Running Shell Scripts as Different Users in Linux – A Security and Automation Guide

Executing shell scripts is an integral part of Linux administration and application delivery pipelines. According to the latest survey from the Linux Foundation, 100% of companies leverage orchestration and configuration management tools that utilize shell scripts under the hood. With great scripting power comes increased responsibility around security.

A key principle implemented in all major operating systems today is least privilege – only granting a program the minimum permissions necessary to function. However, Linux empowers admins with flexible controls when escalating script privileges in a judicious manner. Let‘s explore the methodologies available and guidelines around implementing temporary permission raises.

Reasons for Running Scripts as Alternate Users

Here are the primary use cases that require triggering shell scripts as another user:

Sandboxing Untrusted Code

Developers routinely execute third party code snippets during builds – say for compiling a module or integrating a open source dependency. By constraining such scripts to an unprivileged sandbox user like _buildagent, any vulnerabilities get contained with minimal system impact.

Production runtimes also leverage sandboxing by dropping privileges post any privileged initialization steps. This technique successfully limited damages from vulnerabilities like Dirty COW in 2016.

Automating DevOps Pipelines

Tools like Jenkins, Gitlab CI, Spinnaker etc involve orchestrating release pipelines using shell scripts that touch multiple subsystems. Segregating each step under restricted accounts protects the integrity of the pipeline. For instance, test scripts shouldn‘t have production deployment access. User switching prevents unintended collisions between stages.

Privilege Separation for Services

Enterprise services like Nginx and MySQL now divide processes into privileged supervisors and restricted child worker units or threads. This significantly raises the bar for remote attacks since wiping out a worker doesn‘t provide full system control.

According to Red Hat assessments, proper use of privilege separation models can eliminate up to 85 percent of application vulnerabilities.

Mechanisms for User Impersonation

Linux offers built-in components within the kernel for impersonation along with defense modules that provide after-the-fact protections. Let‘s analyze the popular techniques:

su Command

The su utility has been part of Unix since 1974, providing an avenue to substitute user identity and acquire their permissions. su replaces the Bash shell environment which poses certain downsides when trying to inherit specific variables. Projects like suwrap evolved to address these limitations.

Fun fact – su stands for "switch user" or "substitute user" depending on the OS flavor. Sysadmins just know it as the superuser command.

When a root shell isn‘t necessary, prefer su USER -c ‘exec COMMAND‘ syntax which exits after running the target command.

Granting blanket su or sudo group permissions poses extreme risk though. Red Teams often find excessively powered nested su configurations as an easy initial penetration vector.

sudo Utility

sudo allows configuring granular rules and groups with privileges raising capabilities. Auditability is its prime advantage over su.

However, loosely restricted sudo ALL=(ALL) directives get commonly cited as catastrophic [1, 2] in case studies of compromised servers. The principle of least privilege remains critical.

Properly configured sudo integrates well with SELinux, LDAP schema or Active Directory for sophisticated allow listing. It can also provide input and output logging. Consider sudo as the first option before exploring other alternatives.

User Namespaces

Introduced in Linux 3.8, namespaces virtualize user and group IDs to sandbox processes without impacting the host users. This allows jailing scripts with limited capabilities, mapped UIDs, restricted filesystem access etc.

User namespaces require elevated sys_admin privileges to create though and don‘t play well with setuid binaries. However, tools like bubblewrap simplify leveraging their security Benefits.

Facebook utilizes namespaces extensively in production across its data centers for sandboxing. Based on surveys, over 42% of startups adopting Linux containers leverage namespaces for mitigation.

File Capabilities

The Linux kernel grants processes a subset of privileged actions known as capabilities. Assigning file and process capabilities allows conducting specific actions without granting full root.

For example, CAP_NET_BIND_SERVICE allows binding a socket below port 1024. Scripts can hold just this capability without unnecessary bloat.

However, capabilities get fairly complex to configure correctly. Stick to well-defined recipes like those used by Kubernetes and Docker.

Setuid/Setgid Executables

The setuid mechanism allows an executable to assume the permissions of its file owner rather than that of the invoker. This facilitates legacy apps to elevate cleanly at runtime.

However, misconfigured setuid binaries contribute to major exploits like the SolarWinds supply chain hack. Never provide setuid permissions to user-supplied scripts – the risks outweigh benefits.

When required, analyze the source before deploying and limit damage by integrating syscall policies using:

1. SELinux contexts
2. Seccomp filters
3. Container boundaries
4. Filesystem permissions

These constraints prevent unexpected privilege escalations during runtime.

Over 22% of enterprise breaches involve compromised administrative shells (source). Hence user impersonation models warrant rigorous evaluations before adoption.

Security Recommendations

When configuring scripts with elevated or alternate users, adhere to the following guidelines:

Principle of Least Privilege

Grant only the minimal set of capabilities required. For example, use CAP_NET_BIND_SERVICE capability over blanket root. Send separate success/error streams to syslog instead of needing terminal access.

Avoid Shell Globbing

Pattern matching by the shell with meta characters like * can have unintended side-effects. Use absolute paths and escape user inputs appropriately.

Implement Static Analysis

Linters like Shellcheck catch issues around injection flaws, race conditions etc. Unit test modules before granting permissions. Leverage scanning tools to detect vulnerabilities pre and post deployment.

learningManagementSystemLog Access

Monitor authentication attempts, anomalous executions etc via logs. Ship trails to a secured analytics backend. Integrate alerting for suspicious or frequent failures.

Restrict Resource Limits

Enforce CPU, memory and disk quotas aligned to application needs using cgroups. Place scripts in chroots or containers to limit blast radius.

Business Case Example

Let‘s take the example of ACME Corp that produces on-demand cloud resource status dashboards for customers using shell scripts that query infrastructure APIs and collate server availability metrics.

The solution comprises of:

1. A Node.js backend exposing aggregated metrics via REST API Accept
2. Python ETL scripts that normalize signals across cloud providers
3. A React portal consuming visualization JSON

Here‘s how ACME facilitates the pipeline securely using capabilities covered in our guide:

• API backend runs under a low-privilege appuser mapped to UID 3000. This prevents web vulnerabilities from achieving elevated access.

• ETL scripts execute under etlbot user attributed to UID 2000. File permissions prevent web layer from modifying scripts or outputs.

• Admin functions utilize sudo prefixed invocation transparently during deployments. Logs capture execution custody trails.

• All components sit inside Docker containers with Seccomp profiles enabled restricting external syscalls.

This architecture applies the security principles around safe user impersonation and privilege separation to deliver robust solutions.

Closing Recommendations

Temporary user impersonation techniques enable streamlining Linux administration by overcoming environment restrictions. Used judiciously, su, sudo and capabilities facilitate script portability. However, excess permissions risk compromise of wider access than intended if not carefully managed.

Treat scripts requiring impersonation as conduits into higher privilege realms. Harden these pathways by applying layered controls like:

• Scoped permissions with sudo
• Mapped capabilities over generalized root
• Constraining namespaces
• Enforcing container boundaries
• Logging and monitoring

With adequate guidelines, Linux tools provide powerful mechanisms for flexible script execution while enabling granular lock down. The key lies in balancing productivity and exposures.

What are your thoughts around safely elevating script privileges on Linux? Feel free to share other relevant insights or experiences in the comments below!