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Reject overlong UTF-8 forms in _Mbrtowc and add test case #6060

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Reject overlong UTF-8 forms in _Mbrtowc and add test case #6060

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1a1f521
Reject overlong forms and out-of-range code points in _Mbrtowc
SiliconA-Z Dec 6, 2025
8105e46
Add test case
cpplearner Jan 31, 2026
8cb140a
Don't need to reject out-of-range values
cpplearner Jan 31, 2026
cb99567
Merge branch 'main' into utf-8
StephanTLavavej Jan 31, 2026
f6883d6
Add test cases.
StephanTLavavej Jan 31, 2026
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5 changes: 5 additions & 0 deletions stl/src/xmbtowc.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -126,6 +126,11 @@ _MRTIMP2 _Success_(return >= 0) int __cdecl _Mbrtowc(
}
}

if ((consumedCount == 2 && wch < 0x80u) || (consumedCount == 3 && wch < 0x800u)) { // overlong forms
errno = EILSEQ;
return -1;
}

if (wch >= 0xD800u && wch <= 0xDFFFu) { // tried to decode unpaired surrogate
errno = EILSEQ;
return -1;
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13 changes: 12 additions & 1 deletion tests/std/tests/VSO_0644691_utf_8_codecvt/test.cpp
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Expand Up @@ -32,7 +32,7 @@ const encoding_test_char utf_test_cases[] = {
{"\xED\x9F\xBF", L"\uD7FF"}, // U+D7FF unencoded from the Hangul Jamo Extended-B block,
// last character before UTF-16 illegal region
{"\xED\xA0\x80", nullptr}, // U+D800 beginning of surrogate range
{nullptr, L"\xD800"},
{nullptr, L"\xD800"}, //
{"\xED\xBF\xBF", nullptr}, // U+DFFF end of surrogate range
{nullptr, L"\xDFFF"},
{"\xEE\x80\x80", L"\uE000"}, // U+E000 unencoded from the private use area, first codepoint after surrogate range
Expand All @@ -48,6 +48,17 @@ const encoding_test_char utf_test_cases[] = {
// ^^^ TRANSITION, VSO-653059 ^^^
{"\xC2\x61", nullptr}, // Wrong number of trailing bytes
{"\xE0\xA0\x61", nullptr},

{"\xC0\x80", nullptr}, // overlong form: 2 bytes for U+0000 NULL
{"\xC0\x81", nullptr}, // overlong form: 2 bytes for U+0001 START OF HEADING

{"\xC1\xBF", nullptr}, // overlong form: 2 bytes for U+007F DELETE
{"\xE0\x81\xBF", nullptr}, // overlong form: 3 bytes for U+007F DELETE
{"\xF0\x80\x81\xBF", nullptr}, // overlong form: 4 bytes for U+007F DELETE

{"\xD0\xAF", L"\u042F"}, // U+042F CYRILLIC CAPITAL LETTER YA
{"\xE0\x90\xAF", nullptr}, // overlong form: 3 bytes for U+042F CYRILLIC CAPITAL LETTER YA
{"\xF0\x80\x90\xAF", nullptr}, // overlong form: 4 bytes for U+042F CYRILLIC CAPITAL LETTER YA
};

void assert_empty_file(const wchar_t* const fileName) {
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