Off-by-one error in the QUtf8Decoder::toUnicode function in Trolltech Qt 3 allows context-dependent attackers to cause a denial of service (crash) via a crafted Unicode string that triggers a heap-based buffer overflow. NOTE: Qt 4 has the same error in the QUtf8Codec::convertToUnicode function, but it is not exploitable.

The UTF-8 decoder in codecs/qutfcodec.cpp in Qt 3.3.8 and 4.2.3 does not reject long UTF-8 sequences as required by the standard, which allows remote attackers to conduct cross-site scripting (XSS) and directory traversal attacks via long sequences that decode to dangerous metacharacters.