On a Wiki where HTML markup is disallowed, a malicious user might introduce HTML markup all the same using Table Of Contents Plugin. I have checked only with harmless markup (text formatting), but I presume arbitrary HTML code might be rendered, including malicious javascript.

Here is an example (as long as the engine running jspwiki.org will allow it :o):

<I>This heading might be italics but the engine forbids HTML markup so it is not#

<I>This text might be italics but it is not

<SUP>This heading might be superscript, but it is not#

<SUP>This text might be superscript, but it is not

And this is normal text, unnaffected

However if we put a table of contents [{TableOfContents }], the HTML markups in the headings are retained in the TOC text:

And this text appears as italics and superscript.

N.B.: I edited Wiki Markup Development because HTML tags in headings were rendered in its TOC, making the whole page unreadable.

Fixed in 2.4.31.