Posts being replaced with "Undefined"

[VirusZero]

After some extensive testing and going some searching through support archives, I've found that one of the major reasons why posts here would get replaced with the single word (Undefined) stems from whether a user has any dodgy extensions or toolbars installed.

If a user has any toolbars (like Wibiya or Attracta) these can conflict with MyBB's own natural Javascript codes and cause anything entered to be replaced while also disabling the edit button (since it uses javascript to offer options between quick edit and standard edit).

Certain extensions can also cause problems ("Unfriend notifier for facebook" and Freecorder 6 are common problem extension. Though they aren't the only ones.)

The only solution here is to remove those toolbars/extensions.

If you haven't installed any toolbars or extensions willingly/knowingly, then this could also be the result of a malware infection that has installed something on it's own.
(Check to see if any new extensions or plugins have been installed. Especially ones you don't recognise. Also check installed programs on your computer to see if any malware may have been installed and is affecting your browser's normal operation.)

As a reminder, Endless Fight encourages all users to:
- Keep antivirus software running and up to date. (There are several free programs which have fairly good protection. Avira, Avast and AVG.)
- Keep your browser up to date. (Out of date browsers have flaws that malware can use to inject itself.)
- Install all security updates/patches to your operating system. (Malware can easily exploit holes in security to install itself without your consent.)
- Avoid installing any toolbars (even ones offered through trusted and legitimate programs like Flash).

Spoiler:

In this case here, we can safely rule out Javascript issues since the version of javascript here hasn't changed since February and the only modification I've made is replaced

Code:

$(code)

to

Code:

jQuery(code)

to stop conflicts. (And even then this was done over a month ago.)
And during this time, no issues with posting have been reported.

Other users aren't reporting any issues and I am unable to replicate it on my own system. The database is reporting the data entered as "Undefined" which means that it's being intercepted and altered before being saved to the database.

This issue also occurs on multiple sites (with a known good copy of this site also being unable to be used).