expat: buffer over-read and crash on XML with malformed UTF-8 sequences

The updatePosition function in lib/xmltokimpl.c in libexpat in Expat 2.0.1, as used in Python, PyXML, w3c-libwww, and other software, allows context-dependent attackers to cause a denial of service application crash via an XML document with crafted UTF-8 sequences that trigger a buffer over-read,...