Securing Sensitive Environment Variables in Linux Systems

Learn comprehensive methods and best practices for protecting sensitive environment variables in Linux, from basic permissions to advanced encryption.

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Securing Sensitive Environment Variables in Linux

Table of Contents

1. [Introduction](#introduction) 2. [Understanding Environment Variables](#understanding-environment-variables) 3. [Security Risks](#security-risks) 4. [Best Practices Overview](#best-practices-overview) 5. [Methods for Securing Environment Variables](#methods-for-securing-environment-variables) 6. [Implementation Examples](#implementation-examples) 7. [Advanced Security Techniques](#advanced-security-techniques) 8. [Monitoring and Auditing](#monitoring-and-auditing) 9. [Common Mistakes and Pitfalls](#common-mistakes-and-pitfalls) 10. [Conclusion](#conclusion)

Introduction

Environment variables are a fundamental component of Linux systems, providing a way to pass configuration data to applications and processes. However, when these variables contain sensitive information such as passwords, API keys, database credentials, or encryption keys, they become potential security vulnerabilities if not properly managed.

This comprehensive guide explores various methods and best practices for securing sensitive environment variables in Linux environments, from basic file permissions to advanced encryption techniques and specialized tools.

Understanding Environment Variables

What are Environment Variables

Environment variables are dynamic named values that can affect the way running processes behave on a computer. They are part of the environment in which a process runs and can be accessed by applications to retrieve configuration information.

Types of Environment Variables

| Type | Scope | Persistence | Example | |------|-------|-------------|---------| | System-wide | All users | Permanent | PATH, HOME | | User-specific | Single user | Session/permanent | USER, SHELL | | Process-specific | Single process | Process lifetime | Custom app variables | | Session-specific | Current session | Session lifetime | Temporary variables |

Common Commands for Environment Variables

`bash

Display all environment variables

env

Display specific variable

echo $VARIABLE_NAME

Set temporary variable (current session only)

export TEMP_VAR="value"

Display variable with default value if not set

echo ${VARIABLE_NAME:-default_value}

Unset variable

unset VARIABLE_NAME

List all variables (including shell variables)

set `

Where Environment Variables are Defined

| Location | Scope | Purpose | |----------|-------|---------| | /etc/environment | System-wide | Global variables for all users | | /etc/profile | System-wide | Login shell initialization | | /etc/bash.bashrc | System-wide | Non-login bash shells | | ~/.bashrc | User-specific | User's bash shell configuration | | ~/.profile | User-specific | User's login shell | | ~/.bash_profile | User-specific | User's bash login shell |

Security Risks

Common Security Vulnerabilities

#### Process Visibility Environment variables are visible to other processes running under the same user context through the /proc filesystem:

`bash

View environment variables of a process

cat /proc/PID/environ

View environment variables of all processes (if permitted)

ps auxe

View environment variables in process tree

pstree -p -a `

#### Log File Exposure Applications may inadvertently log environment variables, exposing sensitive data:

`bash

Check system logs for potential exposure

grep -r "PASSWORD\|API_KEY\|SECRET" /var/log/

Check application logs

journalctl -u service_name | grep -i "env\|variable" `

#### Shell History Exposure Commands containing sensitive data may be stored in shell history:

`bash

Check bash history for sensitive data

grep -i "export.password\|export.key\|export.*secret" ~/.bash_history `

Risk Assessment Matrix

| Risk Factor | Low | Medium | High | Critical | |-------------|-----|--------|------|----------| | Data Sensitivity | Public info | Internal data | Personal data | Credentials/Keys | | Access Level | Restricted user | Standard user | Privileged user | Root access | | Network Exposure | Local only | Internal network | DMZ | Internet-facing | | Logging Level | Minimal | Standard | Verbose | Debug mode |

Best Practices Overview

Fundamental Security Principles

1. Principle of Least Privilege: Grant minimal necessary access 2. Defense in Depth: Multiple layers of security 3. Separation of Concerns: Isolate sensitive data 4. Regular Auditing: Monitor and review access 5. Encryption: Protect data at rest and in transit

Security Hierarchy

| Security Level | Method | Use Case | |----------------|--------|----------| | Basic | File permissions | Development environments | | Intermediate | Encrypted files | Small production systems | | Advanced | Key management systems | Enterprise environments | | Enterprise | Hardware security modules | High-security requirements |

Methods for Securing Environment Variables

Method 1: Restricted File Permissions

#### Basic File Permission Security

`bash

Create secure environment file

sudo touch /etc/secure_env sudo chmod 600 /etc/secure_env sudo chown root:root /etc/secure_env

Add sensitive variables

sudo tee /etc/secure_env << EOF DATABASE_PASSWORD=super_secret_password API_KEY=abcd1234567890 ENCRYPTION_KEY=very_long_encryption_key EOF `

#### Advanced Permission Management

`bash

Create dedicated group for application

sudo groupadd secure_app

Create service user

sudo useradd -r -s /bin/false -g secure_app app_service

Set restrictive permissions

sudo chmod 640 /etc/secure_env sudo chown root:secure_app /etc/secure_env

Load variables securely in script

if [ -r /etc/secure_env ]; then set -a source /etc/secure_env set +a fi `

Method 2: Using systemd for Service Environment Variables

#### Systemd Environment Files

`bash

Create systemd environment directory

sudo mkdir -p /etc/systemd/system/myapp.service.d

Create environment file

sudo tee /etc/systemd/system/myapp.service.d/environment.conf << EOF [Service] EnvironmentFile=-/etc/myapp/environment EOF `

#### Secure Service Configuration

`ini

/etc/systemd/system/myapp.service

[Unit] Description=My Secure Application After=network.target

[Service] Type=simple User=myapp Group=myapp EnvironmentFile=-/etc/myapp/secure.env ExecStart=/usr/local/bin/myapp Restart=always RestartSec=10

Security settings

NoNewPrivileges=true ProtectSystem=strict ProtectHome=true ReadWritePaths=/var/lib/myapp PrivateTmp=true

[Install] WantedBy=multi-user.target `

Method 3: Encryption-Based Solutions

#### GPG Encryption for Environment Files

`bash

Generate GPG key for encryption

gpg --gen-key

Create and encrypt environment file

cat > sensitive.env << EOF DATABASE_URL=postgresql://user:password@localhost/db API_SECRET=very_secret_api_key PRIVATE_KEY=-----BEGIN PRIVATE KEY----- ... -----END PRIVATE KEY----- EOF

Encrypt the file

gpg --cipher-algo AES256 --compress-algo 1 --s2k-mode 3 \ --s2k-digest-algo SHA512 --s2k-count 65536 --symmetric \ --output sensitive.env.gpg sensitive.env

Remove plain text file

shred -vfz -n 3 sensitive.env `

#### Decryption Script

`bash #!/bin/bash

decrypt_and_load.sh

set -euo pipefail

ENV_FILE_ENCRYPTED="/etc/myapp/sensitive.env.gpg" TEMP_DIR=$(mktemp -d) ENV_FILE_TEMP="$TEMP_DIR/environment"

Cleanup function

cleanup() { if [ -f "$ENV_FILE_TEMP" ]; then shred -vfz -n 3 "$ENV_FILE_TEMP" fi rm -rf "$TEMP_DIR" }

Set trap for cleanup

trap cleanup EXIT INT TERM

Decrypt environment file

gpg --quiet --batch --yes --decrypt --output "$ENV_FILE_TEMP" "$ENV_FILE_ENCRYPTED"

Source the decrypted file

set -a source "$ENV_FILE_TEMP" set +a

Execute the application

exec "$@" `

Method 4: Using External Key Management Systems

#### HashiCorp Vault Integration

`bash

Install Vault CLI

wget https://releases.hashicorp.com/vault/1.15.0/vault_1.15.0_linux_amd64.zip unzip vault_1.15.0_linux_amd64.zip sudo mv vault /usr/local/bin/

Configure Vault client

export VAULT_ADDR='https://vault.example.com:8200' export VAULT_TOKEN='your-vault-token'

Store secrets in Vault

vault kv put secret/myapp \ database_password="super_secret" \ api_key="abcd1234567890" `

#### Vault Integration Script

`bash #!/bin/bash

vault_env_loader.sh

set -euo pipefail

VAULT_PATH="secret/myapp" VAULT_ADDR="${VAULT_ADDR:-https://vault.example.com:8200}"

Function to retrieve secret from Vault

get_vault_secret() { local path="$1" local field="$2" vault kv get -field="$field" "$path" 2>/dev/null || { echo "Error: Failed to retrieve $field from $path" >&2 exit 1 } }

Load secrets from Vault

export DATABASE_PASSWORD=$(get_vault_secret "$VAULT_PATH" "database_password") export API_KEY=$(get_vault_secret "$VAULT_PATH" "api_key") export ENCRYPTION_KEY=$(get_vault_secret "$VAULT_PATH" "encryption_key")

Execute application

exec "$@" `

Method 5: Container-Based Security

#### Docker Secrets Management

`dockerfile

Dockerfile with security considerations

FROM alpine:3.18

Create non-root user

RUN addgroup -g 1001 appgroup && \ adduser -D -s /bin/sh -u 1001 -G appgroup appuser

Copy application

COPY --chown=appuser:appgroup app /usr/local/bin/app

Switch to non-root user

USER appuser

Use ENTRYPOINT for better signal handling

ENTRYPOINT ["/usr/local/bin/app"] `

#### Docker Compose with Secrets

`yaml

docker-compose.yml

version: '3.8'

services: app: image: myapp:latest environment: - DATABASE_PASSWORD_FILE=/run/secrets/db_password - API_KEY_FILE=/run/secrets/api_key secrets: - db_password - api_key deploy: resources: limits: memory: 512M cpus: '0.5'

secrets: db_password: file: ./secrets/db_password.txt api_key: file: ./secrets/api_key.txt `

Implementation Examples

Example 1: Secure Database Connection Script

`bash #!/bin/bash

secure_db_connect.sh

set -euo pipefail

Configuration

SECRETS_DIR="/etc/myapp/secrets" DB_HOST="${DB_HOST:-localhost}" DB_PORT="${DB_PORT:-5432}" DB_NAME="${DB_NAME:-myapp}"

Function to read secret from file

read_secret() { local secret_file="$1" if [ ! -f "$secret_file" ]; then echo "Error: Secret file $secret_file not found" >&2 exit 1 fi # Verify file permissions local perms=$(stat -c "%a" "$secret_file") if [ "$perms" != "600" ] && [ "$perms" != "400" ]; then echo "Warning: Secret file $secret_file has insecure permissions: $perms" >&2 fi cat "$secret_file" }

Load database credentials

DB_USER=$(read_secret "$SECRETS_DIR/db_user") DB_PASSWORD=$(read_secret "$SECRETS_DIR/db_password")

Construct connection string

export DATABASE_URL="postgresql://${DB_USER}:${DB_PASSWORD}@${DB_HOST}:${DB_PORT}/${DB_NAME}"

Clear sensitive variables from memory when possible

unset DB_USER DB_PASSWORD

Execute application

exec "$@" `

Example 2: Systemd Service with Encrypted Environment

`bash #!/bin/bash

setup_secure_service.sh

set -euo pipefail

SERVICE_NAME="myapp" SERVICE_USER="myapp" SECRETS_DIR="/etc/${SERVICE_NAME}/secrets"

Create service user

if ! id "$SERVICE_USER" &>/dev/null; then sudo useradd -r -s /bin/false -d "/var/lib/$SERVICE_NAME" "$SERVICE_USER" fi

Create directories

sudo mkdir -p "$SECRETS_DIR" sudo mkdir -p "/var/lib/$SERVICE_NAME" sudo mkdir -p "/var/log/$SERVICE_NAME"

Set permissions

sudo chown root:root "$SECRETS_DIR" sudo chmod 700 "$SECRETS_DIR" sudo chown "$SERVICE_USER:$SERVICE_USER" "/var/lib/$SERVICE_NAME" sudo chown "$SERVICE_USER:$SERVICE_USER" "/var/log/$SERVICE_NAME"

Create encrypted environment file

sudo tee "$SECRETS_DIR/environment.enc" << 'EOF'

This would be encrypted content

DATABASE_PASSWORD=encrypted_password_here API_KEY=encrypted_api_key_here EOF

Create decryption script

sudo tee "/usr/local/bin/${SERVICE_NAME}-decrypt" << 'EOF' #!/bin/bash set -euo pipefail

SECRETS_DIR="/etc/myapp/secrets" TEMP_ENV=$(mktemp)

cleanup() { [ -f "$TEMP_ENV" ] && shred -vfz -n 3 "$TEMP_ENV" } trap cleanup EXIT

Decrypt environment file (implement your decryption method)

decrypt_file "$SECRETS_DIR/environment.enc" > "$TEMP_ENV"

Load environment

set -a source "$TEMP_ENV" set +a

Execute application

exec "$@" EOF

sudo chmod 755 "/usr/local/bin/${SERVICE_NAME}-decrypt" `

Example 3: Kubernetes Secret Management

`yaml

kubernetes-secret-example.yaml

apiVersion: v1 kind: Secret metadata: name: myapp-secrets namespace: production type: Opaque stringData: database-password: "super_secret_password" api-key: "abcd1234567890" --- apiVersion: apps/v1 kind: Deployment metadata: name: myapp namespace: production spec: replicas: 3 selector: matchLabels: app: myapp template: metadata: labels: app: myapp spec: securityContext: runAsNonRoot: true runAsUser: 1001 fsGroup: 1001 containers: - name: myapp image: myapp:latest env: - name: DATABASE_PASSWORD valueFrom: secretKeyRef: name: myapp-secrets key: database-password - name: API_KEY valueFrom: secretKeyRef: name: myapp-secrets key: api-key securityContext: allowPrivilegeEscalation: false readOnlyRootFilesystem: true capabilities: drop: - ALL resources: limits: memory: "512Mi" cpu: "500m" requests: memory: "256Mi" cpu: "250m" `

Advanced Security Techniques

Hardware Security Module (HSM) Integration

`bash #!/bin/bash

hsm_integration.sh

set -euo pipefail

PKCS#11 HSM integration example

PKCS11_MODULE="/usr/lib/x86_64-linux-gnu/pkcs11/libsofthsm2.so" HSM_SLOT="0" HSM_PIN="1234"

Initialize HSM token

softhsm2-util --init-token --slot "$HSM_SLOT" --label "MyAppToken" --pin "$HSM_PIN" --so-pin "5678"

Function to encrypt data using HSM

hsm_encrypt() { local plaintext="$1" local key_label="$2" echo "$plaintext" | pkcs11-tool --module "$PKCS11_MODULE" \ --slot "$HSM_SLOT" --pin "$HSM_PIN" \ --encrypt --mechanism RSA-PKCS \ --input-format binary --output-format base64 }

Function to decrypt data using HSM

hsm_decrypt() { local ciphertext="$1" local key_label="$2" echo "$ciphertext" | base64 -d | pkcs11-tool --module "$PKCS11_MODULE" \ --slot "$HSM_SLOT" --pin "$HSM_PIN" \ --decrypt --mechanism RSA-PKCS \ --input-format binary } `

SELinux Integration

`bash

SELinux policy for secure environment variables

myapp.te

policy_module(myapp, 1.0.0)

require { type unconfined_t; type admin_home_t; class file { read write create }; }

Define custom type for application

type myapp_t; type myapp_exec_t; init_daemon_domain(myapp_t, myapp_exec_t)

Define type for secret files

type myapp_secret_t; files_type(myapp_secret_t)

Allow application to read secret files

allow myapp_t myapp_secret_t:file read_file_perms;

Prevent other processes from accessing secrets

neverallow { domain -myapp_t } myapp_secret_t:file *; `

AppArmor Profile

`bash

/etc/apparmor.d/usr.local.bin.myapp

#include

/usr/local/bin/myapp { #include # Application binary /usr/local/bin/myapp mr, # Allow reading configuration /etc/myapp/config r, # Allow reading secrets with specific permissions /etc/myapp/secrets/* r, # Application data directory /var/lib/myapp/ rw, # Temporary files /tmp/myapp-* rw, # Network access network inet tcp, # Deny access to sensitive system files deny /etc/shadow r, deny /etc/passwd w, deny /proc/*/environ r, # Deny capability to change user deny capability setuid, deny capability setgid, } `

Monitoring and Auditing

Audit Configuration

`bash

/etc/audit/rules.d/sensitive-env.rules

Monitor access to sensitive environment files

-w /etc/myapp/secrets/ -p ra -k sensitive_env_access

Monitor environment variable access through /proc

-a always,exit -F arch=b64 -S open,openat -F dir=/proc -F success=1 -k proc_env_access

Monitor systemd environment file access

-w /etc/systemd/system/ -p wa -k systemd_config_change

Monitor GPG operations

-w /usr/bin/gpg -p x -k gpg_execution -w /usr/bin/gpg2 -p x -k gpg_execution `

Monitoring Script

`bash #!/bin/bash

env_security_monitor.sh

set -euo pipefail

LOG_FILE="/var/log/env-security.log" ALERT_EMAIL="admin@example.com"

Function to log security events

log_event() { local level="$1" local message="$2" local timestamp=$(date '+%Y-%m-%d %H:%M:%S') echo "[$timestamp] [$level] $message" >> "$LOG_FILE" if [ "$level" = "CRITICAL" ]; then echo "$message" | mail -s "Security Alert: Environment Variable Access" "$ALERT_EMAIL" fi }

Check for suspicious process environment access

check_proc_access() { local suspicious_processes=$(lsof +D /proc 2>/dev/null | grep environ | wc -l) if [ "$suspicious_processes" -gt 0 ]; then log_event "WARNING" "Detected $suspicious_processes processes accessing /proc/*/environ" fi }

Check file permissions on secret files

check_secret_permissions() { local secrets_dir="/etc/myapp/secrets" if [ -d "$secrets_dir" ]; then find "$secrets_dir" -type f ! -perm 600 ! -perm 400 | while read -r file; do local perms=$(stat -c "%a" "$file") log_event "CRITICAL" "Insecure permissions $perms on secret file: $file" done fi }

Check for environment variables in process lists

check_process_env_exposure() { if ps auxe | grep -q "PASSWORD\|SECRET\|KEY" 2>/dev/null; then log_event "CRITICAL" "Sensitive environment variables detected in process list" fi }

Main monitoring loop

main() { while true; do check_proc_access check_secret_permissions check_process_env_exposure sleep 60 done }

Run monitoring

main `

Log Analysis Commands

`bash

Search audit logs for environment variable access

sudo ausearch -k sensitive_env_access -ts today

Check for failed access attempts

sudo ausearch -m AVC -ts today | grep environ

Monitor real-time audit events

sudo ausearch -k sensitive_env_access -i --start now

Generate audit report

sudo aureport -x --summary -i

Search system logs for environment variable exposure

sudo journalctl --since="1 hour ago" | grep -i "password\|secret\|key" `

Common Mistakes and Pitfalls

Security Anti-patterns

| Anti-pattern | Risk Level | Better Alternative | |--------------|------------|-------------------| | Hardcoded secrets in scripts | Critical | External secret files | | World-readable secret files | Critical | Restrictive permissions (600) | | Secrets in version control | Critical | Git hooks and .gitignore | | Plain text environment files | High | Encrypted storage | | Overprivileged service accounts | High | Principle of least privilege | | No audit logging | Medium | Comprehensive monitoring |

Common Configuration Errors

`bash

WRONG: World-readable secret file

chmod 644 /etc/myapp/secrets.env

CORRECT: Restrictive permissions

chmod 600 /etc/myapp/secrets.env chown myapp:myapp /etc/myapp/secrets.env

WRONG: Exposing secrets in command line

myapp --password=secret123

CORRECT: Using environment variables or files

export APP_PASSWORD_FILE=/etc/myapp/password myapp

WRONG: Logging environment variables

env | logger

CORRECT: Selective logging

echo "Application starting with user: $USER" | logger `

Troubleshooting Common Issues

`bash

Debug permission issues

namei -l /etc/myapp/secrets/password

Check file ownership and permissions

ls -la /etc/myapp/secrets/

Verify group membership

groups myapp

Test file accessibility

sudo -u myapp cat /etc/myapp/secrets/password

Check SELinux context

ls -Z /etc/myapp/secrets/

Verify systemd service environment loading

systemctl show myapp.service | grep Environment `

Conclusion

Securing sensitive environment variables in Linux requires a multi-layered approach combining proper file permissions, encryption, access controls, and monitoring. The choice of security method should align with your threat model, compliance requirements, and operational complexity.

Key takeaways for implementing secure environment variable management:

1. Never store secrets in plain text in easily accessible locations 2. Use restrictive file permissions (600 or 400) for secret files 3. Implement proper access controls using users, groups, and system policies 4. Encrypt sensitive data at rest using GPG, Vault, or similar tools 5. Monitor and audit access to sensitive environment data 6. Use specialized tools like HashiCorp Vault for enterprise environments 7. Follow the principle of least privilege for all service accounts 8. Regularly rotate sensitive credentials and keys 9. Implement proper cleanup of temporary files containing secrets 10. Test your security measures regularly through audits and penetration testing

By implementing these practices and continuously monitoring your environment, you can significantly reduce the risk of sensitive data exposure while maintaining operational efficiency. Remember that security is an ongoing process that requires regular review and updates as threats and technologies evolve.

Tags

  • Linux
  • encryption
  • environment-variables
  • security
  • system-administration

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Securing Sensitive Environment Variables in Linux Systems