Session Hijacking - How It Works and How to
Prevent It

A session hijacking attack is one of the more common ways in which malicious actors can commit fraud. It allows black hat hackers to completely bypass secure authentication mechanisms, including multi-factor authentication (MFA) and others. This, in turn, grants access to a user’s secured accounts and systems, which can give attackers free reign to steal sensitive data. These types of attacks pose a serious threat to cybersecurity, both on an individual and organizational scale. The ramifications can include extensive financial losses and long-term damage to an organization’s reputation.

You may not be able to prevent your organization from being targeted by session hijacking attacks, but there are steps you can take to recognize these attacks and stop them in their tracks. Keep reading to explore the hallmarks of session hijacking, the various ways it can be attempted, and the prevention methods you can deploy to protect your users and your business.

Session hijacking is when an attacker takes control of someone’s online activity by stealing or guessing the information that lets the website know the person is still logged in. Specifically, it takes advantage of how websites keep track of logged-in users by targeting the system that websites use to recognize and verify active user sessions.

Most servers need to manage multiple ongoing user sessions. This is typically handled by assigning a session token to each verified user. These can be either opaque tokens (e.g., session ID) or self-describing tokens (e.g., JSON web tokens). The web server sends the token to the client browser after the client has been authenticated. Through session hijacking, attackers can intercept, predict, or brute-force a legitimate session token in order to pose as the authenticated user and gain access to their secured data. In this way session hijacking often functions as a vector for account takeover (ATO) attacks and opens the door to fraud.

Session hijacking is particularly effective because it can be used to bypass multi-factor authentication (MFA). The stolen token allows the attacker to duplicate a target’s logged-in session on a separate browser, thus granting them access without needing to go through MFA and other security checkpoints. This attack vector can be particularly tricky for organizations that want to enable long-lasting sessions to keep users signed in longer and improve ease of use

There are many ways in which a malicious actor can carry out a successful session hijacking attack. Broadly speaking, they can be categorized as either an active or passive attack. Another way to think about session hijacking methods is stealing a valid session token vs. faking a valid session token. Stealing usually involves intercepting user traffic, while faking requires the attacker to spoof a valid token or force a client to use a predetermined token.

Passive Attacks

Passive attacks use various server and browser exploits to monitor client activity in order to gather data, including a session token.

Active Attacks

In an active attack, the cybercriminal might use phishing or malware to obtain a session token and even alter web page content. 

Let’s take a look at several specific methods that can be used to carry out a session hijacking attack.

Man-in-the-Middle (MITM) Attacks

Type: Passive Attack

Cybercriminals can use man-in-the-middle (MITM) attacks to intercept a session token. The attacker may use a compromised network device or unsafe Wi-Fi connection to place themself between the target and the web server. From there they monitor the target’s network traffic, waiting for them to log in to a secured account. This exposes the user’s authenticated session token; the attacker then steals it and gains access to the account on their own web browser.

There are multiple types of MITM attacks that malicious actors can use to execute session hijacking, including:

• IP Spoofing: The attacker uses modified Internet Protocol (IP) packets to pose as an authenticated client or server.
• Session Sidejacking: The attacker exploits a vulnerability found in a website’s SSL (Secure Sockets Layer) encryption to expose and steal session tokens.
• DNS Spoofing: The attacker exploits the caching process of a web server’s Domain Name System (DNS) by replacing the stored IP address of a trusted website with an address that leads to an unsafe server they control.

Adversary-in-the-Middle (AITM) Attacks

Type: Active Attack

Adversary-in-the-middle (AITM) attacks can be thought of as an evolution of MITM. While this attack initially takes the same approach to stealing session tokens – by eavesdropping on network traffic – the attacker then becomes active by manipulating the data being passed between user and server. This might include injecting malware into data the user receives, which can in turn grant them access to even more secure data.

Cross-Site Scripting (XSS)

Type: Active Attack

Cross-site scripting takes advantage of web application vulnerabilities to send malicious browser side scripts to unsuspecting users. The attacker can inject the script into a trusted website, which then relays that script to the user. The target’s browser sees the script as coming from a trusted source and so has no defenses against it. The malicious script then accesses the target’s session token and returns it to the attacker, who can use it to gain control of the target’s account. This type of attack is particularly prevalent, as it can be used on any web application that incorporates user input into its output data.

Session Fixation

Type: Active Attack

Rather than steal a client’s session token, session fixation gains access to an account by forcing the target to use a token that is already known to the attacker. This is done via a simple phishing scam. The cybercriminal sends the client a link to the target web server that already contains the chosen token (typically in the URL). When the client logs in using that link, their session token becomes the one chosen by the attacker. By visiting that same URL, the attacker now has access to the target’s account.

This can be done even if the web server uses server-generated session tokens. The attacker simply visits the web server to generate an ID, then sends the client a link containing that same session token in the URL. Once the client has logged in, the attacker gains access.

Cookie Theft

Type: Active Attack

Cookie theft uses phishing and malware to steal a client’s session token once they have logged into a secured web server.

Session Token Prediction

Type: Active Attack

In this type of session hijacking attack, the cybercriminal does not need to steal an authenticated session token. Instead, they gain access by predicting what a valid token will look like.

Web applications that generate session tokens pull from a specific set of data to create the token, sometimes using predictable information such as the client username or IP address. For example, a session token string might include a variable like “client501” that is altered sequentially for each new token generated. By monitoring this activity, the attacker can understand the session token structure and pair this information with machine learning to predict what a valid ID from the web server will look like (e.g., “client502” and so on). From there it’s only a matter of trying possible IDs until one is successful.

Session hijacking is a way for attackers to achieve complete account takeover. The consequences of this can be devastating, both for the user client and for an organization whose security has been compromised. In both cases, unauthorized access to sensitive information can lead to serious financial losses and stolen identity.

The ramifications for an organization extend even further. Not only can a session hijack attack significantly compromise overall data security, but the fallout of this for an organization’s clients and customers can seriously harm that organization’s reputation for years to come. Brand loyalty and trust can plummet, affected parties may bring legal cases against the organization, and the exposed cybersecurity vulnerabilities could lead to regulatory fines.

Recognizing failed attempts at session hijacking is crucial if an organization is going to prevent them. This can be difficult, since malicious actors will take great pains to be as discreet as possible. That being said, there are a number of telltale signs that cybersecurity professionals should be on the lookout for.

Watch out for unusual network activity. This might include authentic sessions being forced to logout, or those same sessions expiring unexpectedly. Logins at unusual times or from unusual locations, as well as multiple failed attempts by a user to login, usually coming from multiple unrecognized IP addresses, are also key indicators of an attempted session hijack. Monitor user activity so that you can recognize odd changes to behavioral patterns. You should also be on the lookout for suspicious activity in system logs, security monitoring tools, and session data.

Strong bot detection systems can also help identify and deter session hijacking attacks. Malicious actors often use bots in conjunction with a stolen session token to carry out their attacks.

Should you recognize attempts at session hijacking, it is imperative to take swift action. Mitigation methods will vary depending on when you detect session hijacking. Good general steps to take include:

• Terminate the affected session immediately. This should be followed by resetting session tokens to “close the breach.”
• Notify the user whose session was hijacked and advise them to change their password.
• Log out all active sessions associated with the compromised account.
• Notify relevant stakeholders, including IT security teams and management. Security IT should attempt to identify any security vulnerabilities that were exploited and close them to prevent future attacks.
• Contact relevant authorities and implement damage control measures. This might involve notifying all clients and users of the attempted breach and the type of data that may have been exposed.

Mitigation During Authentication

Early detection of a session hijacking attempt is preferable, but not always feasible. If you do manage to catch the attempt while the session is being created, here are some steps you can take:

• Conduct a thorough investigation to determine the extent of the breach. This should include an audit of potentially-compromised network devices.
• Implement secured transfer and data-storage methods (e.g., TLS, http-only secured cookies).
• Consider implementing more rigorous authentication methods, such as FIDO.

Mitigation After Authentication

It’s much more likely that you will detect a session hijacking attack after the malicious actor has already achieved account takeover and executed transactions in that context. In that case, additional mitigation steps should include:

• Implementing shorter authenticated user sessions (although this could negatively impact user experience)
• Increasing token security with encryption methods like Demonstrated Proof of Possession (DPoP) or mTLS

Catching an attempted session hijacking attack in the act is one thing, but it is also important to look at long-term strategies to strengthen security and mitigate future attempts. Organizations should practice regular cybersecurity audits to test the strength of their network protection and identify ways to fortify security. Make sure you are using the latest encryption, authentication methods, and secure web protocols. Consider implementing intrusion detection systems (IDS) or prevention systems (IPS) to help monitor network traffic and identify suspicious activity.

Prevention should include the human element as well. Educate clients and users on how to recognize phishing scams and suspicious web content. Organizations might also consider implementing security tokens: peripheral devices that supplement passwords to provide an additional layer of protection to secured networks.

Session hijacking is becoming more and more prevalent. When it comes to protecting your business from fraud, early detection is key. Ping Identity can help you recognize suspicious user activity in real time and isolate fraudulent sessions without causing friction for your trusted users.