diff --git a/stl/src/format.cpp b/stl/src/format.cpp
index 67a91356a9d..24033999edb 100644
--- a/stl/src/format.cpp
+++ b/stl/src/format.cpp
@@ -31,8 +31,15 @@ extern "C" [[nodiscard]] __std_win_error __stdcall __std_get_cvt(
             break;
         }
 
-        for (unsigned char _First = _Info.LeadByte[_Idx], _Last = _Info.LeadByte[_Idx + 1]; _First != _Last; ++_First) {
-            _Pcvt->_Isleadbyte[_First >> 3] |= 1u << (_First & 0b111u);
+        const unsigned int _First = _Info.LeadByte[_Idx];
+        const unsigned int _Last  = _Info.LeadByte[_Idx + 1];
+
+        if (_Last < _First) {
+            continue;
+        }
+
+        for (unsigned int _Byte = _First; _Byte <= _Last; ++_Byte) {
+            _Pcvt->_Isleadbyte[_Byte >> 3] |= 1u << (_Byte & 0b111u);
         }
     }
 
diff --git a/tests/tr1/tests/cwchar2/test.cpp b/tests/tr1/tests/cwchar2/test.cpp
index a8d8e43d749..5bfb4e72860 100644
--- a/tests/tr1/tests/cwchar2/test.cpp
+++ b/tests/tr1/tests/cwchar2/test.cpp
@@ -5,7 +5,9 @@
 #define TEST_NAME "<cwchar>, part 2"
 
 #include "tdefs.h"
+#include <clocale>
 #include <cwchar>
+#include <errno.h>
 #include <limits.h>
 #include <time.h>
 
@@ -50,6 +52,27 @@ void test_cpp() { // test C++ header
         CHECK_INT(STDx mbrtowc(&wc, abc + 3, 9, &mbst), 0);
         CHECK_INT(wc, L'\0');
 
+        { // UTF-8 overlong sequences should be rejected
+            const char overlong_nul[] = "\xC0\x80";
+            const char overlong_A[]   = "\xC1\x81";
+
+            if (std::setlocale(LC_CTYPE, ".UTF-8") != nullptr) {
+                STDx mbstate_t utf8_state{};
+                wchar_t wide_out;
+
+                errno = 0;
+                CHECK_INT(STDx mbrtowc(&wide_out, overlong_nul, sizeof(overlong_nul) - 1, &utf8_state), -1);
+                CHECK_INT(errno, EILSEQ);
+
+                utf8_state = STDx mbstate_t{};
+                errno      = 0;
+                CHECK_INT(STDx mbrtowc(&wide_out, overlong_A, sizeof(overlong_A) - 1, &utf8_state), -1);
+                CHECK_INT(errno, EILSEQ);
+
+                std::setlocale(LC_CTYPE, "C");
+            }
+        }
+
         CHECK(STDx mbsinit(&mbst) != 0);
         pc = &abc[0];
         CHECK_INT(STDx mbsrtowcs(wcs, &pc, 10, &mbst), 3);