What Is SSL Sniffing and How to Avoid It?

SSL Sniffing

Since the Internet of Things becomes more and more secure, cybercriminals are forced to think of new methods of intercepting and stealing sensitive data. They usually take advantage of the existing cybersecurity tools and try to deceive users to trust them. Therefore, this article will help you find out what’s an SSL Sniffing attempt. It will also show you how to protect your SSL connection from it. But first, let’s define what’s a TLS/SSL termination proxy.

Table of Contents

  1. What Is a TLS/SSL Termination Proxy?
  2. What Is SSL Sniffing?
  3. How Does SSL Sniffing Work?
  4. How to Avoid SSL Sniffing?
  5. How to Prevent SSL Sniffing?

What Is a TLS/SSL Termination Proxy?

A TLS/SSL termination proxy is a proxy server that is particularly used by an entity to intercept and handle incoming TLS/SSL connections, decrypt the TLS/SSL, and then pass on the unencrypted request to one of its highly secure servers, such as Apache HTTP Server, Nginx or HAProxy.

The purpose of having a TLS/SSL termination proxy is to reduce the load on the entity’s main servers. But before implementing this proxy, the entity should ensure that its network is so secure that it doesn’t have to encrypt the session data for its users.

Unfortunately, cybercriminals have learned how to use the capabilities of a TLS/SSL termination proxy to sniff secure SSL connections. They can now easily intercept the messages exchanged within it.

What Is SSL Sniffing?

SSL sniffing is a malicious cyber-attack when a TLS/SSL termination proxy acts as a MitM proxy which hijacks the secure SSL connection

Why MitM (man-in-the-middle)? Because a proxy is by definition a man-in-the-middle third party. The proxy connects to the server, and then the client connects to the proxy.

How Does SSL Sniffing Work?

You may already know that SSL certificates prevent MitM attacks. How come MitM can intervene and sniff an SSL connection?

Sniffing SSL connection involves the MitM proxy acting as an untrusted Certificate Authority. The MitM proxy dynamically generates SSL Certificates to whatever hostname is needed for establishing the SSL connection. For example, if the browser wants to connect to https://www.mywebsite.com, the MitM proxy will generate an SSL Certificate for www.mywebsite.com and sign it as an independent CA.

By deliberately acting as a separate CA, the malicious MitM proxy takes advantage of this possibility and attempts to hijack a trusted SSL connection. So, when the browser tries to connect to the web server, the MitM proxy intervenes by issuing and signing its own certificate. Therefore, while the MitM proxy receives the original SSL Certificate of the server, the browser gets the fake SSL certificate issued by the MitM proxy.

But, here comes the crucial part of an SSL Sniffing attempt:

The browser thinks that it connects to the server, but it actually connects to the MitM proxy. Here comes the hiccup: for the sniffing to take place, the browser must accept this proxy as a genuine, trusted CA.

If the user ignores any security warnings and accepts the fake SSL Certificate and the MitM proxy CA as trusted, the user will successfully connect to the MitM proxy, initiate the SSL Sniffing and the SSL connection will become insecure.

How to Avoid SSL Sniffing?

Let’s suppose that the user didn’t install any malicious software. When their browser receives the fake SSL Certificate, the browser will display a security message warning users to not trust the issuer of the SSL Certificate and will ask whether to include the fake MitM proxy certificate in its list of trusted certificates. By rejecting the above request, the user stops the SSL Sniffing attempt, successfully protecting its secure SSL connection.

How to Prevent SSL Sniffing?

To prevent HTTPS sniffing and ensure the confidentiality and integrity of your SSL/TLS-protected communications, follow these guidelines:

  1. Use the latest TLS protocol versions. Ensure you use the latest TLS protocols (e.g., TLS 1.3) and the most recent, well-maintained TLS libraries or frameworks on both the server and client sides. These updates often include security enhancements and vulnerability fixes. With time, the older SSL/TLS releases could become a backdoor for an SSL sniffer to exploit. Don’t leave attackers any room for maneuvering and directing their SSL sniff attacks through outdated protocols.
  2. Implement Strict Transport Security (HSTS). HSTS is a security mechanism that instructs web browsers to always connect to a website over HTTPS, preventing any downgrade attacks. It helps protect against SSL stripping attacks. By enabling HSTS, you ensure that even if a user types “http://” instead of “https://,” the browser will automatically switch to a secure connection.
  3. Get Certs from a Trusted Certificate Authority. Obtain SSL/TLS certificates from reputable CAs. These third-party entities validate the identity of the server and issue trusted certificates, preventing attackers from using rogue or self-signed certificates to intercept traffic. Regularly check for certificate validity and expiration dates, as an expired certificate is another significant loophole hackers love to explore.
  4. Implement Certificate Pinning. Certificate pinning allows you to specify which certificates or public keys are trusted for a particular domain. This ensures that only the expected certificates or keys are accepted during the SSL/TLS handshake.

    Certificate pinning prevents such attacks by ensuring that the client verifies the server’s identity using only the specified certificate or key. If an attacker attempts to use a different certificate to sniff SSL traffic, the client will reject it, preventing the interception and decryption of the traffic.
  5. Secure Your Device With a Reliable Anti-Virus. Sadly, if attackers hacked the user’s computer and installed a virus, the browser may trust the MitM proxy as a valid CA. This process usually happens in the background and is invisible to the user. For this reason, it is crucial to secure your computer with reliable anti-virus software.

    If you have anti-virus software protecting your computer, then your web browsers will alert you about the untrusted Certificate Authority and its certificate. Please be careful and don’t trust any malicious MitM proxies. Always check the SSL certificate and its Certificate Authority in case of doubt. An SSL Sniffing attack is only possible because of users’ gullibility. For example, “I’ll just click OK to finally visit that website”.

Final Thoughts

Remember that the purpose of an SSL Certificate is to secure the connection between you and the web server. This is why you should not allow any third-party interventions. SSL Sniffing may only happen if you ignore the warnings or make your computer susceptible to viruses and malware. Make sure to avoid any of these occurrences.

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Experienced content writer specializing in SSL Certificates. Transforming intricate cybersecurity topics into clear, engaging content. Contribute to improving digital security through impactful narratives.