files.
• Configuration Management Tools: Utilize configuration management tools like Ansible, Puppet, Chef, or SaltStack to define and enforce desired PAM configurations. This automates the process, ensures consistency across a fleet of servers, and helps prevent drift from the secure baseline.
• Version Control: Store your /etc/pam.d/ configurations in a version control system (e.g., Git). This allows for easy tracking of changes, rollbacks, and diffing to identify unauthorized modifications.
• Security Scanners/Auditing Tools: Employ automated security scanners and compliance auditing tools that can parse PAM configurations and flag known insecure patterns or deviations from best practices. While not exhaustive, they can provide a good baseline check.

6.4. Harden PolicyKit Rules

Given that CVE-2025-6018 often leads to privilege escalation through PolicyKit, hardening Polkit rules is a critical complementary mitigation. Review and modify PolicyKit rules (.pkla files in directories like /usr/share/polkit-1/rules.d/ or /etc/polkit-1/localauthority/50-local.d/) to restrict actions that can be performed by users, particularly those deemed ‘active’.

• Identify Vulnerable Rules: Focus on rules that use <allow_active>yes</allow_active> or <allow_active>auth_active for sensitive operations without further authentication, especially for services like udisks, systemd-logind, network-manager, or power-manager.

• Modify Authorization States: Change the authorization state to require explicit administrative authentication (auth_admin) or user authentication (auth_self) for sensitive actions, even for ‘active’ users. For instance, for udisks operations:

  • Original (potentially vulnerable) example in a .pkla file:
    xml
<action id="org.freedesktop.udisks2.modify-device">
<description>Modify a storage device</description>
<message>Authentication is required to modify a storage device.</message>
<defaults>
<allow_any>no</allow_any>
<allow_inactive>no</allow_inactive>
<allow_active>yes</allow_active> <!-- This is the problematic setting -->
</defaults>
</action>
  • Mitigated example:
    xml
<action id="org.freedesktop.udisks2.modify-device">
<description>Modify a storage device</description>
<message>Authentication is required to modify a storage device.</message>
<defaults>
<allow_any>no</allow_any>
<allow_inactive>no</allow_inactive>
<allow_active>auth_admin</allow_active> <!-- Requires admin password -->
</defaults>
</action>
  • Other options like auth_self (requires the user’s own password) or auth_self_keep (user’s password, then cached) can also be used.

• Implement Custom Rules: Consider creating your own .pkla files in /etc/polkit-1/localauthority/50-local.d/ to override default rules with stricter permissions, especially if you cannot modify the vendor-supplied files directly due to package updates.

6.5. Implement Additional Security Controls

• SELinux/AppArmor: Utilize mandatory access control (MAC) frameworks like SELinux or AppArmor to enforce granular permissions on processes and files. Policies can be written to restrict what udisks or other services can do, even if exploited, by limiting their ability to execute arbitrary binaries or access sensitive files. For example, AppArmor profiles could prevent udisks from executing SUID binaries from arbitrary mount points.
• Restrict SSH Access: Limit SSH access to only necessary users and IP addresses. Implement SSH key-based authentication, disable password authentication, and consider multi-factor authentication (MFA).
• User Education and Policy: Educate users about the risks of manipulating environment variables, especially within files like ~/.pam_environment. Establish clear organizational security policies regarding user-controlled configuration files and system interaction.
• System-wide Environment Variables: For critical environment variables, prefer setting them system-wide in /etc/environment or /etc/profile.d/ scripts, which are typically less prone to user manipulation in a privileged context compared to ~/.pam_environment.

7. Best Practices for Secure PAM Configuration

Beyond mitigating specific vulnerabilities, adopting a holistic approach to PAM configuration, based on established security best practices, can significantly enhance overall system security and resilience against future threats.

7.1. Principle of Least Privilege (PoLP)

Apply the Principle of Least Privilege rigorously to PAM configurations. This means configuring PAM modules to grant the absolute minimum necessary privileges to users for their specific roles and tasks. For example:

• Restrict Module Usage: Only load PAM modules that are genuinely required for a given service. Remove or comment out modules that are not needed, reducing the attack surface.
• Granular Control Flags: Carefully select control flags (requisite, required, sufficient, optional) to enforce strict authentication flows. Avoid sufficient where required is more appropriate for critical steps. Understand that optional modules will not fail the stack even if they fail, which can be a security risk if not intended.
• Module Arguments: Use module arguments to tighten security wherever possible (e.g., audit to force logging, debug for troubleshooting but disable in production, use_first_pass for efficiency, try_first_pass for flexibility).

7.2. Comprehensive Logging and Monitoring

Enable detailed logging for all PAM activities and integrate these logs into a centralized security information and event management (SIEM) system. Robust logging is crucial for detecting anomalous behavior, identifying failed authentication attempts, and aiding in forensic analysis during a security incident.

• Monitor auth.log / secure.log: These files (or journalctl -u systemd-logind for systemd-based systems) record authentication successes and failures, session openings/closings, and other PAM-related events. Look for:
  • Excessive failed login attempts from a single user or IP address.
  • Unusual login times or locations.
  • Unexpected session changes or escalations.
  • Messages indicating PAM module failures.
• Auditd Configuration: Configure the Linux Audit system (auditd) to monitor changes to PAM configuration files (/etc/pam.d/), PolicyKit rules (/etc/polkit-1/ and /usr/share/polkit-1/), and critical PAM module paths (/lib/security/, /usr/lib/security/).
• Alerting: Set up alerts for critical PAM-related events, such as multiple failed login attempts, successful logins from unusual sources, or changes to core authentication files.

7.3. Regular Updates and Patching

Maintain a rigorous schedule for applying security updates and patches to all PAM modules, the underlying PAM library, the kernel, and all related system components. This goes beyond just PAM; a holistic patching strategy is vital. Many vulnerabilities (like CVE-2025-6019) are not in PAM itself but in services that interact with PAM’s understanding of session context.

• Automate Updates: Where appropriate and after thorough testing, automate the application of non-critical security updates.
• Subscription Services: For enterprise environments, leverage vendor-specific security update services and tools to stay informed and manage patches effectively.

7.4. User Education and Awareness

Educate users, particularly system administrators and developers, about the implications of environment variables and the importance of maintaining secure configurations. Users should be aware of:

• Risks of ~/.pam_environment: Explain why manipulating this file can lead to security risks and why it’s generally discouraged unless absolutely necessary and understood.
• Strong Password Practices: Reinforce the importance of strong, unique passwords.
• Phishing and Social Engineering: Awareness of common attack vectors targeting credentials.

7.5. Configuration Management and Version Control

Treat PAM configuration files as critical infrastructure code. Employ configuration management tools and version control to manage them.

• Desired State Enforcement: Configuration management tools (Ansible, Puppet, Chef) ensure that PAM configurations across all systems adhere to a defined, secure baseline and automatically remediate any deviations.
• Change Tracking and Rollback: Version control (Git) for /etc/pam.d/ allows for clear tracking of who made what changes, when, and why. It also provides the ability to quickly roll back to a known good configuration if a change introduces a problem or vulnerability.

7.6. Understand Your Distribution’s Defaults

Different Linux distributions have varying default PAM configurations and module behaviors. What is secure by default on one distribution (e.g., Red Hat/CentOS often have user_readenv disabled by default in sshd) might be insecure on another (e.g., openSUSE). Always review the PAM configuration after a fresh installation or major upgrade, specifically for your chosen distribution.

7.7. Regular Security Audits and Penetration Testing

Beyond internal audits, engage third-party security professionals to conduct regular security audits and penetration tests. These external perspectives can often uncover misconfigurations or vulnerabilities that internal teams might overlook.

8. Conclusion

CVE-2025-6018 serves as a critical, contemporary reminder of the inherent complexities and potential security pitfalls within the Pluggable Authentication Module (PAM) framework. While PAM’s modularity offers unparalleled flexibility in tailoring authentication and session management, this power comes with a significant responsibility for meticulous configuration and ongoing oversight. The vulnerability, rooted in the default behavior of the pam_env module on certain distributions, highlights how seemingly minor environmental context changes can be weaponized, especially when chained with other services like udisks and PolicyKit, to achieve full privilege escalation.

This extensive analysis has delved into the technical intricacies of CVE-2025-6018, its reliance on specific XDG_ environment variables to manipulate session context, and its pivotal role in enabling a multi-stage attack that culminates in root access. By examining historical precedents, it becomes clear that vulnerabilities related to environment variable handling and the intricate interplay of PAM modules are recurring challenges that demand continuous attention.

To effectively counter such threats, system administrators and security professionals must adopt a proactive, multi-faceted approach. Key mitigation strategies include the immediate disabling of the user_readenv option in relevant PAM configurations, diligently applying vendor-supplied security updates for PAM and related components, and rigorously hardening PolicyKit rules to prevent unauthorized actions based on a false ‘active session’ premise. Furthermore, embracing comprehensive best practices for PAM configuration—such as adhering to the principle of least privilege, implementing robust logging and monitoring, utilizing configuration management tools, and fostering a culture of continuous security auditing and user education—is paramount.

Ultimately, securing the PAM framework is not a one-time task but an ongoing commitment. Continuous vigilance, regular audits, a deep understanding of module interactions, and adherence to established security practices are indispensable in safeguarding against privilege escalation and maintaining the integrity and confidentiality of Unix-like operating systems in an evolving threat landscape.

References

• CVE-2025-6018: Local Privilege Escalation via pam_env with Non-Default Configuration. Gentoo Bugzilla. (https://bugs.gentoo.org/958338)
• CVE-2025-6018 and CVE-2025-6019: Root Access on Linux. SOCRadar® Cyber Intelligence Inc. (https://socradar.io/cve-2025-6018-and-cve-2025-6019-root-access-on-linux/)
• CVE-2025-6018: Linux openSUSE Vulnerability Analysis and Mitigation. Wiz. (https://www.wiz.io/vulnerability-database/cve/cve-2025-6018)
• CVE-2010-4708: NVD. (https://nvd.nist.gov/vuln/detail/CVE-2010-4708)
• CVE-2011-3149: NVD. (https://security-tracker.debian.org/tracker/CVE-2011-3149)
• CVE-2025-6018, CVE-2025-6019: New Linux Vulnerabilities Allow Instant Root Access. OP Innovate. (https://op-c.net/blog/linux-root-access-cve-2025-6018-6019/)
• Security Update for pam SUSE-SU-2025:02013-1. SUSE Support. (https://www.suse.com/support/update/announcement/2025/suse-su-202502013-1)
• FreeDesktop.org XDG Base Directory Specification. (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html)
• PolicyKit Documentation. (https://www.freedesktop.org/software/polkit/docs/latest/)
• udisks2 Documentation. (https://www.freedesktop.org/wiki/Software/udisks/)