You can invest in a great firewall, EDR, new AI detection, and a completely clean dashboard, but a bug can nevertheless run in the background, without any detection or notice. This could mean that someone is impersonating someone else or logging in as a legitimate user, without any alert.

This is what happens when you’re not aware of broken authentication or session management. It’s an interesting bug that’s common in web vulnerabilities, but if unnoticed, it is very critical, and the business risk is huge.

What does “broken authentication” really mean?

Authentication is the process of proving you are who you say you are. Broken authentication is when that process, or the logic around it, fails in a way that lets attackers assume someone else’s identity.

To be clear, it’s when a system lets an attacker impersonate a legitimate user without the attacker having to steal or crack the user’s password.

This can happen in different ways:

  • Weak or bypassable MFA like SMS-based OTP can be intercepted or logic flaws can skip MFA under certain conditions.
  • Predictable password reset links allow attackers to guess or brute force tokens to reset accounts. This happens a lot when a token is easy to guess or predictable.
  • Session fixation forces a victim to use a session ID that the attacker already knows.

You may think that this is not a big deal. Unlike a ransomware attack that is loud and disruptive, a broken authentication exploit can be totally silent. The attacker simply “becomes” a valid user, engaging in normal user behavior.

 

 

What about session management?

Authentication is very important, but session management is its equally critical twin. Even if you authenticate a user securely, your defense is bad if sessions are not handled properly afterward.

A session is how a system remembers that a user is logged in, often through a token or cookie. If that session is stolen, reused, or never properly invalidated, an attacker doesn’t need the user password at all.

Here are some issues to look out for:

  • Long-lived sessions—When a token remains valid for months, it give attackers a huge exploitation window.
  • Bad cryptography—When using the wrong protocol or when it’s not properly configured, you can craft session tokens.
  • Failure to invalidate on logout or password change—A compromised session stays active even after a user tries to secure their account.
  • Tokens are stored in insecure locations—If tokens are stored in the local storage of a browser, malicious scripts can easily read them.
  • Lack of rotation—Session tokens don’t change after privilege escalation or MFA completion.

Authentication is the lock, and the session management process is how you keep the door closed once inside.

 

The business risk

From a business perspective, broken authentication and session management can be devastating. The risks associated with these vulnerabilities are typically devastating leaks of sensitive data.

From a compliance and regulatory perspective, it can trigger GDPR or CCPA penalties because it concerns customers’ data. Also, the brand and trust in the company will be damaged. If it’s easy to go through all your systems and be logged in as any user without any detection, you’ll have a tough time coming back from that. Your PR team will be working overtime.

Don’t forget about downstream impact. Chaining these vulnerabilities can result in more advanced attacks. If you start by compromising a simple account, you may be able to then compromise an admin account and gain access to the internals of a company.

Let’s take a look at how hackers work to build a better defense.

 

From a hacker’s perspective

There are different ways that hackers test your authentication and session system:

  • Logic flaws in MFA—Hackers will attempt to skip the MFA step by manipulating the request, trying to use the token you had before being asked to enter the MFA, or simply brute forcing the MFA token if there are no rate-limiting protections.
  • Weak password reset processes—This means guessing tokens, trying to send the token to different emails, or maybe exploiting a token using a bad crypto protocol like the MD5 hash of the user email.
  • SSO and OAuth misconfigurations—Hackers attempt to find issues in the OAuth process or look for an open redirect to leak the user token.
  • Session fixation—This is where a user is tricked into logging in with a session already controlled by the attacker.

As you can see, there are multiple creative ways to defeat a system. To stay prepared, It’s important to be aware of all these possible attack paths.

How can we detect attacks?

Broken authentication and session management attacks are silent, but they’re still possible to find. Question the following:

  • Token invalidation—Are session tokens immediately revoked on logout, password change, or account deletion?
  • Session lifetime—Are high-privilege areas protected with shorter sessions?
  • MFA coverage—Does MFA protect all sensitive flows?
  • Session rotation—Are tokens regenerated after a privilege change?
  • Anomaly detection—Can we detect impossible travel, concurrent logins, or suspicious IP changes during a session?

For instance, if a user is connected in France and a few minutes later in Japan, that’s likely a red flag that indicates a bug is present. To reduce the risk of being compromised, you can enforce MFA by default. You can also monitor authentication logs for anomalies and respond in real time.

The best way to protect yourself is to go through your whole authentication process and ask, “What can go wrong at this step?” Be creative in your defense to better understand what flaws can happen to your system.

Broken authentication and session management flaws are not exotic but can be devastating. They bypass expensive perimeter defenses and deliver attackers directly to your most sensitive systems.

So ask yourself who is at your door; who have you already let inside without realizing it?

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