What is Session Fixation and What are the Ways to Prevent It?

Quick Summary: Session fixation involves using a predefined identifier to log in to a web application. In this article, you can get a complete understanding of session fixation and how the corresponding attack takes place. You can also find ways to prevent session fixation attacks. Keep reading to get all the information.

Sessions are useful for keeping track of user’s interactions with the server. They track information like user preferences, login status, and data entered into forms. In this process, session IDs are assigned to users to identify them. It is quite a useful process to ensure a secure exchange of information between the browser and server.

However, attackers can exploit flaws in session management and launch a session fixation attack. It happens when an attacker induces a user into authenticating with a legitimate web application with a predetermined session ID.

With the flaws in the session mechanism, the web application will trust a user with an existing session ID. Consequently, the attacker can use the same URL to gain unauthorized access to a user’s account, which is known as a session fixation attack.

Keep reading to know more about it, how it occurs, and what you can do for session fixation protection. Let’s dive into the details.

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What is Session Fixation?

Session fixation is a kind of cyberattack in which a user is tricked into signing in with a predefined session ID or token. This happens when an attacker has a session ID and induces a user to log in to the application using this existing session ID.

Let’s understand it with an example. Suppose an attacker has a session ID like “1234567890” and crafts a URL using this ID like:

http://bankwebapp.com/?sid=1234567890

Now, the attacker can use social engineering techniques such as phishing to induce a user to open the link and log in to the bank application. Since the attacker already knows the session ID, they can hijack the session or use the same link to gain unauthorized access.

So, in a session fixation attack, a user is forced to log in with a specific session ID that can be later used to gain unauthorized access. The following image illustrates the attack more accurately.

How Session Fixation Affects Web App Security?

Session fixation attack has enough potential to deteriorate the effectiveness of your web application. Let’s check out how this attack affects web app security.

1. Unauthorized Access

If attackers are aware of session IDs, they are capable of obtaining access to user's account or confidential data without authorization. This becomes riskier if the session ID is not regenerated upon login.

2. Account Takeover

The attacker can obtain control over the user’s session, which can lead to a full account takeover. This can allow attacks to leak sensitive data, perform their nasty actions, impersonate users, and alter settings of accounts.

3. Data Theft

If confidential data like personal details, financial data, or proprietary business information is accessible during the session, the attacker can easily steal that data to fulfill his purpose.

4. Privilege Escalation

If the user has privileged access to confidential resources, the attacker can obtain access to the sensitive areas of the system, stealing users’ credentials and causing further security compromise and notable damage.

5. Regulatory Compliance Issues

Depending on the nature of the data involved, session fixation can lead to non-compliance with data protection regulations. This can cause businesses to bear legal consequences and huge penalties.

6. Session Hijacking

If an attacker creates a session ID and the user logs in with it, the attacker can gain control or hijack the session and execute actions on behalf of an authenticated user.

How Web Applications Become Vulnerable to Session Fixation Attacks?

Let’s check out the key factors that cause web applications to experience session fixation attacks.

1. Static Session IDs

If the application uses a session ID that remains unchanged throughout the session, attackers can misuse it by creating a session ID before the legitimate user logs in.

2. Inappropriate Session Management

Failure to recreate session IDs after authentication or during sensitive operations enables attackers to use a fixed session ID.

3. Insecure Cookie Handling

Using less secured cookies or not setting proper attributes (like HttpOnly and Secure flags) can cause exposure of session IDs to attackers which they can manipulate to succeed in their aim.

4. Lack of Input Validation

Improper input validation of session ID values provided by users can enable attackers to inject and manipulate session IDs.

5. Session Fixation in Login Flaws

If users can pass a session ID through URL parameters or other methods during the login process, it can lead to session fixation vulnerabilities.

Techniques Used by an Attacker to Launch a Session Fixation Attack

Attackers use a wide range of techniques to launch a successful session fixation attack. Mostly, this happens due to flaws in the session mechanism. The attacker will exploit the vulnerabilities in this mechanism to deceive users and web applications to establish a connection after login. Let’s see how the attacker does it with the tactics below.

Crafted URL

In this attack method, the attacker will create a URL with the predefined session ID. Next, this URL is sent to a victim user through techniques like phishing emails or social media posts. After this, when the victim user opens the link and provides credentials, login starts with the given session ID. The attacker can also access the account by using the same link.

Hidden Form Field

In this method, an attacker can leverage a fake form with a hidden field that contains a session identifier. The hidden field will contain a value that will match the expected session identifier. After this, when a user is tricked into submitting the form, the user will log in with the predefined session ID. The attacker might host the web form at an evil website or send it via email.

Using Meta Tag

Meta tag contains additional information about a web page. However, an attacker can also use it to launch a session fixation attack by setting the “Set-Cookie” parameter in an HTML meta tag like http-equiv=”Set-Cookie.” After this, the attacker can manipulate the cookie and take control of a user’s session. The bad part is that it works even if JavaScript is disabled.

Using HTTP Response Header

In this method, an attacker takes advantage of the server response to set the session ID in the web browser of a victim user. The attacker will tamper with the HTTP header’s “Set-Cookie” parameter. It works if a web application is vulnerable to HTTP header injection. With this, the attacker will inject a predefined cookie into the header. As a result, the attack can gain unauthorized access to a user’s account.

Cross-Site Scripting

Today, almost every browser can execute client-side scripts. An attacker can take advantage of a cross-site scripting (XSS) attack to launch a fixed session attack. The attacker can insert a malicious script into the URL sent to a user and set the session ID with its cookie. The browser that executes the script can fix the cookie value by using the document.cookie function.

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How to Assess Session Fixation Vulnerabilities?

Let’s find out viable ways to assess potential session fixation vulnerabilities and ensure robust security for web applications.

1. Test Session ID Regeneration

Ensure whether the session ID changes after user authentication and sensitive actions or not. Optimize automated tools or manual testing to check if the session ID remains the same before and after login.

2. Examine Session ID Management

Constantly monitor how session IDs are transmitted (e.g., through cookies, URLs, or headers). Verify that session IDs are not exposed or predictable to attackers.

3. Review Session Management Code

Analyze the application’s session management implementation for proper session ID management, regeneration mechanisms, and secure storage practices.

4. Perform Penetration Testing

Conduct manual or automated penetration testing to imitate session fixation attacks and capture potential vulnerabilities in a controlled environment.

5. Check Cookie Security Attributes

Verify if the cookies are set with apt security attributes (e.g., HttpOnly, Secure) to avoid unauthorized access or manipulation.

How to Prevent Session Fixation Attack?

Bad coding practices and insufficient security measures are the main reasons for the session fixation vulnerability. You need to combine different strategies for session management to make it secure. The following are session fixation mitigation techniques to minimize the threat.

1. Use Best Practices

By following the best practices to develop secure web apps, you can minimize most of the risks. It involves using a robust session management mechanism and many other ways that you can implement at the development stage.

• Validate Session Identifier: A common security weakness is not validating the session identifier. When this flaw is present in a web application, it can be exploited by an attacker by giving a random string of a specific format as a session identifier. Avoid such a case by implementing a proper session validation mechanism. You can allow session IDs only if they are generated by the server.
• Change Session ID: Another situation that makes a web application vulnerable occurs when a session ID is assigned before login, and it remains the same after login. It will allow an attacker to hijack the session. The safe way is by changing the session ID. Make sure that the session identifier is changed after login. You can implement a way to test if the user-agent or IP address has changed and then modify the session identifier accordingly.
• Check for Timeout: Attackers can take advantage of a session token that doesn’t expire after a certain period. Consequently, an attacker can use this token to trick a victim user into authenticating in a web app with an existing session token. However, you can avoid a potential security issue by regenerating the session token after a certain period using the timeout method. For example, if the user session expires after 10 minutes, the existing token should be invalidated, and the user must be assigned a new one.
• Use Session Cookies: When session IDs are received via HTTP headers or HTTP requests, they might be intercepted by an attacker. You should use the session identifiers to manage sessions. Using HTTPS, appropriate session flags like SameSite or Secure, and anti-CSRF tokens also help avoid session fixation.

2. Regular Vulnerability Scanning

Vulnerability scanning is quite useful in preventing various cyberattacks like SQL injection, cross-site scripting, and session fixation. It is a way to identify, prioritize, and mitigate potential security weaknesses. Vulnerabilities are a big issue in cybersecurity. Attackers can exploit vulnerabilities to launch attacks like session fixation.

You can mitigate security risks with a comprehensive vulnerability assessment. It will enable you to identify weak spots within your applications and APIs. By identifying and detecting vulnerabilities, you can ensure a robust security posture.

How Can Session Fixation Be Dangerous?

Session identifiers play a crucial role in web session management. Without these identifiers, life would be harder because users would have to log in to applications more frequently. But thanks to these identifiers, the session remains active without any problem, and a user doesn’t have to log in more often.

But like a password, a session identifier can also pose security challenges when leaked or stolen. An attacker can gain unauthorized access to a user’s account with this identifier. Plus, the attacker can cause privilege escalation or attempt more attacks. The attacker may also steal confidential data.

Session fixation is one of the critical security risks for web applications that can have a drastic impact. The impact is not limited to the targeted users, but it can also affect organizations. An attacker can even take control of the server by gaining access to an account with administrative privileges. After this, the attack can disrupt services or damage an organization’s reputation.

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How Can ZeroThreat Mitigate the Session Fixation Risk?

While following the best coding practices can help you mitigate the risk of session fixation, a strong security posture is also important to prevent such threats. You need to identify and resolve session fixation vulnerabilities to ensure a robust security posture.

Finding vulnerabilities requires a feature-rich DAST tool like ZeroThreat. It can thoroughly analyze your web application and discover a myriad of vulnerabilities, such as OWASP Top 10. You can scan your web application to detect vulnerabilities that can cause session fixation risk and fix them to avoid it.

ZeroThreat is an advanced vulnerability scanning tool that can identify hard-to-detect vulnerabilities like out-of-band and zero-day exploits. It offers zero false positives, 5x faster scanning, and many other benefits.