SSL: A little-known security leak

In January, I wrote an article named "SSL: Really broken this time", which mentioned that if the certificate issuing Authority uses the MD5 Algorithm for digital signature, the certificate can be forged. Of course, this vulnerability can be easily corrected as long as the Certificate Authority uses SHA-1 instead of the MD5 algorithm. Even so, many people still think that SHA-1 will be followed by MD5 soon.

SSLsniff

Actually, cracking SHA-1 is only a matter of time. Maybe you still remember Moxie Marlinspike, who once developed a very advanced hacker tool SSLsniff. This tool detects vulnerabilities in Internet Explorer and can launch MitM even when using SSL ). Microsoft finally fixed the vulnerability.

However, even if the vulnerability is fixed, SSLsniff is still a powerful tool. For example, SSLsniff can be used to demonstrate MitM attacks in the password cracking organization (for example, the MD5 cracking organization I mentioned above.

SSLstrip

The upgraded hacker tool developed by Moxie Marlinspike is called SSLstrip. The purpose of this tool is to allow malicious attackers to obtain sensitive user information under any encryption conditions. Moxie Marlinspike noticed that most users do not require HTTPS when accessing the Internet. Even some websites that are sensitive to information do not use encrypted pages, such as websites in, therefore, he decided to bypass the encryption process.

The original homepage is not encrypted. HTTPS is enabled only after the user inputs some information, as shown in:

SSLstrip is a MitM Protocol. In this example, it captures a vulnerability between a user and a bank website server. Next, let's see how this vulnerability was exploited by SSLstrip:

1. First, I enter the http://www.usbank.com in the browser to enter the home page of the bank website

2. Enter your username and press Enter.

3. SSLstrip captures the bank website URL and my username

4. SSLstrip connects to the bank website server and submits the user name

5. SSLstrip then returns the new page returned by the Bank Web server to the browser

6. enter your password on the new page.

7. SSLstrip again obtains my password and submits it to the bank website server. At this time, the bank website server considers that I have logged on.

8. SSLstrip again submits the new page returned by the bank website to the browser. In my opinion, I have logged on normally and can perform the next step.

Maybe you will ask, why is "s" not added after HTTP, and the bank website links are not all in the form of HTTPS? In this example, there is no "s" because the SSL connection is established between the attacked computer and the bank website server. I only returned all the correct pages to the browser through SSLstrip, but this process does not have a secure tunnel. So who has obtained my password can imagine it?

I have noticed that some users with strong vigilance and good observation may find such tricks, but most users may be unaware of them. For more details about this vulnerability, refer to Moxie Marlinspike's New Tricks for Defeating SSL in Practice (PDF) presentation slides at Black Hat ). The entire process is explained in detail.

Additional features

In Moxie Marlinspike's slides, he also pointed out several other features of SSLstrip, which can make insecure websites look safer, thus deceiving users. Because most websites Add a website icon in front of their website domain names, and SSLstrip utilizes this feature, attackers can create their own icons to display them in front of insecure websites, for example, the following icon.

A lock represents a safe object. If a lock icon is added to the front of the website, most users will think that the website they visit is safe.

If someone asks, attackers may even provide a real SSL connection, but the URL is only slightly different from what users should access. The difference is that there are several more characters at the end of the URL. Moxie Marlinspike explains this in the slides:

Upgrade your browser

We humans have some inherent habits that no one can refute. Therefore, I can only honestly say that not every time I discover the traps set by hackers. Fortunately, this issue has been discussed. Previously, Jason Hiner, chief editor of TechRepublic, sent me a George Ou article about HTTPS web page hijacking from theory to reality.

In this article, developers are advised to give Web browsers more intelligence, so that the browser can know under what circumstances the SSL encryption protocol should be used. If SSL is not used, disconnect from the Web server. George also said that Google has developed this issue in its Chrome 2.o browser. We hope other browser developers can take this into consideration.

Summary

First, I would like to thank Black Hat for allowing me to use their Logo and Moxie Marlinspike for allowing me to use his demo slides as part of the article. In addition, he hoped that the efforts made by the majority of Internet users to understand SSL attacks would also be admirable.

In addition, I have noticed that the attack mentioned in this Article happens only when the user does not care. Fortunately, after my reminder, many people have noticed the nuances on the URL, which may prevent hackers from falling into the trap. If you feel alert, do you have noticed the HTTPS mark before the URL when you visited the website using SSL last time? At least I did not.