diff --git a/scripts/build.sh b/scripts/build.sh
index d653f32..4e73892 100755
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -70,12 +70,13 @@ $TOOL $DBG_GCC $CPP_FLAGS -c $SRC/src/wrappers/datetime.cpp -o $OUTPUT/lib/$BUIL
 $TOOL $DBG_GCC $CPP_FLAGS -c $SRC/src/wrappers/regex.cpp -o $OUTPUT/lib/$BUILD/$MM/regex.o
 $TOOL $DBG_GCC $CPP_FLAGS -c $SRC/src/wrappers/thread.cpp -o $OUTPUT/lib/$BUILD/$MM/thread.o
 $TOOL $DBG_GCC $CPP_FLAGS -c $SRC/src/wrappers/http_linux.cpp -o $OUTPUT/lib/$BUILD/$MM/http_linux.o
+$TOOL $DBG_GCC $CPP_FLAGS -c $SRC/src/wrappers/memory.cpp -o $OUTPUT/lib/$BUILD/$MM/memory.o
 
 $BIN_PATH/$TOOL_NAME $DBG_OPTS $MM_OPT --emit=obj --export=none --nowarn --no-default-lib $SRC/src/lib.linux.ts $PIC -o $OUTPUT/lib/$BUILD/$MM/lib.linux.o
 
 # Build Lib
 $BIN_PATH/$TOOL_NAME $DBG_OPTS $MM_OPT --emit=obj --export=none --nowarn --no-default-lib $SRC/src/lib.ts $PIC -o $OUTPUT/lib/$BUILD/$MM/lib.o
-$ARC rcs $OUTPUT/lib/$BUILD/$MM/libTypeScriptDefaultLibCore.a $OUTPUT/lib/$BUILD/$MM/lib.o $OUTPUT/lib/$BUILD/$MM/lib.linux.o $OUTPUT/lib/$BUILD/$MM/io.o $OUTPUT/lib/$BUILD/$MM/datetime.o $OUTPUT/lib/$BUILD/$MM/regex.o $OUTPUT/lib/$BUILD/$MM/thread.o $OUTPUT/lib/$BUILD/$MM/http_linux.o
+$ARC rcs $OUTPUT/lib/$BUILD/$MM/libTypeScriptDefaultLibCore.a $OUTPUT/lib/$BUILD/$MM/lib.o $OUTPUT/lib/$BUILD/$MM/lib.linux.o $OUTPUT/lib/$BUILD/$MM/io.o $OUTPUT/lib/$BUILD/$MM/datetime.o $OUTPUT/lib/$BUILD/$MM/regex.o $OUTPUT/lib/$BUILD/$MM/thread.o $OUTPUT/lib/$BUILD/$MM/http_linux.o $OUTPUT/lib/$BUILD/$MM/memory.o
 
 # A static archive cannot record its own dependencies on ELF (there is no ld.bfd equivalent of
 # the `#pragma comment(lib, ...)` http.cpp uses on Windows), so programs linking
@@ -93,7 +94,7 @@ EOF
 # undefined on purpose: they must bind to the one collector already in the process
 # (libTypeScriptRuntime.so under the JIT, libgc.a in an exe); linking libgc.a in here would
 # give the library a second, separate heap.
-g++ -shared $DBG_GCC $OUTPUT/lib/$BUILD/$MM/lib.o $OUTPUT/lib/$BUILD/$MM/lib.linux.o $OUTPUT/lib/$BUILD/$MM/io.o $OUTPUT/lib/$BUILD/$MM/datetime.o $OUTPUT/lib/$BUILD/$MM/regex.o $OUTPUT/lib/$BUILD/$MM/thread.o $OUTPUT/lib/$BUILD/$MM/http_linux.o -lcurl -lm -o $OUTPUT/dll/$BUILD/$MM/libTypeScriptDefaultLib.so
+g++ -shared $DBG_GCC $OUTPUT/lib/$BUILD/$MM/lib.o $OUTPUT/lib/$BUILD/$MM/lib.linux.o $OUTPUT/lib/$BUILD/$MM/io.o $OUTPUT/lib/$BUILD/$MM/datetime.o $OUTPUT/lib/$BUILD/$MM/regex.o $OUTPUT/lib/$BUILD/$MM/thread.o $OUTPUT/lib/$BUILD/$MM/http_linux.o $OUTPUT/lib/$BUILD/$MM/memory.o -lcurl -lm -o $OUTPUT/dll/$BUILD/$MM/libTypeScriptDefaultLib.so
 
 # Copy
 # Stage into a single shared defaultlib tree with per-build subfolders under dll/ and lib/,
diff --git a/scripts/build_android.bat b/scripts/build_android.bat
index 04b14c8..8daed45 100644
--- a/scripts/build_android.bat
+++ b/scripts/build_android.bat
@@ -102,14 +102,14 @@ if exist "%OBJ%" rd /s /q "%OBJ%"
 if exist "%LIB_OUT%" rd /s /q "%LIB_OUT%"
 md "%OBJ%" "%LIB_OUT%"
 
-for %%w in (io datetime regex thread http_stub) do (
+for %%w in (io datetime regex thread http_stub memory) do (
     %CXX% -c "%ROOT%\src\wrappers\%%w.cpp" -o "%OBJ%\%%w.o" || exit /b 1
 )
 
 %TSC% "%ROOT%\src\lib.linux.ts" -o "%OBJ%\lib.linux.o" || exit /b 1
 %TSC% "%ROOT%\src\lib.ts" -o "%OBJ%\lib.o" || exit /b 1
 
-set OBJS="%OBJ%\lib.o" "%OBJ%\lib.linux.o" "%OBJ%\io.o" "%OBJ%\datetime.o" "%OBJ%\regex.o" "%OBJ%\thread.o" "%OBJ%\http_stub.o"
+set OBJS="%OBJ%\lib.o" "%OBJ%\lib.linux.o" "%OBJ%\io.o" "%OBJ%\datetime.o" "%OBJ%\regex.o" "%OBJ%\thread.o" "%OBJ%\http_stub.o" "%OBJ%\memory.o"
 %AR% rcs "%LIB_OUT%\libTypeScriptDefaultLib.a" %OBJS% || exit /b 1
 
 rd /s /q "%OBJ%"
diff --git a/scripts/build_core.bat b/scripts/build_core.bat
index d546d4d..9a8069b 100644
--- a/scripts/build_core.bat
+++ b/scripts/build_core.bat
@@ -205,6 +205,7 @@ rem Build native wrappers for C++ code
 %TSLANG_CC% %DBG_CL% /EHsc /Wall /c /Fo%OUTPUT%\%LIB_OUT%\ %SRC%\src\wrappers\regex.cpp
 %TSLANG_CC% %DBG_CL% /EHsc /Wall /c /Fo%OUTPUT%\%LIB_OUT%\ %SRC%\src\wrappers\thread.cpp
 %TSLANG_CC% %DBG_CL% /EHsc /Wall /c /Fo%OUTPUT%\%LIB_OUT%\ %SRC%\src\wrappers\http.cpp
+%TSLANG_CC% %DBG_CL% /EHsc /Wall /c /Fo%OUTPUT%\%LIB_OUT%\ %SRC%\src\wrappers\memory.cpp
 
 rem Build OS-specific Lib
 echo Build OS-specific Lib
@@ -212,7 +213,7 @@ echo Build OS-specific Lib
 
 rem Build DLL
 echo Build DLL
-%TOOL_PATH%\%TOOL_NAME%.exe %TRIPLE_OPT% %DBG% %MM_OPT% --emit=dll --gc-lib-path=%GC_SHARED_LIB_PATH% %TSLANG_RUNTIME_OPT% --embed-declarations=false --nowarn --no-default-lib %SRC%\src\lib.ts --obj=%OUTPUT%\%LIB_OUT%\lib.win32.obj --obj=%OUTPUT%\%LIB_OUT%\io.obj --obj=%OUTPUT%\%LIB_OUT%\datetime.obj --obj=%OUTPUT%\%LIB_OUT%\regex.obj --obj=%OUTPUT%\%LIB_OUT%\thread.obj --obj=%OUTPUT%\%LIB_OUT%\http.obj -o %OUTPUT%\%DLL_OUT%\TypeScriptDefaultLib.dll
+%TOOL_PATH%\%TOOL_NAME%.exe %TRIPLE_OPT% %DBG% %MM_OPT% --emit=dll --gc-lib-path=%GC_SHARED_LIB_PATH% %TSLANG_RUNTIME_OPT% --embed-declarations=false --nowarn --no-default-lib %SRC%\src\lib.ts --obj=%OUTPUT%\%LIB_OUT%\lib.win32.obj --obj=%OUTPUT%\%LIB_OUT%\io.obj --obj=%OUTPUT%\%LIB_OUT%\datetime.obj --obj=%OUTPUT%\%LIB_OUT%\regex.obj --obj=%OUTPUT%\%LIB_OUT%\thread.obj --obj=%OUTPUT%\%LIB_OUT%\http.obj --obj=%OUTPUT%\%LIB_OUT%\memory.obj -o %OUTPUT%\%DLL_OUT%\TypeScriptDefaultLib.dll
 
 rem Build Lib
 echo Build Lib
@@ -220,7 +221,7 @@ echo Build Lib
 rem %TOOL_PATH%\%TOOL_NAME%.exe %TRIPLE_OPT% %DBG% %MM_OPT% --emit=llvm --export=none %SRC%\src\lib.ts -o %OUTPUT%\%LIB_OUT%\lib.ll
 rem %TOOL_PATH%\%TOOL_NAME%.exe %TRIPLE_OPT% %DBG% %MM_OPT% --emit=mlir --export=none %SRC%\src\lib.ts 2> %OUTPUT%\%LIB_OUT%\lib.mlir
 
-%TSLANG_AR% /out:%OUTPUT%\%LIB_OUT%\TypeScriptDefaultLib.lib %OUTPUT%\%LIB_OUT%\lib.obj %OUTPUT%\%LIB_OUT%\lib.win32.obj %OUTPUT%\%LIB_OUT%\io.obj %OUTPUT%\%LIB_OUT%\datetime.obj %OUTPUT%\%LIB_OUT%\regex.obj %OUTPUT%\%LIB_OUT%\thread.obj %OUTPUT%\%LIB_OUT%\http.obj
+%TSLANG_AR% /out:%OUTPUT%\%LIB_OUT%\TypeScriptDefaultLib.lib %OUTPUT%\%LIB_OUT%\lib.obj %OUTPUT%\%LIB_OUT%\lib.win32.obj %OUTPUT%\%LIB_OUT%\io.obj %OUTPUT%\%LIB_OUT%\datetime.obj %OUTPUT%\%LIB_OUT%\regex.obj %OUTPUT%\%LIB_OUT%\thread.obj %OUTPUT%\%LIB_OUT%\http.obj %OUTPUT%\%LIB_OUT%\memory.obj
 
 del %OUTPUT%\%LIB_OUT%\lib.obj
 del %OUTPUT%\%LIB_OUT%\lib.win32.obj
@@ -229,6 +230,7 @@ del %OUTPUT%\%LIB_OUT%\datetime.obj
 del %OUTPUT%\%LIB_OUT%\regex.obj
 del %OUTPUT%\%LIB_OUT%\thread.obj
 del %OUTPUT%\%LIB_OUT%\http.obj
+del %OUTPUT%\%LIB_OUT%\memory.obj
 
 rem Stage into a single shared defaultlib tree with per-build subfolders under
 rem dll\ and lib\. Only the current build's subfolders are refreshed so the
diff --git a/src/lib.ts b/src/lib.ts
index 5c62175..af7e9eb 100644
--- a/src/lib.ts
+++ b/src/lib.ts
@@ -576,7 +576,7 @@ export class RegExp
             // get formatted replacement
             const len = r.length;
             let result = "".clone().resize(len);
-            memcpy(Ref(result[0]), r, len);
+            boundedCopy(Ref(result[0]), len, r, len);
             
             regexp_free_string(r);
             regexp_free(cmatch);
@@ -592,7 +592,7 @@ export class RegExp
         const r = regexp_replace(this.source, this.flags, s, replacement);
         const len = r.length;
         let result = "".clone().resize(len);
-        memcpy(Ref(result[0]), r, len);
+        boundedCopy(Ref(result[0]), len, r, len);
         regexp_free_string(r);
         return result;
     }
@@ -655,7 +655,7 @@ namespace __String {
         const newString = this.clone().resize(count);        
         let index = this.length;
         for (const item of other) {
-            memcpy(Ref(newString[index]), Ref(item[0]), sizeof<char>() * item.length);
+            boundedCopy(Ref(newString[index]), sizeof<char>() * (count - index), Ref(item[0]), sizeof<char>() * item.length);
             index += item.length;
         }
 
@@ -670,7 +670,7 @@ namespace __String {
         const newString = "".clone().resize(count);        
         let index = 0;
         for (const item of parts) {
-            memcpy(Ref(newString[index]), Ref(item[0]), sizeof<char>() * item.length);
+            boundedCopy(Ref(newString[index]), sizeof<char>() * (count - index), Ref(item[0]), sizeof<char>() * item.length);
             index += item.length;
         }
 
@@ -881,7 +881,7 @@ namespace __String {
         // the clone holds the first copy already: count - 1 more fill the string, and one more
         // wrote past its end
         for (let i = 1; i < count; i++) {
-            memcpy(Ref(newString[index]), this, byteSize);
+            boundedCopy(Ref(newString[index]), sizeof<char>() * (newSize - index), this, byteSize);
             index += this.length;
         }
 
@@ -980,7 +980,7 @@ namespace __String {
 
         const count = indexEnd - indexStart;
         const newString = "".clone().resize(count);
-        memcpy(Ref(newString[0]), Ref(this[indexStart]), sizeof<char>() * count);
+        boundedCopy(Ref(newString[0]), sizeof<char>() * count, Ref(this[indexStart]), sizeof<char>() * count);
         return newString;
     }
 
@@ -1075,7 +1075,7 @@ namespace __String {
 
         const count = indexEnd - indexStart;
         const newString = "".clone().resize(count);
-        memcpy(Ref(newString[0]), Ref(this[indexStart]), sizeof<char>() * count);
+        boundedCopy(Ref(newString[0]), sizeof<char>() * count, Ref(this[indexStart]), sizeof<char>() * count);
         return newString;        
     }
 
diff --git a/src/native/lib.native.d.ts b/src/native/lib.native.d.ts
index 1f73db5..4ee225c 100644
--- a/src/native/lib.native.d.ts
+++ b/src/native/lib.native.d.ts
@@ -85,6 +85,9 @@ declare function trunc(x: number): number;
 // memory
 declare function memcpy(dest: Opaque, src: Opaque, bytes: index): Opaque;
 declare function memmove(dest: Opaque, src: Opaque, bytes: index): Opaque;
+// wrappers/memory.cpp: memcpy_s on Windows, __memcpy_chk elsewhere - ends the process rather
+// than copy more than `destSize` bytes
+declare function boundedCopy(dest: Opaque, destSize: index, src: Opaque, count: index): void;
 declare function memcmp(dest: Opaque, src: Opaque, bytes: index): int;
 
 // gc weak references
diff --git a/src/wrappers/memory.cpp b/src/wrappers/memory.cpp
new file mode 100644
index 0000000..db0eb3e
--- /dev/null
+++ b/src/wrappers/memory.cpp
@@ -0,0 +1,22 @@
+#include <cstddef>
+#include <cstring>
+
+#include "tslang_export.h"
+
+// A copy of `count` bytes into `dest`, which has `destSize` bytes left, that ends the process
+// rather than write past them. Windows has the CRT's memcpy_s: its invalid parameter handler ends
+// the process. glibc and Bionic have no memcpy_s, but have the fortified copy _FORTIFY_SOURCE
+// uses, which aborts with "buffer overflow detected"; the builtin calls it, or a plain memcpy
+// where the compiler can prove the copy fits.
+//
+// In C++, not in lib.win32.ts / lib.linux.ts: the compiler emits memcpy_s / __memcpy_chk for
+// copies of its own, and a TypeScript declaration of the same function in a module that makes such
+// a copy before the declaration is lowered is a second definition of the symbol.
+extern "C" TSLANG_EXPORT void boundedCopy(void *dest, size_t destSize, const void *src, size_t count)
+{
+#ifdef _WIN32
+    memcpy_s(dest, destSize, src, count);
+#else
+    __builtin___memcpy_chk(dest, src, count, destSize);
+#endif
+}
diff --git a/src/wrappers/regex.cpp b/src/wrappers/regex.cpp
index a7dd922..3328da0 100644
--- a/src/wrappers/regex.cpp
+++ b/src/wrappers/regex.cpp
@@ -53,7 +53,11 @@ extern "C" TSLANG_EXPORT char* regexp_match_results_format(std::cmatch *cm, cons
     std::allocator<char> alloc;
     char* buffer = alloc.allocate(len + 1);
 
+#ifdef _WIN32
+    memcpy_s(buffer, len + 1, result.c_str(), len);
+#else
     std::memcpy(buffer, result.c_str(), len);
+#endif
 
     buffer[len] = 0;
     
@@ -114,7 +118,11 @@ extern "C" TSLANG_EXPORT char* regexp_replace(const char *expr, const char *flag
         std::allocator<char> alloc;
         char* buffer = alloc.allocate(len + 1);
 
+#ifdef _WIN32
+        memcpy_s(buffer, len + 1, result.c_str(), len);
+#else
         std::memcpy(buffer, result.c_str(), len);
+#endif
 
         buffer[len] = 0;