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An Example of how Cybersecurity actors can Compromise Shell Scripting code

Cybersecurity attackers can compromise shell scripting code by exploiting command injection, race conditions, improper file permissions, and environment variable manipulation. Below is an example of how an attacker can compromise a shell script using command injection.​​

Example: Command Injection in a Vulnerable Shell Script

Vulnerable Script (backup.sh):

   #!/bin/bash

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   echo "Enter the directory to back up:"
    read user_input

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    # Execute the tar command to back up the specified directory
    tar -czf backup.tar.gz $user_input

How an Attack Works

Step 1: Malicious Input

An attacker can enter the following input:

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    /home/user; rm -rf /

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The script will execute:

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     tar -czf backup.tar.gz /home/user; rm -rf /

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Since ; allows multiple commands to run in sequence, the rm -rf / command wipes the entire filesystem, destroying the system.

More Ways Shell Scripts Can Be Exploited

1. Race Condition Exploits (Symlink Attack)

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     ln -s /etc/passwd /tmp/tempfile

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  • If a script writes to /tmp/tempfile, it could overwrite /etc/passwd, leading to privilege escalation.

 

2. Environment Variable Manipulation

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​​     export PATH="/malicious_dir:$PATH"

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  • If a script executes ls or rm without an absolute path (/bin/ls), a malicious binary could be executed instead.

 

3. Weak File Permissions

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​     chmod 777 /etc/sensitive_script.sh

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  • Allows attackers to modify scripts and insert backdoors.

Mitigation Strategies

1. Validate User Input to Prevent Command Injection

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        if [[ "$user_input" =~ ^/home/[a-zA-Z0-9/_-]+$ ]]; then tar -czf backup.tar.gz "$user_input" else echo "Invalid directory." exit 1 fi

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2. Use Absolute Paths for Commands

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        /bin/tar -czf backup.tar.gz "$user_input"

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3. Secure File Permissions

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​        chmod 700 backup.sh

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4. Use Temporary Files Securely

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       TEMP_FILE=$(mktemp)

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