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User PerfektesChaos from dewiki has wrote a example lua module, which produces a fatal in the php preprocessor.

The module is:

-- Cause a server crash.
-- includes/parser/Preprocessor_DOM.php line 1692:
--         "Call to a member function item() on a non-object"
-- Crashes any 1.24wmf1 wiki, not only beta.wmflabs.org
 
local p = { }
 
function p.f( frame )
    local story = "ö"                  -- non-ASCII char
    local sub   = story:sub( 1, 1 )    -- ASCII substring at half of UTF-8
                                       -- should have been mw.ustring.sub()
    return frame:callParserFunction( "#tag:nowiki", { sub } )
end -- p.f
 
return p


The error is:
includes/parser/Preprocessor_DOM.php line 1692: "Call to a member function item() on a non-object"

I have run it with warnings on on my dev machine and the output is:
Warning: DOMDocument::loadXML() [domdocument.loadxml]: Input is not proper UTF-8, indicate encoding ! Bytes: 0xC3 0x3C 0x2F 0x76 in Entity, line: 1 in \includes\parser\Preprocessor_DOM.php on line 85

Notice: Trying to get property of non-object in \includes\parser\Preprocessor_DOM.php on line 88

Fatal error: Call to a member function item() on a non-object in \includes\parser\Preprocessor_DOM.php on line 1692


See http://de.wikipedia.beta.wmflabs.org/wiki/MakeTheServerCrash for more information and a link to the example page.

Looks like the half byte is mangeled into the builded xml in the preprocessor and than it gives a invalid xml.