Improving Linux Security Through Better Access Control Configuration
Defining Access Control
Access control is a security technique that regulates which users or processes have the ability to access a system resource like a file, directory, or network service. The goals of access control are to prevent unauthorized access and malicious activity while enabling legitimate use. On Linux systems, access control policies are implemented in the kernel and applied across users, file systems, and networks.
Linux uses several methods to control access, including traditional Unix-style permissions for files and directories, access control lists (ACLs) for setting complex policies, and mandatory access control (MAC) systems like SELinux and AppArmor. These tools allow administrators to configure policies ranging from user-based read, write, execute permissions to sophisticated security models and context-aware controls.
Types of Access Control Policies
Linux supports several key types of access control policies, including:
- Discretionary Access Control (DAC) – Controls access based on user identity and permissions.
- Mandatory Access Control (MAC) – Restricts access using security labels and security contexts.
- Role-Based Access Control (RBAC) – Grants access based on user roles and responsibilities.
- Attribute-Based Access Control (ABAC) – Defines rules using user attributes like location and device.
Analyzing Current Access Controls
Improving security through access control starts with auditing and analyzing how access is currently configured. Examining permission settings, reviewing logs for issues, and assessing needs can reveal where controls should be added or modified.
Reviewing File and Directory Permissions
The basic building block of access control on Linux are the read, write, execute permissions defined for files, directories, and devices. Using the ls -l command or file browsers, current permissions can be reviewed and evaluated for appropriateness. Too many write permissions or world executable bits may indicate areas for improvement.
Identifying Risks from Open Access
Unprotected directories like /tmp and broad write permissions on /var or /etc can leave systems exposed. Audit permission settings in key locations like /etc, user home directories, application directories, and databases. Identify areas where users or processes have unnecessary access that could lead to data loss, breach, or violation.
Assessing Needs for Access
Aligning access controls with legitimate business needs for access is key for security and compliance. Analyzing how users, processes, and applications require access enables setting appropriate controls. Processes running as root when less privilege would suffice indicates potential for improving protections.
Configuring Access Control Policies
Once risks from excessive permissions have been identified, access controls can be implemented to reduce exposure. Define rules restricting access only to approved entities based on needs, using tools like permissions, ACLs, and SELinux/AppArmor policies.
Setting User, Group, and Other Permissions
The most basic access controls are the read (r), write(w) and execute(x) permissions assigned to files, directories, devices. Using chmod these can be modified to ensure users only have necessary access.
Applying Access Control Lists (ACLs)
For more complex controls, access control lists enable different permissions per user or group. ACLs supplement standard Unix permissions. Filesystem ACLs have the advantage of specificity over coarse grained permission bits.
Leveraging SELinux/AppArmor Policies
SELinux and Apparmor provide mandatory access controls based on security policy frameworks. These access decisions are based on the security context of subjects and objects. Tighter controls can be achieved by defining and applying policies to processes and resources.
Managing Access Changes
To avoid permission risks from accumulating over time as systems evolve, maintaining and validating controls should become part of operational habits.
Automating Access Control Configuration
Using tools like Ansible, Puppet and Chef infrastructure as code automation can consistently apply and validate permissions, ACLs, SELinux policies rather relying on fragile manual processes.
Testing Access for Issues
Regularly testing access by validating file permissions against an approved baseline using scripts helps detect risky changes or excessive permissions before damage occurs.
Monitoring Access Control Changes
Monitoring file integrity checking tools like Tripwire and auditing capability changes via Auditd ensures new risks don’t arise over time as systems morph via poor change control.
Example Access Control Improvements
Some examples of where tuning Linux access controls reduces risk include:
Restricting Access to Sensitive Files
Remove unnecessary permissions and groups on sensitive data like cryptocurrency wallets, credentials files, financial data, logs, and personally identifiable information (PII) to limit exposure.
Isolating Application Access
Run processes like databases, web servers, and applications using isolated users and groups to construct access boundaries between services and data to contain breaches.
Protecting Administration Utilities
Tools that alter system state like useradd, shutdown, ssh must be protected by removing world access. Utilities that read system state like ps and netstat often don’t require execute permissions.
Achieving Defense-in-Depth with Access Controls
In a multilayered security strategy, access controls provide fundamental protections limiting damage from compromised users or malware. Tuning policies to enforce least privilege access hardens systems against unnecessary risk.
