What is Shellshock vulnerability?

Neda Ali
Reviewed by
Abey Koshy Itty
Published on
08 Nov 2023
8 min read

Shellshock, also known as the Bash bug, is a critical vulnerability in the Bash shell.

It affects all operating systems (Linux and Unix based), which allows an attacker to execute arbitrary commands on a vulnerable system by sending specially crafted environment variables to a Bash-based application.

It is a severe threat to the system and causes extreme data breaches.

History of Shellshock vulnerability

Shellshock or Bash vulnerability was discovered in 2014. Upon its discovery, Shellshock posed a serious threat to the security of many systems worldwide.

It affected Bash versions 1.0.3 through 4.3. Patches and updates were quickly released by operating system vendors and software developers to address the vulnerability.

But many systems were still vulnerable at the time of the disclosure.

As a result, the Shellshock vulnerability was widely exploited by attackers, and it is estimated that millions of systems were compromised.

How can Shellshock vulnerability be exploited?

Shellshock can be exploited in the following ways:

  1. An attacker can send a specially crafted environment variable to a Bash-based application. This environment variable will contain a function definition that will be executed by Bash (Bash is a Command language interpreter). The function definition can contain any arbitrary command, which will be executed with the privileges of the user running the Bash-based application.

  2. An attacker can create a malicious web page that contains a specially crafted environment variable. When a user visits the web page, the environment variable will be sent to the web server, which will then execute the arbitrary command.

  3. An attacker can send a specially crafted email message that contains a specially crafted environment variable. When the user opens the email message, the environment variable will be sent to the email client, which will then execute the arbitrary command.


GET http://example.beaglesecurity.com/cgi-bin/beaglesecurity.cgi HTTP/1.1
User-Agent: AppFabs
Host: shellshock.appfabs.com
Referer: () { :;}; echo "NS:" $(</etc/passwd)

What is the impact of Shellshock vulnerability on affected systems?

Some of the key impacts of the Shellshock vulnerability include:

1. Remote code execution

Shellshock allowed attackers to execute arbitrary commands on targeted systems remotely.

By exploiting the vulnerability, attackers could send specially crafted HTTP requests, emails, or other forms of input to a vulnerable system and gain unauthorized access. This could lead to the execution of malicious code, unauthorized data access, and potential system compromise.

2. System compromise

Once an attacker gained access to a vulnerable system, they could potentially take control of it.

This could result in various malicious activities, such as installing backdoors, malware, or other forms of malicious software.

Attackers could also increase their privileges and gain administrative control over the compromised system.

3. Information disclosure

Shellshock could be used to extract sensitive information from vulnerable systems. By executing commands, attackers could access files, databases, configuration files, and other resources containing sensitive data.

This could lead to the exposure of personal information, financial data, or intellectual property.

4. Exploitation in networks

Since Bash is commonly used in network devices like routers, switches, and firewalls, the Shellshock vulnerability posed a risk to network infrastructure. Attackers could exploit the vulnerability to gain unauthorized access to network devices, bypass security measures, and potentially compromise the entire network.

5. Service disruption

In some cases, attackers could use Shellshock to disrupt services or launch denial-of-service (DoS) attacks.

By executing malicious commands, they could overwhelm system resources, leading to service degradation or complete unavailability.

6. Widespread impact

The Shellshock vulnerability affected a wide range of systems, including Linux servers, Mac computers, and various embedded devices.

Its widespread nature made it a significant concern, as a large number of systems were potentially at risk.

To mitigate the impact of Shellshock, it was crucial for affected organizations to promptly apply security patches and updates to their systems, ensuring they were running non-vulnerable versions of Bash.

Additionally, network and system administrators needed to monitor their environments for any signs of exploitation and implement appropriate security measures to detect and prevent unauthorized access.

What precautions should be taken to mitigate the Shellshock vulnerability?

To mitigate the Shellshock vulnerability and reduce the risk of exploitation, the following precautions and actions can be taken:

1. Apply patches and updates

It is crucial to apply the latest security patches and updates provided by operating system vendors and software developers.

These patches address the Shellshock vulnerability and help protect systems from potential exploitation. Regularly check for updates and ensure they are promptly applied.

2. Update bash version

Update the Bash shell to a non-vulnerable version. Check with your operating system vendor or software provider for the latest patched version of Bash and upgrade accordingly.

Older versions of Bash, especially those released prior to September 2014, are more likely to be vulnerable.

3. Monitor vendor notifications

Stay informed about security advisories and notifications from operating system vendors, software developers, and security organizations.

These notifications often provide information on vulnerabilities, patches, and recommended actions. Subscribe to relevant mailing lists or security alert services to receive timely updates.

4. Implement Intrusion Detection Systems (IDS) and Intrusion Prevention Systems (IPS)

IDS and IPS solutions can help detect and block attempted exploits targeting Shellshock.

These systems monitor network traffic and system activity, identifying suspicious behavior or exploit attempts. Configure IDS/IPS rules to detect and prevent Shellshock-related attacks.

5. Restrict access to bash

Limit access to the Bash shell, particularly in cases where it is not required.

Disable or restrict shell access for user accounts that do not need it, reducing the attack surface and potential avenues for exploitation.

6. Web server configuration

When running a web server, configure it to sanitize and validate user-supplied input.

Web applications and CGI scripts should be reviewed and modified to ensure they are not susceptible to Shellshock attacks. Regularly update and patch web server software.

7. Network device hardening

For network devices that use Bash or Bash-based tools, ensure they are running the latest patched versions.

Follow security best practices for network device hardening, including using strong passwords, disabling unnecessary services, and regularly updating firmware.

8. Security awareness and training

Educate users and system administrators about the Shellshock vulnerability and the importance of applying patches and updates.

Promote best practices such as not clicking on suspicious links or opening email attachments from unknown sources.

9. Regular security audits

Conduct regular security audits and vulnerability assessments to identify potential weaknesses in systems and applications.

Use automated scanning tools or engage third-party security professionals to assess the security posture of your infrastructure.

By implementing these precautions and mitigation measures, organizations can significantly reduce the risk of exploitation and protect their systems from the Shellshock vulnerability.

Is Shellshock still a problem?

The Shellshock vulnerability was discovered in 2014 and the necessary patches and updates were released to address the issue. Since then, there have been many years for organizations to apply the necessary fixes and protect their systems.

Therefore, it is expected that the prevalence of the Shellshock vulnerability has significantly decreased.

However, it’s important to note that the overall security landscape is constantly evolving, and new vulnerabilities and exploits can emerge over time.

While Shellshock vulnerability may not be as rampant as it was immediately after its discovery, it is still possible for some systems to remain unpatched or have outdated software versions. This could potentially leave them vulnerable to exploitation.

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Written by
Neda Ali
Neda Ali
Product Marketing Specialist
Abey Koshy Itty
Abey Koshy Itty
Marketing Manager
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