Operating SystemsProtection Domains

Protection Domains

LevelAdvanced

Duration75 mins

TopicProtection Domains

4 / 5

Least Privilege

The Wisdom of Minimal Authority

"A program that needs to read a file should not have the power to format the disk."

This seemingly obvious statement encapsulates one of the most important principles in security engineering: the Principle of Least Privilege (PoLP). First formally articulated by Jerome Saltzer and Michael Schroeder in their landmark 1975 paper "The Protection of Information in Computer Systems," this principle states that every program and every user of the system should operate using the least set of privileges necessary to complete the job.

The principle seems intuitive, yet it is violated constantly in practice. Running services as root, granting applications full disk access, giving users administrator privileges "just in case"—these common shortcuts undermine security at its foundation. Understanding least privilege is understanding how to design systems that remain secure even when components fail or are compromised.

What You Will Learn

By the end of this page, you will understand the theoretical basis of least privilege, its benefits for security and reliability, practical implementation techniques in real operating systems, challenges and trade-offs, and how modern systems attempt to achieve least privilege through capabilities, containers, and mandatory access controls.

Formal Statement of Least Privilege

The principle of least privilege can be stated formally:

Definition:

Every subject (process, user, system) should be granted only those privileges that are essential for performing its authorized functions, and those privileges should be held only for the minimum duration necessary.

This definition has several important components:

"Only those privileges that are essential":

Not "convenient" or "might be needed" or "easier to configure"
The minimum set required for legitimate operation
Defined by the task, not by the identity of the actor

"Performing its authorized functions":

Privileges match intended behavior
Unauthorized actions are impossible, not just forbidden
Policy is enforced at the mechanism level

"Minimum duration necessary":

Temporary elevation when needed
Immediate return to lower privilege
No standing privileges beyond current task

Mathematical Formulation:

For a subject S performing task T, let:

R(T) = set of rights required to complete T
G(S) = set of rights granted to S

Least privilege requires:

G(S) = R(T)    (granted rights exactly equal required rights)

In practice, achieving exact equality is often infeasible, so we aim for:

G(S) ⊇ R(T)    (granted rights ≥ required rights)
minimize |G(S) - R(T)|    (minimize excess privilege)

Corollary: Fail-Safe Defaults

A related principle states that the default answer to access requests should be denial. Absence of a specific permission implies no access. This ensures that:

New objects are secure by default
Configuration errors result in denied access (safe) not granted access (unsafe)
Permissions must be explicitly granted, never assumed

Privilege vs. Permission

Though often used interchangeably, 'privilege' typically refers to the ability to perform operations (execute code, change system state), while 'permission' refers to access rights over objects (read file, write memory). Least privilege applies to both concepts.

Benefits of Least Privilege

Least privilege provides multiple layers of benefit, affecting security, reliability, and maintainability:

Security Benefits:

Security Advantages

•Limits Breach Impact — If a component is compromised, the attacker gains only that component's limited privileges. An exploited web server can't read /etc/shadow if it never had that right.
•Reduces Attack Surface — Fewer privileges mean fewer ways to cause harm. An attacker can't abuse a capability the program doesn't have.
•Contains Malware — Malicious code runs with the privileges of its host process. Limited host privileges mean limited malware power.
•Prevents Privilege Escalation Chains — If each component has minimal privileges, attackers can't use one compromise to bootstrap to the next.
•Enables Meaningful Auditing — When privileges are minimal, access logs are meaningful. When everything is root, logs just say 'root did everything.'

Reliability Benefits:

Reliability Advantages

•Limits Bug Impact — A bug in an unprivileged component cannot corrupt critical system data. The damage is bounded by the component's privileges.
•Improves Fault Isolation — When components have separate, minimal privilege sets, failures don't cascade. One component's crash doesn't bring down others.
•Enables Easier Recovery — Lower-privilege components can often be restarted without affecting the system. High-privilege failures may require full system restart.
•Supports Defensive Programming — Knowing your code has limited privileges encourages checking return values and handling failures gracefully.

Privilege Level vs. Impact Comparison
Scenario	Without Least Privilege	With Least Privilege
Web server RCE exploit	Attacker gets root, owns system	Attacker gets www-data, limited to web directory
Database driver bug	Corrupts kernel, system crash	Corrupts DB process, service restart needed
Malicious email attachment	Ransomware encrypts all files	Ransomware limited to user's files, backups safe
Supply chain compromise	Backdoor has full system access	Backdoor limited to application sandbox
Configuration error	Accidentally deletes system files	Cannot access files outside owned directories

Implementing Least Privilege in Unix

Traditional Unix provides several mechanisms for implementing least privilege, though each has limitations:

User IDs and File Permissions:

The basic Unix permission model:

Each file has owner (UID), group (GID), and permission bits
Processes run with a specific UID/GID determining access
Services run as dedicated users (www-data, postgres, nobody)

# View service user
$ ps aux | grep nginx
www-data  1234  nginx: worker process

# nginx runs as www-data, can only access:
#   - Files owned by www-data
#   - Files with world or group (www-data) permissions
#   - Directories with appropriate traversal permissions

setuid/setgid Bits:

Programs can temporarily acquire elevated privileges:

// Classic pattern: setuid program
int main() {
    FILE *sensitive = fopen("/etc/shadow", "r");  // Needs root
    
    // Immediately drop privileges after opening
    if (setuid(getuid()) < 0) {
        perror("setuid");
        exit(1);
    }
    
    // Process file with normal user privileges
    process_file(sensitive);
    fclose(sensitive);
}

Linux Capabilities:

Linux capabilities split root's monolithic privilege into ~40 distinct capabilities:

# Instead of running as root, grant specific capabilities
$ setcap 'cap_net_bind_service=+ep' /usr/bin/myserver

# myserver can now bind to ports < 1024 without being root

Key capabilities:

Capability	Allows
CAP_NET_BIND_SERVICE	Bind to privileged ports (< 1024)
CAP_NET_RAW	Use raw sockets
CAP_SYS_ADMIN	Various admin operations (too broad!)
CAP_SYS_PTRACE	Trace/debug other processes
CAP_DAC_OVERRIDE	Bypass file read/write permission checks
CAP_SETUID	Manipulate process UIDs
CAP_NET_ADMIN	Network configuration
CAP_SYS_TIME	Set system clock

capabilities_example.c
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#include <stdio.h>
#include <sys/capability.h>
#include <unistd.h>
 
// Drop all capabilities except those needed
int drop_to_minimum_caps(void) {
    cap_t caps = cap_get_proc();
    if (!caps) return -1;
    
    // Clear all capabilities
    if (cap_clear(caps) < 0) {
        cap_free(caps);
        return -1;
    }
    
    // Only keep capability to bind to low ports
    cap_value_t keep_caps[] = { CAP_NET_BIND_SERVICE };
    
    if (cap_set_flag(caps, CAP_PERMITTED, 1, keep_caps, CAP_SET) < 0 ||
        cap_set_flag(caps, CAP_EFFECTIVE, 1, keep_caps, CAP_SET) < 0) {
        cap_free(caps);
        return -1;
    }
    
    if (cap_set_proc(caps) < 0) {
        cap_free(caps);
        return -1;
    }
    
    cap_free(caps);
    
    // Lock capabilities - prevent further changes
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
        return -1;
    }
    
    return 0;
}
 
int main() {
    // Bind to privileged port 80
    int sockfd = create_socket_and_bind(80);
    
    // Now drop all unnecessary capabilities
    if (drop_to_minimum_caps() < 0) {
        fprintf(stderr, "Failed to drop privileges\n");
        return 1;
    }
    
    // Server loop runs with minimal privileges
    serve_forever(sockfd);
    return 0;
}

Seccomp and System Call Filtering

Even with reduced capabilities, a process can still make any system call. Seccomp (Secure Computing) allows further restriction by filtering which system calls a process can execute.

Seccomp Modes:

Mode	Description	Use Case
Strict	Only read, write, exit, sigreturn allowed	Pure computation
Filter	BPF program decides allow/deny/kill	Custom per-application filtering

How Seccomp Works:

Process installs a BPF (Berkeley Packet Filter) program
The BPF program examines each syscall (number, arguments)
BPF returns: ALLOW, KILL, ERRNO, TRAP, or LOG
Kernel enforces the decision
Filters cannot be relaxed after installation

seccomp_filter.c
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#include <stdio.h>
#include <seccomp.h>
#include <unistd.h>
#include <sys/prctl.h>
 
// Install a seccomp filter allowing only essential syscalls
int install_seccomp_filter(void) {
    scmp_filter_ctx ctx;
    
    // Default action: kill the process on disallowed syscall
    ctx = seccomp_init(SCMP_ACT_KILL);
    if (!ctx) return -1;
    
    // Explicitly allow required syscalls
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(read), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fstat), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0);
    
    // Allow mmap but only for anonymous mappings (no files)
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap), 1,
                     SCMP_A3(SCMP_CMP_MASKED_EQ, MAP_ANONYMOUS, MAP_ANONYMOUS));
    
    // Load the filter
    if (seccomp_load(ctx) < 0) {
        seccomp_release(ctx);
        return -1;
    }
    
    seccomp_release(ctx);
    return 0;
}
 
int main() {
    printf("Installing seccomp filter...\n");
    
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
        perror("prctl");
        return 1;
    }
    
    if (install_seccomp_filter() < 0) {
        fprintf(stderr, "Failed to install seccomp filter\n");
        return 1;
    }
    
    printf("Filter installed. Now restricted to allowed syscalls.\n");
    
    // This works:
    write(1, "Hello from seccomp sandbox\n", 27);
    
    // This would kill the process (open is not allowed):
    // open("/etc/passwd", O_RDONLY);
    
    return 0;
}

Defense in Depth

Seccomp combines with capabilities, namespaces, and file permissions. A process might run as unprivileged user (UID), with only one capability (cap_net_bind_service), restricted syscalls (seccomp), in an isolated namespace (container), and with minimal file access (chroot or mount namespace). Each layer reinforces the others.

Sandboxing Techniques

Modern applications achieve least privilege through sandboxing—restricting a process to a limited view of system resources. Several technologies provide sandboxing:

chroot Jails:

The original Unix sandbox, chroot changes the apparent root directory:

# Create a minimal root filesystem
mkdir -p /jail/{bin,lib,etc}
cp /bin/bash /jail/bin/
cp -L /lib/x86_64-linux-gnu/libc.so.6 /jail/lib/

# Run program in jail
chroot /jail /bin/bash
# Program sees /jail as /
# Cannot access /etc/passwd (real one), only /jail/etc/passwd

Limitations of chroot:

Root can escape (mknod, mount, chroot again)
Doesn't isolate network, PIDs, users
Same kernel, same UIDs visible
Not a security boundary against root

Linux Namespaces:

Namespaces provide comprehensive isolation by giving each process its own view of system resources:

Namespace	Isolates	Use Case
mnt	Mount points	Filesystem isolation
pid	Process IDs	Separate PID trees
net	Network stack	Virtual network interfaces
user	UIDs/GIDs	Map container root to host unprivileged
uts	Hostname	Separate hostname per container
ipc	IPC objects	Separate semaphores, message queues
cgroup	Cgroup visibility	Limit cgroup view
time	System clocks	Different boot time per container

# Unshare namespaces and run command
unshare --mount --pid --fork --user /bin/bash

# Now running in isolated namespace
# PID 1 in this namespace, different mount tree, mapped UID

Container Runtimes (Docker, Podman):

Containers combine namespaces, cgroups, and seccomp for comprehensive isolation:

# Dockerfile implementing least privilege
FROM alpine:latest

# Run as unprivileged user, not root
RUN adduser -D appuser
USER appuser

# Read-only filesystem
VOLUME ["/data"]  # Only /data is writable

# Minimal capabilities
# docker run --cap-drop=ALL --cap-add=NET_BIND_SERVICE ...

# Run container with extensive restrictions
docker run \
    --user 1000:1000 \           # Non-root user
    --cap-drop=ALL \              # Drop all capabilities
    --read-only \                 # Read-only root filesystem
    --security-opt no-new-privileges \  # Can't gain privs
    --security-opt seccomp=profile.json \ # Syscall filter
    myapp:latest

Containers Are Not VMs

Containers share the host kernel. A kernel vulnerability exploited from inside a container affects the host. For strong isolation, combine containers with VMs (e.g., Kata Containers, gVisor's sentry, or Firecracker microVMs).

Privilege Separation Pattern

A powerful design pattern for achieving least privilege is privilege separation: splitting an application into multiple processes with different privilege levels, communicating over a narrow, well-defined interface.

The Pattern:

┌─────────────────────────────────────────────────────┐
│                   Application                        │
│  ┌─────────────────┐   IPC   ┌─────────────────┐    │
│  │  Privileged     │◄───────►│  Unprivileged   │    │
│  │  Component      │         │  Component      │    │
│  │  (small, simple,│         │  (large,complex,│    │
│  │   runs as root) │         │  runs as user)  │    │
│  └─────────────────┘         └─────────────────┘    │
└─────────────────────────────────────────────────────┘

Privileged Component: Opens files, binds ports, authenticates
Unprivileged Component: Parses input, processes data, renders output

OpenSSH Example:

OpenSSH pioneered privilege separation in Unix:

┌──────────────────────────────────────────────────────┐
│                    sshd                               │
│  ┌────────────────┐        ┌─────────────────────┐   │
│  │ sshd (root)    │◄──────►│ sshd (unprivileged) │   │
│  │                │  pipe   │                     │   │
│  │ - Auth users   │        │ - Parse SSH proto   │   │
│  │ - Pty alloc    │        │ - Crypto operations │   │
│  │ - User switch  │        │ - Key exchange      │   │
│  └────────────────┘        └─────────────────────┘   │
│         ▲                            │               │
│         │                            ▼               │
│         └───────►┌─────────────────────────────────┐ │
│                  │ User session (user's UID)       │ │
│                  │ - Run user's shell              │ │
│                  │ - Full network access revoked   │ │
│                  └─────────────────────────────────┘ │
└──────────────────────────────────────────────────────┘

Benefits of Privilege Separation

•Reduced Attack Surface — Complex parsing code runs unprivileged. Vulnerabilities in parsers (common!) don't grant root.
•Clear Security Boundary — The IPC interface is narrow and can be audited. Only sanctioned operations cross the boundary.
•Defense in Depth — Attacker must compromise unprivileged process, then escape to privileged process, then escalate further.
•Easier Auditing — The privileged component is small and can be thoroughly reviewed.
•Graceful Degradation — Unprivileged component can crash/restart without losing privileged state.

Chrome Browser Architecture:

Chrome uses extensive privilege separation:

Process	Privilege	Role
Browser	Full user	UI, process management, file access
Renderer	Sandboxed	Parse HTML, execute JavaScript
GPU	Limited hardware access	Graphics rendering
Network	Network only	Fetch resources
Plugin	Sandboxed or privileged	Third-party plugins

Renderers (where untrusted web content runs) are heavily sandboxed: no file access, no network (requests go through browser), seccomp limits syscalls, separate PID namespace.

Challenges and Trade-offs

Implementing least privilege is not without challenges. Understanding these difficulties helps navigate real-world constraints:

Challenge 1: Determining Minimum Privileges

How do you know what privileges a program actually needs?

Static analysis: Examine code for resource access patterns
Dynamic analysis: Run program, trace resource access, infer requirements
Documentation: Read specs, but real behavior may differ
Trial and error: Remove privileges until it breaks

Problem: Programs often need different privileges at different times or for different inputs. The "minimum" set might vary by use case.

Challenge 2: Granularity Mismatch

Somtimes the available privilege granularity doesn't match needs:

Need: Write to /var/log/myapp.log
Available: CAP_DAC_OVERRIDE (write to ANY file)

Need: Bind to port 443 only
Available: CAP_NET_BIND_SERVICE (bind to ANY port < 1024)

Solution: Use additional mechanisms (seccomp, AppArmor, SELinux) to narrow broad capabilities.

Challenge 3: Usability vs. Security

Strict least privilege can make systems hard to use:

Users forced to enter passwords constantly
Administrators can't debug easily
Developers resist security that slows them down
Error messages may be unhelpful ("permission denied" without context)

Challenge 4: Legacy Compatibility

Older software often assumes root privileges:

// Legacy code that assumes root
void legacy_function() {
    FILE *f = fopen("/etc/secret", "r");
    if (!f) {
        // Programmer assumed they'd always be root
        abort();  // No graceful fallback
    }
}

Challenge 5: Ambient Authority

In many systems, authority is ambient—processes inherit "who they are" and that determines access. This makes least privilege harder because:

Programs get privileges from identity, not need
Temporary privilege is awkward (setuid, sudo)
Can't delegate subset of privileges without delegating identity

Least Privilege Trade-offs
Dimension	More Restriction	Less Restriction
Security	Higher (smaller blast radius)	Lower (larger blast radius)
Usability	More authentication prompts, errors	Smoother user experience
Debugging	Harder (can't access needed info)	Easier (full visibility)
Development Speed	Slower (must define permissions)	Faster (grant all, ship)
Maintenance	More complex (policy updates)	Simpler (no policy to maintain)

Least Privilege in Modern Systems

Modern operating systems and platforms have increasingly embraced least privilege, often making it the default:

Mobile Platforms (iOS, Android):

Mobile OSes lead in least privilege enforcement:

┌─────────────────────────────────────────────────────┐
│                 Mobile App Sandbox                   │
│                                                     │
│  ┌───────────────────────────────────────────────┐  │
│  │ App runs in isolated container               │  │
│  │ - Own UID (Android) or sandbox (iOS)         │  │
│  │ - No access to other apps' data              │  │
│  │ - Explicit permission grants for:            │  │
│  │   • Camera, Microphone (prompt)              │  │
│  │   • Location (prompt with options)           │  │
│  │   • Contacts (prompt)                        │  │
│  │   • Files (Scoped Storage/File Picker)       │  │
│  │ - Permissions revocable at any time          │  │
│  └───────────────────────────────────────────────┘  │
└─────────────────────────────────────────────────────┘

Apps start with almost no privileges. Each capability must be explicitly requested and granted.

macOS App Sandboxing:

Mac App Store requires sandboxing with entitlements:

<!-- App.entitlements -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "...">
<plist version="1.0">
<dict>
    <key>com.apple.security.app-sandbox</key>
    <true/>
    
    <!-- Only network access requested -->
    <key>com.apple.security.network.client</key>
    <true/>
    
    <!-- User-selected files only -->
    <key>com.apple.security.files.user-selected.read-write</key>
    <true/>
    
    <!-- No access to: camera, microphone, contacts, location, 
         arbitrary files, etc. -->
</dict>
</plist>

Cloud IAM (AWS, GCP, Azure):

Cloud platforms implement fine-grained least privilege:

// AWS IAM Policy - Least privilege for Lambda function
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject"
      ],
      "Resource": "arn:aws:s3:::my-bucket/input/*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::my-bucket/output/*"
    }
  ]
}
// Function can only read from input/ and write to output/
// Not admin, not full S3 access - minimum needed rights

The Trend Is Clear

Every major platform is moving toward least privilege by default. Mobile led the way, cloud followed, and desktop is catching up. The 'run everything as root' era is ending. Understanding least privilege is essential for modern system design.

Summary: Least Privilege

We've explored the principle of least privilege from theory to practice. Let's consolidate the key insights:

Key Takeaways

•Least privilege grants only essential rights — Every subject should have only the privileges needed for its current task, held only for the necessary duration
•Benefits span security and reliability — Limited privileges contain breaches, limit bug impact, and enable meaningful auditing
•Unix provides multiple mechanisms — User IDs, capabilities, seccomp, namespaces, and chroot each contribute to privilege reduction
•Privilege separation isolates risk — Splitting applications into privileged and unprivileged components limits attacker capabilities
•Sandboxing enforces least privilege — Containers, namespaces, and platform sandboxes make restriction the default
•Trade-offs exist — Security improvement comes at cost of complexity, usability friction, and development effort
•Modern platforms embrace least privilege — Mobile, cloud, and increasingly desktop platforms enforce least privilege by default

What's Next:

We've covered what protection domains are, how to switch between them, the hierarchical ring model, and the principle of least privilege. The final page examines Domain Implementation—how real operating systems implement these concepts through access control lists, capabilities, and mandatory access controls like SELinux.

Page Complete

You now understand the principle of least privilege and its critical role in secure system design. This principle guides the design of secure systems from operating systems to cloud platforms.

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Operating SystemsProtection Domains

Protection Domains

LevelAdvanced

Duration75 mins

TopicProtection Domains

4 / 5

Least Privilege

The Wisdom of Minimal Authority

"A program that needs to read a file should not have the power to format the disk."

What You Will Learn

Formal Statement of Least Privilege

The principle of least privilege can be stated formally:

Definition:

Every subject (process, user, system) should be granted only those privileges that are essential for performing its authorized functions, and those privileges should be held only for the minimum duration necessary.

This definition has several important components:

"Only those privileges that are essential":

Not "convenient" or "might be needed" or "easier to configure"
The minimum set required for legitimate operation
Defined by the task, not by the identity of the actor

"Performing its authorized functions":

Privileges match intended behavior
Unauthorized actions are impossible, not just forbidden
Policy is enforced at the mechanism level

"Minimum duration necessary":

Temporary elevation when needed
Immediate return to lower privilege
No standing privileges beyond current task

Mathematical Formulation:

For a subject S performing task T, let:

R(T) = set of rights required to complete T
G(S) = set of rights granted to S

Least privilege requires:

G(S) = R(T)    (granted rights exactly equal required rights)

In practice, achieving exact equality is often infeasible, so we aim for:

G(S) ⊇ R(T)    (granted rights ≥ required rights)
minimize |G(S) - R(T)|    (minimize excess privilege)

Corollary: Fail-Safe Defaults

A related principle states that the default answer to access requests should be denial. Absence of a specific permission implies no access. This ensures that:

New objects are secure by default
Configuration errors result in denied access (safe) not granted access (unsafe)
Permissions must be explicitly granted, never assumed

Privilege vs. Permission

Benefits of Least Privilege

Least privilege provides multiple layers of benefit, affecting security, reliability, and maintainability:

Security Benefits:

Security Advantages

•Limits Breach Impact — If a component is compromised, the attacker gains only that component's limited privileges. An exploited web server can't read /etc/shadow if it never had that right.
•Reduces Attack Surface — Fewer privileges mean fewer ways to cause harm. An attacker can't abuse a capability the program doesn't have.
•Contains Malware — Malicious code runs with the privileges of its host process. Limited host privileges mean limited malware power.
•Prevents Privilege Escalation Chains — If each component has minimal privileges, attackers can't use one compromise to bootstrap to the next.
•Enables Meaningful Auditing — When privileges are minimal, access logs are meaningful. When everything is root, logs just say 'root did everything.'

Reliability Benefits:

Reliability Advantages

•Limits Bug Impact — A bug in an unprivileged component cannot corrupt critical system data. The damage is bounded by the component's privileges.
•Improves Fault Isolation — When components have separate, minimal privilege sets, failures don't cascade. One component's crash doesn't bring down others.
•Enables Easier Recovery — Lower-privilege components can often be restarted without affecting the system. High-privilege failures may require full system restart.
•Supports Defensive Programming — Knowing your code has limited privileges encourages checking return values and handling failures gracefully.

Privilege Level vs. Impact Comparison
Scenario	Without Least Privilege	With Least Privilege
Web server RCE exploit	Attacker gets root, owns system	Attacker gets www-data, limited to web directory
Database driver bug	Corrupts kernel, system crash	Corrupts DB process, service restart needed
Malicious email attachment	Ransomware encrypts all files	Ransomware limited to user's files, backups safe
Supply chain compromise	Backdoor has full system access	Backdoor limited to application sandbox
Configuration error	Accidentally deletes system files	Cannot access files outside owned directories

Implementing Least Privilege in Unix

Traditional Unix provides several mechanisms for implementing least privilege, though each has limitations:

User IDs and File Permissions:

The basic Unix permission model:

Each file has owner (UID), group (GID), and permission bits
Processes run with a specific UID/GID determining access
Services run as dedicated users (www-data, postgres, nobody)

# View service user
$ ps aux | grep nginx
www-data  1234  nginx: worker process

# nginx runs as www-data, can only access:
#   - Files owned by www-data
#   - Files with world or group (www-data) permissions
#   - Directories with appropriate traversal permissions

setuid/setgid Bits:

Programs can temporarily acquire elevated privileges:

// Classic pattern: setuid program
int main() {
    FILE *sensitive = fopen("/etc/shadow", "r");  // Needs root
    
    // Immediately drop privileges after opening
    if (setuid(getuid()) < 0) {
        perror("setuid");
        exit(1);
    }
    
    // Process file with normal user privileges
    process_file(sensitive);
    fclose(sensitive);
}

Linux Capabilities:

Linux capabilities split root's monolithic privilege into ~40 distinct capabilities:

# Instead of running as root, grant specific capabilities
$ setcap 'cap_net_bind_service=+ep' /usr/bin/myserver

# myserver can now bind to ports < 1024 without being root

Key capabilities:

Capability	Allows
CAP_NET_BIND_SERVICE	Bind to privileged ports (< 1024)
CAP_NET_RAW	Use raw sockets
CAP_SYS_ADMIN	Various admin operations (too broad!)
CAP_SYS_PTRACE	Trace/debug other processes
CAP_DAC_OVERRIDE	Bypass file read/write permission checks
CAP_SETUID	Manipulate process UIDs
CAP_NET_ADMIN	Network configuration
CAP_SYS_TIME	Set system clock

capabilities_example.c
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#include <stdio.h>
#include <sys/capability.h>
#include <unistd.h>
 
// Drop all capabilities except those needed
int drop_to_minimum_caps(void) {
    cap_t caps = cap_get_proc();
    if (!caps) return -1;
    
    // Clear all capabilities
    if (cap_clear(caps) < 0) {
        cap_free(caps);
        return -1;
    }
    
    // Only keep capability to bind to low ports
    cap_value_t keep_caps[] = { CAP_NET_BIND_SERVICE };
    
    if (cap_set_flag(caps, CAP_PERMITTED, 1, keep_caps, CAP_SET) < 0 ||
        cap_set_flag(caps, CAP_EFFECTIVE, 1, keep_caps, CAP_SET) < 0) {
        cap_free(caps);
        return -1;
    }
    
    if (cap_set_proc(caps) < 0) {
        cap_free(caps);
        return -1;
    }
    
    cap_free(caps);
    
    // Lock capabilities - prevent further changes
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
        return -1;
    }
    
    return 0;
}
 
int main() {
    // Bind to privileged port 80
    int sockfd = create_socket_and_bind(80);
    
    // Now drop all unnecessary capabilities
    if (drop_to_minimum_caps() < 0) {
        fprintf(stderr, "Failed to drop privileges\n");
        return 1;
    }
    
    // Server loop runs with minimal privileges
    serve_forever(sockfd);
    return 0;
}

Seccomp and System Call Filtering

Even with reduced capabilities, a process can still make any system call. Seccomp (Secure Computing) allows further restriction by filtering which system calls a process can execute.

Seccomp Modes:

Mode	Description	Use Case
Strict	Only read, write, exit, sigreturn allowed	Pure computation
Filter	BPF program decides allow/deny/kill	Custom per-application filtering

How Seccomp Works:

Process installs a BPF (Berkeley Packet Filter) program
The BPF program examines each syscall (number, arguments)
BPF returns: ALLOW, KILL, ERRNO, TRAP, or LOG
Kernel enforces the decision
Filters cannot be relaxed after installation

seccomp_filter.c
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#include <stdio.h>
#include <seccomp.h>
#include <unistd.h>
#include <sys/prctl.h>
 
// Install a seccomp filter allowing only essential syscalls
int install_seccomp_filter(void) {
    scmp_filter_ctx ctx;
    
    // Default action: kill the process on disallowed syscall
    ctx = seccomp_init(SCMP_ACT_KILL);
    if (!ctx) return -1;
    
    // Explicitly allow required syscalls
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(read), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fstat), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0);
    
    // Allow mmap but only for anonymous mappings (no files)
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap), 1,
                     SCMP_A3(SCMP_CMP_MASKED_EQ, MAP_ANONYMOUS, MAP_ANONYMOUS));
    
    // Load the filter
    if (seccomp_load(ctx) < 0) {
        seccomp_release(ctx);
        return -1;
    }
    
    seccomp_release(ctx);
    return 0;
}
 
int main() {
    printf("Installing seccomp filter...\n");
    
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
        perror("prctl");
        return 1;
    }
    
    if (install_seccomp_filter() < 0) {
        fprintf(stderr, "Failed to install seccomp filter\n");
        return 1;
    }
    
    printf("Filter installed. Now restricted to allowed syscalls.\n");
    
    // This works:
    write(1, "Hello from seccomp sandbox\n", 27);
    
    // This would kill the process (open is not allowed):
    // open("/etc/passwd", O_RDONLY);
    
    return 0;
}

Defense in Depth

Sandboxing Techniques

Modern applications achieve least privilege through sandboxing—restricting a process to a limited view of system resources. Several technologies provide sandboxing:

chroot Jails:

The original Unix sandbox, chroot changes the apparent root directory:

# Create a minimal root filesystem
mkdir -p /jail/{bin,lib,etc}
cp /bin/bash /jail/bin/
cp -L /lib/x86_64-linux-gnu/libc.so.6 /jail/lib/

# Run program in jail
chroot /jail /bin/bash
# Program sees /jail as /
# Cannot access /etc/passwd (real one), only /jail/etc/passwd

Limitations of chroot:

Root can escape (mknod, mount, chroot again)
Doesn't isolate network, PIDs, users
Same kernel, same UIDs visible
Not a security boundary against root

Linux Namespaces:

Namespaces provide comprehensive isolation by giving each process its own view of system resources:

Namespace	Isolates	Use Case
mnt	Mount points	Filesystem isolation
pid	Process IDs	Separate PID trees
net	Network stack	Virtual network interfaces
user	UIDs/GIDs	Map container root to host unprivileged
uts	Hostname	Separate hostname per container
ipc	IPC objects	Separate semaphores, message queues
cgroup	Cgroup visibility	Limit cgroup view
time	System clocks	Different boot time per container

# Unshare namespaces and run command
unshare --mount --pid --fork --user /bin/bash

# Now running in isolated namespace
# PID 1 in this namespace, different mount tree, mapped UID

Container Runtimes (Docker, Podman):

Containers combine namespaces, cgroups, and seccomp for comprehensive isolation:

# Dockerfile implementing least privilege
FROM alpine:latest

# Run as unprivileged user, not root
RUN adduser -D appuser
USER appuser

# Read-only filesystem
VOLUME ["/data"]  # Only /data is writable

# Minimal capabilities
# docker run --cap-drop=ALL --cap-add=NET_BIND_SERVICE ...

# Run container with extensive restrictions
docker run \
    --user 1000:1000 \           # Non-root user
    --cap-drop=ALL \              # Drop all capabilities
    --read-only \                 # Read-only root filesystem
    --security-opt no-new-privileges \  # Can't gain privs
    --security-opt seccomp=profile.json \ # Syscall filter
    myapp:latest

Containers Are Not VMs

Privilege Separation Pattern

The Pattern:

┌─────────────────────────────────────────────────────┐
│                   Application                        │
│  ┌─────────────────┐   IPC   ┌─────────────────┐    │
│  │  Privileged     │◄───────►│  Unprivileged   │    │
│  │  Component      │         │  Component      │    │
│  │  (small, simple,│         │  (large,complex,│    │
│  │   runs as root) │         │  runs as user)  │    │
│  └─────────────────┘         └─────────────────┘    │
└─────────────────────────────────────────────────────┘

Privileged Component: Opens files, binds ports, authenticates
Unprivileged Component: Parses input, processes data, renders output

OpenSSH Example:

OpenSSH pioneered privilege separation in Unix:

┌──────────────────────────────────────────────────────┐
│                    sshd                               │
│  ┌────────────────┐        ┌─────────────────────┐   │
│  │ sshd (root)    │◄──────►│ sshd (unprivileged) │   │
│  │                │  pipe   │                     │   │
│  │ - Auth users   │        │ - Parse SSH proto   │   │
│  │ - Pty alloc    │        │ - Crypto operations │   │
│  │ - User switch  │        │ - Key exchange      │   │
│  └────────────────┘        └─────────────────────┘   │
│         ▲                            │               │
│         │                            ▼               │
│         └───────►┌─────────────────────────────────┐ │
│                  │ User session (user's UID)       │ │
│                  │ - Run user's shell              │ │
│                  │ - Full network access revoked   │ │
│                  └─────────────────────────────────┘ │
└──────────────────────────────────────────────────────┘

Benefits of Privilege Separation

•Reduced Attack Surface — Complex parsing code runs unprivileged. Vulnerabilities in parsers (common!) don't grant root.
•Clear Security Boundary — The IPC interface is narrow and can be audited. Only sanctioned operations cross the boundary.
•Defense in Depth — Attacker must compromise unprivileged process, then escape to privileged process, then escalate further.
•Easier Auditing — The privileged component is small and can be thoroughly reviewed.
•Graceful Degradation — Unprivileged component can crash/restart without losing privileged state.

Chrome Browser Architecture:

Chrome uses extensive privilege separation:

Process	Privilege	Role
Browser	Full user	UI, process management, file access
Renderer	Sandboxed	Parse HTML, execute JavaScript
GPU	Limited hardware access	Graphics rendering
Network	Network only	Fetch resources
Plugin	Sandboxed or privileged	Third-party plugins

Renderers (where untrusted web content runs) are heavily sandboxed: no file access, no network (requests go through browser), seccomp limits syscalls, separate PID namespace.

Challenges and Trade-offs

Implementing least privilege is not without challenges. Understanding these difficulties helps navigate real-world constraints:

Challenge 1: Determining Minimum Privileges

How do you know what privileges a program actually needs?

Static analysis: Examine code for resource access patterns
Dynamic analysis: Run program, trace resource access, infer requirements
Documentation: Read specs, but real behavior may differ
Trial and error: Remove privileges until it breaks

Problem: Programs often need different privileges at different times or for different inputs. The "minimum" set might vary by use case.

Challenge 2: Granularity Mismatch

Somtimes the available privilege granularity doesn't match needs:

Need: Write to /var/log/myapp.log
Available: CAP_DAC_OVERRIDE (write to ANY file)

Need: Bind to port 443 only
Available: CAP_NET_BIND_SERVICE (bind to ANY port < 1024)

Solution: Use additional mechanisms (seccomp, AppArmor, SELinux) to narrow broad capabilities.

Challenge 3: Usability vs. Security

Strict least privilege can make systems hard to use:

Users forced to enter passwords constantly
Administrators can't debug easily
Developers resist security that slows them down
Error messages may be unhelpful ("permission denied" without context)

Challenge 4: Legacy Compatibility

Older software often assumes root privileges:

// Legacy code that assumes root
void legacy_function() {
    FILE *f = fopen("/etc/secret", "r");
    if (!f) {
        // Programmer assumed they'd always be root
        abort();  // No graceful fallback
    }
}

Challenge 5: Ambient Authority

In many systems, authority is ambient—processes inherit "who they are" and that determines access. This makes least privilege harder because:

Programs get privileges from identity, not need
Temporary privilege is awkward (setuid, sudo)
Can't delegate subset of privileges without delegating identity

Least Privilege Trade-offs
Dimension	More Restriction	Less Restriction
Security	Higher (smaller blast radius)	Lower (larger blast radius)
Usability	More authentication prompts, errors	Smoother user experience
Debugging	Harder (can't access needed info)	Easier (full visibility)
Development Speed	Slower (must define permissions)	Faster (grant all, ship)
Maintenance	More complex (policy updates)	Simpler (no policy to maintain)

Least Privilege in Modern Systems

Modern operating systems and platforms have increasingly embraced least privilege, often making it the default:

Mobile Platforms (iOS, Android):

Mobile OSes lead in least privilege enforcement:

┌─────────────────────────────────────────────────────┐
│                 Mobile App Sandbox                   │
│                                                     │
│  ┌───────────────────────────────────────────────┐  │
│  │ App runs in isolated container               │  │
│  │ - Own UID (Android) or sandbox (iOS)         │  │
│  │ - No access to other apps' data              │  │
│  │ - Explicit permission grants for:            │  │
│  │   • Camera, Microphone (prompt)              │  │
│  │   • Location (prompt with options)           │  │
│  │   • Contacts (prompt)                        │  │
│  │   • Files (Scoped Storage/File Picker)       │  │
│  │ - Permissions revocable at any time          │  │
│  └───────────────────────────────────────────────┘  │
└─────────────────────────────────────────────────────┘

Apps start with almost no privileges. Each capability must be explicitly requested and granted.

macOS App Sandboxing:

Mac App Store requires sandboxing with entitlements:

<!-- App.entitlements -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "...">
<plist version="1.0">
<dict>
    <key>com.apple.security.app-sandbox</key>
    <true/>
    
    <!-- Only network access requested -->
    <key>com.apple.security.network.client</key>
    <true/>
    
    <!-- User-selected files only -->
    <key>com.apple.security.files.user-selected.read-write</key>
    <true/>
    
    <!-- No access to: camera, microphone, contacts, location, 
         arbitrary files, etc. -->
</dict>
</plist>

Cloud IAM (AWS, GCP, Azure):

Cloud platforms implement fine-grained least privilege:

// AWS IAM Policy - Least privilege for Lambda function
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject"
      ],
      "Resource": "arn:aws:s3:::my-bucket/input/*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::my-bucket/output/*"
    }
  ]
}
// Function can only read from input/ and write to output/
// Not admin, not full S3 access - minimum needed rights

The Trend Is Clear

Summary: Least Privilege

We've explored the principle of least privilege from theory to practice. Let's consolidate the key insights:

Key Takeaways

•Least privilege grants only essential rights — Every subject should have only the privileges needed for its current task, held only for the necessary duration
•Benefits span security and reliability — Limited privileges contain breaches, limit bug impact, and enable meaningful auditing
•Unix provides multiple mechanisms — User IDs, capabilities, seccomp, namespaces, and chroot each contribute to privilege reduction
•Privilege separation isolates risk — Splitting applications into privileged and unprivileged components limits attacker capabilities
•Sandboxing enforces least privilege — Containers, namespaces, and platform sandboxes make restriction the default
•Trade-offs exist — Security improvement comes at cost of complexity, usability friction, and development effort
•Modern platforms embrace least privilege — Mobile, cloud, and increasingly desktop platforms enforce least privilege by default

What's Next:

Page Complete

You now understand the principle of least privilege and its critical role in secure system design. This principle guides the design of secure systems from operating systems to cloud platforms.

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