Bash command injection

Nash N Sulthan
Published on
01 Apr 2024
6 min read

The technique of “command injection” involves exploiting weaknesses within a vulnerable application to execute arbitrary commands, thereby compromising its security. Bash command injection attack differs from code injection in its approach and objectives.

Bash Command Injection is a type of security vulnerability that occurs when an attacker can inject malicious code into a system’s command execution process by manipulating input that gets passed to a shell (usually the Bash shell) as part of a command.

This type of vulnerability is particularly prevalent in web applications and software that interact with the underlying operating system.

In command injection, the attacker extends the default functionality of the application, which executes system commands, without the necessity of injecting code host operating system.

In other words, command injection is an injection technique by which a set of arbitrary commands on the host operating system through a vulnerable application to attack the system.

This type of attack is possible when a web application passes unsafe user-supplied data to a system shell.

This includes forms, cookies, HTTP headers etc. In this attack, the operating system commands sent by the attacker are executed with the execution privileges of the vulnerable application. This type of attack is largely due to insufficient input validation.

The server-side scripts allow bash to run in the background. This allows the vulnerability to be exploited remotely over the network which may lead to severe damage to the server.

How does bash command injection occur?

Here’s how Bash command injection occurs:

1. User input handling

The vulnerable application takes user input (e.g., in forms, query parameters) and incorporates it into a shell command that will be executed by the operating system.

2. Lack of validation

The application fails to properly validate or sanitize the user input, allowing the attacker to inject additional commands or characters.

3. Command injection

The attacker inputs specially crafted strings that include additional commands, separators, or characters that the shell will interpret as commands.

4. Command execution

The application unwittingly executes the injected malicious commands in the context of the operating system, potentially giving the attacker unauthorized access.

What are the impacts of Bash command injection?

Bash Command Injection vulnerabilities can have severe consequences for the security and functionality of a system.

When successfully exploited, these vulnerabilities can lead to a wide range of negative impacts. Here are some of the key impacts of Bash Command Injection:

1. Unauthorized access

Attackers can execute arbitrary commands on the target system, potentially gaining unauthorized access with the privileges of the user or application executing the command.

2. Data breach

Attackers can access and exfiltrate sensitive data, including user credentials, personal information, financial records, and confidential files.

3. System compromise

Successful command injection can lead to complete control over the target system, allowing attackers to modify configurations, install malware, and compromise the integrity of the system.

4. Malware deployment

Attackers can use command injection to deploy malicious software, backdoors, or other types of malwares on the compromised system, further extending their control.

5. Data manipulation

Attackers can manipulate, modify, or delete data on the system, leading to data corruption, data loss, or inaccurate information.

6. Denial of Service (DoS)

Attackers can execute resource-intensive commands that overload the system’s resources, causing slowdowns, crashes, or complete unavailability of services.

7. Vertical and lateral movement

Once inside the system, attackers can use command injection to escalate privileges, pivot to other systems within the network, and potentially compromise the entire infrastructure.

Data breaches resulting from command injection attacks can lead to legal liabilities, regulatory violations, and potential legal actions.

9. Financial losses

Organizations can incur financial losses due to remediation costs, legal fees, regulatory fines, loss of business, and customer compensation.

10. Third-party impact

If the compromised system interacts with third-party systems, the attack could extend to those systems, affecting partners and customers.

How can you prevent Bash command injection?

Preventing and mitigating Bash Command Injection vulnerabilities requires a combination of secure coding practices, input validation, and defense mechanisms.

Here’s how to prevent and mitigate Bash Command Injection:

1. Input validation and sanitization

Validate and sanitize all user inputs and data coming from external sources before using them in command construction.

Use whitelists of allowed characters or formats and reject inputs that don’t conform.

2. Avoid string concatenation

Do not directly concatenate user inputs to form shell commands.

Use prepared statements, parameterized queries, or libraries that handle user input securely.

3. Use safe APIs

Whenever possible, use APIs and libraries that interact with external systems without resorting to direct command execution.

4. Escape user inputs

If you need to include user input in commands, escape the input to neutralize any special characters.

Different programming languages provide functions for this purpose.

5. Input encoding

If you need to include user input in URLs or other contexts, ensure proper encoding to prevent interpretation as commands.

6. Least privilege principle

Limit the permissions of the user or service executing commands to the minimum required for the task.

7. Regular security testing

Conduct regular security assessments, including code reviews and penetration testing, to identify and address vulnerabilities.

8. Security training

Educate your development team about the risks of command injection and secure coding practices.

9. Web Application Firewalls (WAFs)

Deploy a WAF to detect and block malicious inputs that could lead to command injection.

Configure the WAF to enforce rules that identify suspicious command-like patterns.

10. Intrusion Detection/Prevention Systems (IDS/IPS)

Implement IDS/IPS solutions that can detect and block command injection attempts in real-time.

By combining these preventive measures and proactive security practices, you can significantly reduce the risk of Bash Command Injection vulnerabilities and protect your applications and systems from potential exploitation.

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Written by
Nash N Sulthan
Nash N Sulthan
Cyber Security Lead Engineer
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