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I installed NinjaFirewall plugin on WP.org v4.7.2. It locked me out of my site.

They asked for the log from my hosting company's cPanel, which I got and they report:

You have a cookie that contains the NULL byte character (0x00). That’s very odd and unusual.

You can log in to WordPress using another browser (or delete your cookies first), then go to “NinjaFirewall > Firewall Policies > Various > Block ASCII character 0x00 (NULL byte)” and select “No”. Then, scroll down to the bottom of the page and click “Save Firewall Policies”.

This is all new ground for me. Does anyone know if there are any issues I should be aware of?

  • Do you know which cookie was affected (wordpress sets quite a lot of different cookies)? – tim Mar 3 '17 at 11:53
  • I'm afraid not. NinjaFirewall helped earlier: *The cookie is related to “themify” (I’m not familiar with this product) . The NULL byte character is usually blocked by Web Application Firewalls when it is found inside a user input (GET, POST, cookie, user-agent etc) because there aren’t a lof of reasons to use it, except in a few cases such as in binary files etc. Here are some more info: (resources.infosecinstitute.com/null-byte-injection-php). I'll ask Themify now. – James Mar 3 '17 at 11:59
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Does anyone know if there are any issues I should be aware of?

The main issue is that the scripting language (for example PHP) that is parsing the cookie may be implemented in a language (for example C) that treats the NULL byte character (0x00) as a special control character.

For example, in the C language a "string" of length N is implemented as an array of N+1 characters, where the (N+1)th character at the end of the array is 0x00, which is used to indicate the end of the string.

The general security principle being violated here is the unexpected mixing of instructions (the instruction-like control character 0x00) with data. Cookies are generally thought of as strings of data, but because the null byte has a special control property in C (it indicates the end of strings and can cause string parsing routines to return) it functions somewhat more like an instruction than data.

Here's a specific example of how the null byte could cause problems: It might be possible to upload a file called "backdoor.php\x00.png" that contains PHP backdoor code. Even if php files are blacklisted from being uploaded, this file might still be uploaded since the extension is ".png." If the PHP filename parsing (which is implemented in C) improperly returns at the null byte character the file might be saved with the name "backdoor.php" and could then be executed by accessing the file in the uploads folder. This type of exploit is called "Null Byte Injection."

I'm not aware of a specific exploit based on a null character in a cookie, but the idea is similar to above and would exploit the special properties of the null byte in the implementation language of the code parsing the cookie.

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