Understanding And Preventing Command Injection Vulnerabilities In Shell Scripts

The Risks of Unchecked User Input in Shell Scripts

Shell scripts can execute system commands via user input without proper validation. This allows attackers to inject additional malicious commands giving them unauthorized system access. Potential damages include sensitive data theft, malware installation, and container escape.

Shell scripts use interpretation to execute commands from user input. The script execution environment parses these commands vulnerably. Attackers exploit this by appending crafted input with additional harmful commands. Without input sanitization and output encoding, the script interpreter blindly executes everythingtogether as one system command.

Command injection permits unauthorized access for attackers to steal passwords, encryption keys, personal data, and intellectual property. Sensitive data like customer information, healthcare records, and financial data faces exposure and theft. Attackers can also utilize discovered system vulnerabilities to distribute malware payloads through infected command execution.

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On container platforms, command injection gives adversaries a path to escape sandboxes. Containers rely on process isolation mechanisms that get bypassed. This leads to compromise of host machines and pivoting to wider container environments, clouds, and on-premise networks.

Detecting Command Injection Vulnerabilities

Static and dynamic analysis uncover command injection flaws in shell scripts. Static analysis scans source code for improper user input handling based on patterns. Dynamic analysis observes real-time behavior via input fuzzing to trigger vulnerabilities.

The following source code samples illustrate dangerous practices that enable command injection:

# Vulnerable echo command with unquoted user input
echo $USERINPUT

# Unsafe use of eval() executing arbitrary commands   
USERINPUT="ls; malicious_code" 
eval $USERINPUT

Tools like shellcheck, RATS, flawfinder, and Brakeman scan for these types of patterns. Fuzzers like wfuzz manipulate input combinations to detect command injection. Combining static and dynamic methods provides optimal assurance.

Safe Coding Practices for Shell Scripts

Safe shell script coding entails proper user input handling, avoiding risky functions, and correctly quoting/escaping arguments:

Avoid evaluation functions like eval(), exec(), system()
Use whitelists over blacklists for allowed input characters/patterns
Enclose arguments in single or double quotes to avoid special interpretation
Escape special characters like semicolons that can allow command chaining

Compare the following secure and insecure code examples:

# UNSAFE
NAME=$1
echo "Hello $NAME"

# SAFE 
NAME=$1
echo "Hello ${NAME@Q}" # Quotes special characters

# UNSAFE
FILES=$(ls $USERINPUT) 

# SAFE
REGEX="[A-Za-z/_]{1,20}"
if [[ "$USERINPUT" =~ $REGEX ]]; then
  FILES=$(ls $USERINPUT)
fi

Validating User Input in Shell Scripts

Input validation prevents command injection by sanitizing, whitelisting, and encoding user input:

Blacklisting – Strip/replace characters like semicolons
Whitelisting – Only permit approved input formats
Encoding – Escape special characters

Implementation options include custom validations, input pattern matching, native shell facilities, and libraries:

# Custom validation
CLEANSED=$(sed 's/[^a-zA-Z]//g' <<< "$INPUT")

# Pattern matching  
if [[ "$INPUT" =~ ^[0-9]{1,5}$ ]]; then
   LENGTH=$INPUT
fi

# Native shell capabilities  
LENGTH=${#INPUT}
QUOTED_INPUT="${INPUT@Q}"

# Validation libraries
import validator

v = Validator()
v.check($INPUT, validators=[RegexValidator(regex='^[\w.@-]*$')])

Process Sandboxing and Containerization

Sandboxing isolates processes under restricted permissions and resources to reduce command injection impacts. Containerization depends on similar mechanisms for separation.

System-level virtualization containers like Docker constrain processes. This limits the scope of compromise from container escape. Sandboxing tools like Firejail use security policies to enforce:

Filesystem, networking, user access restrictions
Read-only mounts
Seccomp syscall filters

By jailing processes to minimal required resources, threats have reduced lateral movement. Any command injection gets properly contained. Using tools like Firejail and Docker sandbox interpreters running vulnerable shell scripts:

# Firejail sandbox
$ firejail --profile=python my_script.py
 
# Docker containerization 
$ docker run -it --rm my_script bash

Additional Layers of Protection

Supplementary controls strengthen defense against command injection:

Script auditing and linting - Identifies bad practices requiring remediation
Restrictive permissions - Limits execution contexts vulnerable to exploitation
Application firewalls - Block follow-on exploitation like reverse shells
Intrusion detection - Signals post-compromise malicious activities

Direct exploitation resistance combines with detective controls after footholds establish. This provides defense-in-depth mitigating command injection risks in shell scripts:

# Script validation
$ shellcheck my_script.sh

# Removed setuid permissions  
$ chmod 0755 my_script.sh 

# Web application firewall blocks payloads
INCOMING PAYLOADS: Reverse Shell Blocked

# Network intrusion detection
ALERT: Attempted Data Exfiltration Detected