Session fixation vs. session hijacking attacks: Prevention and the main differences

Session fixation and session hijacking are two major threats that exploit vulnerabilities in web application session management. These attacks allow cybercriminals to take over user sessions, potentially gaining unauthorized access to sensitive information. Since session identifiers (IDs) serve as the key to maintaining user authentication, they become a prime target for attackers. In this article, we'll break down how session hijacking and session fixation work, highlight their key differences, explore other session-based threats, and discuss best practices to defend against them.

What is session hijacking?

Session hijacking is a type of attack where hackers take control of an active user session by stealing and exploiting the session ID. The session ID is a unique token that identifies the user and maintains state across requests, often stored in cookies, passed in URLs, or embedded in hidden form fields. In session hijacking attacks, once the attacker obtains the session ID, they can access the user’s account without needing credentials, allowing them to read sensitive data, make unauthorized changes, or escalate privileges. Timing is critical in these attacks, as session IDs are only valid for a limited period.

How session hijacking works

Session hijacking exploits weak points in how web sessions are managed. A typical session hijacking works like this:

1. A user logs in, and the server assigns a session ID, usually stored in a cookie or HTTP header.
2. An attacker intercepts or guesses the session ID using methods like packet sniffing, cross-site scripting (XSS), or malware.
3. With the stolen ID, the attacker creates requests that look legitimate and bypasses the login process entirely.
4. Now, acting as the user, they can steal data, change settings, or escalate privileges. This step is especially dangerous in business environments.
5. Session IDs can be stolen through unsecured Wi-Fi, infected endpoints, exposed query strings, or insecure web apps vulnerable to cross-site scripting. Even systems using HTTPS aren't immune if the session management is sloppy. That's why effective session hijacking prevention solutions are key to securing web applications.

Real-world examples of session hijacking

Session hijacking attacks have been used in high-profile breaches. One early example was Firesheep, an extension for the Firefox browser released in 2010, which allowed anyone on the same network to hijack sessions of users logged into sites like Facebook or Twitter over HTTP.

More recently, attackers have targeted internal business apps by injecting session-stealing scripts into vulnerable web portals. That led to a full account takeover, access to sensitive internal systems, and data breaches.

What is session fixation?

Session fixation is a type of attack where the attacker sets the session identifier before the victim logs in. When the user authenticates with the same session ID, the attacker can reuse it to access the session without needing credentials. This exploit takes advantage of poor session management practices, such as not regenerating session IDs after login.

How session fixation works

A session fixation attack typically follows this process:

1. The attacker generates or obtains a valid session identifier from the target application (usually from a login or pre-login page).
2. They get the victim to use the same session ID. The specific technique depends on how the application handles session IDs. It could be via a link with the ID embedded or a fake site that passes it through.
3. The victim logs in using that session ID. If the application doesn't regenerate the session ID after login, the attacker now shares access to the authenticated session.
4. With that user's session ID, the attacker can interact with the app as if they were logged in themselves.

Session fixation attacks rely on weak session management — specifically, accepting session IDs from untrusted sources (like URLs or form data) and failing to issue new IDs after login. If a system lets one user set or reuse another's session ID, it's vulnerable.

Real-world example of session fixation

A session fixation vulnerability was discovered in Schneider Electric's EcoStruxure™ Power Monitoring Expert (PME). In this case, the system allowed a session ID to be set in advance via the login URL. An attacker could send a specially crafted link containing a predefined session ID to a victim. If the victim logged in using that link, the attacker could then use the same session ID to access the authenticated session — effectively hijacking it without needing to steal credentials or intercept tokens. This attack highlighted how improper session handling can lead to serious security breaches, even in industrial and enterprise environments.

Session hijacking vs. session fixation: The main differences

Both session fixation and session hijacking take advantage of improper session management and have a similar goal: gaining access to a web server session ID. However, they differ in the way that attackers achieve this end goal.

In a session hijacking attack, the attacker waits for the user to log in and then steals the session ID to slip into the existing session unnoticed. In a session fixation attack, the attacker tricks the user into using a predetermined session ID.

Let’s see how session hijacking and session fixation compare side by side:

Other session-based attack types

Beyond session fixation and hijacking, several related session attacks exist. While not always identical, they often overlap in risk and impact.

• Session predictions. The attacker guesses or predicts valid session identifiers based on weak generation algorithms. This can be surprisingly effective if session tokens follow a pattern or are not randomized properly.
• Session replay. In this attack, the attacker captures a valid session request and replays it later to impersonate a user. It often overlaps with hijacking, especially in API-based applications.
• Session spoofing. Here, an attacker manually crafts session data or headers to impersonate a session, typically when session validation is weak or token structure is predictable.

These techniques are often chained with session hijacking or fixation to gain access, escalate privileges, or maintain persistence. If your session handling is weak, attackers will find a way in.

Risks of session-based attacks for businesses

Session-based attacks are a serious threat because they target one of the core mechanisms nearly all web applications rely on: session management. The fallout can affect everything from customer trust to regulatory standing.

Direct risks

These are the consequences when an attacker gains control of a session:

• Data breach. Hijacked sessions can expose customer data, financial records, or internal documents.
• Account takeover. The risk of account takeover is especially dangerous in admin or privileged user accounts.
• Financial theft. A session hijack in e-commerce or banking platforms can lead to unauthorized transactions.

Indirect and long-term risks

Even after the attack is over, the damage often continues with:

• Legal compliance violations: Under GDPR, PCI DSS, and other regulations, failure to secure session data can trigger fines or audits.
• Reputational damage: Customers lose trust quickly when unauthorized access or data leaks are reported.
• Incident response costs: Time, resources, and recovery operations after an attack can be significant.

How to protect against session hijacking and fixation attacks

Most session-based attacks come down to poor session management. The fixes aren't complicated, but they need to be implemented consistently.

Here's how to secure the sessions of your web application:

1. Regenerate session IDs after login. Always create a new session ID once a user logs in. This invalidates any pre-authentication tokens and neutralizes session fixation.
2. Use HTTPS. Encrypt all traffic using HTTPS, ideally with HSTS enforced. Without it, session IDs can be intercepted in plaintext.
3. Use long, random session IDs. Generate random session tokens with enough entropy to prevent guessing or brute-force attacks.
4. Enforce strict session ID expiration and rotation. Short expiration times and inactivity timeouts limit how long a stolen session ID is useful. Regular token rotation closes the window even further.
5. Monitor for anomalies. Track unusual session behaviors — like simultaneous logins from different IPs — and respond automatically (such as change the session ID or request re-authentication).
6. Harden your code against XSS. Most session hijacking begins with script injection. Sanitize inputs, use CSP headers, and audit third-party scripts.
7. Avoid embedding session IDs in URLs. Use session cookies or secure headers to pass session data. Never expose tokens in URLs or redirect parameters.
8. Educate users. Help users spot phishing attempts and avoid clicking suspicious links, especially in environments with shared access (such as public computers or libraries).

Even with all the right precautions, session-based attacks can slip through. That's why security monitoring and automation matter.

NordStellar’s session hijacking prevention solution proactively scans the deep and dark web for stolen session cookies linked to an organization’s employees and customers. When a compromised session cookie is detected, the platform immediately alerts the organization with details such as the source, device, and other stolen information. To prevent attackers from exploiting stolen sessions, NordStellar enforces security measures that block unauthorized transactions, impersonation attempts, and other account fraud, ensuring seamless protection without disrupting legitimate user activity.