Description:
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Suppose htmlspecialchars() should check byte sequence more strictly for security reasons. An XSS exploit code has been unveiled.
http://d.hatena.ne.jp/t_komura/20091004/1254665511 [ja]

I wrote a primitive patch.
http://iwamot.com/misc/html.c.patch.20091006
I don't know whether it is useful though :)

Reproduce code:
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// overlong UTF-8 sequence
echo htmlspecialchars("A\xC0\xAF&",     ENT_QUOTES, 'UTF-8');
// invalid Shift_JIS sequence
echo htmlspecialchars("B\x80&",         ENT_QUOTES, 'Shift_JIS');
echo htmlspecialchars("C\x81\x7f&",     ENT_QUOTES, 'Shift_JIS');
// invalid EUC-JP sequence
echo htmlspecialchars("D\x80&",         ENT_QUOTES, 'EUC-JP');
echo htmlspecialchars("E\xA1\xFF&",     ENT_QUOTES, 'EUC-JP');
echo htmlspecialchars("F\x8E\xFF&",     ENT_QUOTES, 'EUC-JP');
echo htmlspecialchars("G\x8F\xA1\xFF&", ENT_QUOTES, 'EUC-JP');

Expected result:
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output nothing

Actual result:
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A_&B_&C&D_&E__&F__&G___&
("_" means an invalid byte)