How to input a null character into a web form?

my suggestion would be to write the null character to an html element you would like it in ex:

    document.getElementByID("my_null_tag").innerHTML += "\0"

I was able to do this using TamperData Firefox plugin.

https://addons.mozilla.org/en-US/firefox/addon/tamper-data/

When given the Tamper Popup I typed "%00" in the Post Parameter Value field.

Still, I cannot find a way to type a null character just using the keyboard.

You can use an HTML entity. Not fully sure of how many zeroes are required but:

�

For an arbitrary Unicode character it's easier to use the hexadecimal notation. E.g., ㍝ prints ㍝ wich is U+335D.

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Update: This question is pretty tricky indeed. I've managed to insert a null character inside an HTML document (using a server-side script and verified with an hexadecimal editor). As expected, there is no difference with the HTML entity, which can be either � or �. But the browser does not send the character in the post request (tested with Firefox and Firebug): it sends %EF%BF%BD, which is REPLACEMENT CHARACTER' (U+FFFD). Exactly, it sends the interrogation mark in a box that's used to print the null in the document (given that null is not printable).

My guess is that your testers need to script the task.