Asked  7 Months ago    Answers:  5   Viewed   30 times

How do you prevent multiple clients from using the same session ID? I'm asking this because I want to add an extra layer of security to prevent session hijacking on my website. If a hacker somehow figures out another user's session ID and makes requests with that SID, how can I detect that there are different clients sharing a single SID on the server and then reject the hijack attempt?


I have accepted Gumbo's answer after careful consideration because I've come to the realization that what I'm asking for is impossible due to the restrictions of a stateless HTTP protocol. I forgot about what is perhaps the most fundamental principle of HTTP, and now that I think about this question seems a bit trivial.

Let me elaborate what I mean:

After User A logs in on, he is given some random session ID, for simplicity's sake, let it be 'abc123'. This session ID is stored as a cookie on the client side and is validated with a server-side session to ensure the user who logged in remains logged in as he moves from one webpage to another. This cookie of course would not need to exist if HTTP were not stateless. For that reason, if User B steals User A's SID, and creates a cookie on his computer with the value 'abc123', he would have successfully hijacked User A's session, but there is simply no way for the server to legitimately recognize that User B's request is any different from User A's requests, and therefore the server has no reason to reject any request. Even if we were to list the sessions that were already active on the server and try to see if someone is accessing a session that is already active, how can we determine that it is another user who is accessing the session illegitimately and not the same user who is already logged in with a session ID, but simply trying to make another request with it (ie navigate to a different webpage). We can't. Checking the user agent? Can be spoofed - but good as a Defense in Depth measure nevertheless. IP Address? Can change for legitimate reasons - but instead of not checking for the IP address at all, I suggest checking something like the first two octets of the IP, as even a user on a data plan network who constantly has a changing IP for perfectly legitimate reasons would only usually have the last two octets of their IP change.

In consclusion, it is the stateless HTTP that condemns us to never being able to fully protect our websites from session hijacking, but good practices (like the ones Gumbo has provided) will be good enough to prevent a good majority of session attacks. Trying to protect sessions from hijacking by denying multiple requests of the same SID is therefore simply ludicrous, and would defeat the whole purpose of sessions.



Unfortunately, there is no effective way to unmistakably identify a request that originates from an attacker in opposite to a genuine request. Because most properties that counter measures check like the IP address or user agent characteristics are either not reliable (IP address might change among multiple requests) or can be forged easily (e. g. User-Agent request header) and thus can yield unwanted false positives (i. e. genuine user switched IP address) or false negatives (i. e. attacker was able to successfully forge request with same User-Agent).

That’s why the best method to prevent session hijacking is to make sure an attacker cannot find out another user’s session ID. This means you should design your application and its session management that (1) an attacker cannot guess a valid session ID by using enough entropy, and (2) that there is no other way for an attacker to obtain a valid session ID by known attacks/vulerabilities like sniffing the network communication, Cross-Site Scripting, leakage through Referer, etc.

That said, you should:

  • use enough random input for generating the session ID (see session.entropy_file, session.entropy_length, and session.hash_function)
  • use HTTPS to protect the session ID during transmission
  • store the session ID in a cookie and not in the URL to avoid leakage though Referer (see session.use_only_cookies)
  • set the cookie with the HttpOnly and Secure attributes to forbid access via JavaScript (in case of XSS vulnerabilities) and to forbid transmission via insecure channel (see session.cookie_httponly and session.cookie_secure)

Besides that, you should also regenerate the session ID while invalidating the old one (see session_regenerate_id function) after certain session state changes (e. g. confirmation of authenticity after login or change of authorization/privileges) and you can additionally do this periodically to reduce the time span for a successful session hijacking attack.

Wednesday, March 31, 2021
answered 7 Months ago

All session variables in PHP are stored server side. The client stores a cookie that references which session should be used, and then the server looks up the values for the session. It is safe to store is_logged_in in your session as well as the user id.

What you should be aware of is if another user gets a hold of another user's session cookie, they will be able to imitate that user until the session times out. One simple solution is to link sessions to IPs.

Saturday, May 29, 2021
answered 5 Months ago

Read OWASP A3-Broken Authentication and Session Management. Also read about OWASP A5-CSRF, which is sometimes called "session riding".

You should use this code in a php header file:


This code prevents session fixation. It also helps protect against xss from access document.cookie which is one way that Session Hijacking can occur. Enforcing HTTPS only cookies is a good way of addressing OWASP A9-Insufficient Transport Layer Protection. This way of using HTTPS is sometimes called "secure cookies", which is a terrible name for it. Also STS is a very cool security feature, but not all browsers support it (yet).

Thursday, June 17, 2021
answered 5 Months ago

Session security risks come from three different possibilities:

  • Prediction
  • Capture
  • Fixation

Prediction would mean that someone that's not the user for whom the session was created guessed their session ID. The chances of that happening are almost 0, although they do grow as more users use the site simultaneously.

With your code, you would make that risk even lower because it would only work if the attacker shared the user agent and the ip of the predicted session. But the difference is trivial in this case.

Fixation would mean that an attacker can create a session and then force another user into using their session. In this case it would depend: If the attacker knows that you are doing it and they fake the user agent and ip of the client, they could fixate the session. Or if they share ip and user agent.

And finally we have session hijacking, probably the most common method of the three. In this case an attacker would somehow gain access to the session id of a valid logged in user, and then use it to log in to their account. As with the previous method, this would only work for them if they know that you are checking the ip and user agent, and faked the same ones as the user. The technique you are using is not unique, and some attackers might fake them just in case.

That being said, is it secure? Yes and no

If you are obsessed with security, the answer is always the same: Use SSL

Unless your code is open source, almost anything you do that changes the behavior of the php sessions will be secure enough.

The only exception to that would be really popular sites that will attract the attention of hackers.

There is some very good documentation on this topic available:

  • PHP Session Security
Wednesday, August 11, 2021
answered 3 Months ago

Check out this snippet from the Session Overview documentation:

System.Web.SessionState.HttpSessionState.SessionID values are sent in clear text, whether as a cookie or as part of the URL. A malicious user could get access to the session of another user by obtaining the SessionID value and including it in requests to the server. If you are storing sensitive information in session state, it is recommended that you use SSL to encrypt any communication between the browser and server that includes the SessionID value.

It seems to me that this means that there is no security baked into Session, so you probably shouldn't use Session as a security measure. Instead, I would recommend relying on ASP.NET security (authentication, authorization).

Here is an article from the Patterns & Practices group on general security recommendations for ASP.NET.

Thursday, August 12, 2021
The Coding Wombat
answered 3 Months ago
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