diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 511abb41c..3db5eea5b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -56,19 +56,16 @@ jobs:
           - target:
               os: linux
             builder: ['self-hosted','ubuntu-22.04']
-            shell: bash
           - target:
               os: macos
             builder: macos-11
-            shell: bash
           - target:
               os: windows
             builder: windows-2019
-            shell: msys2 {0}
 
     defaults:
       run:
-        shell: ${{ matrix.shell }}
+        shell: bash
 
     name: ${{ matrix.target.os }}-${{ matrix.target.cpu }}${{ matrix.branch != '' && ' (Nim ' || '' }}${{ matrix.branch-short }}${{ matrix.branch != '' && ')' || '' }}
     runs-on: ${{ matrix.builder }}
@@ -76,16 +73,25 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
 
-      - name: MSYS2 (Windows amd64)
-        if: runner.os == 'Windows' && matrix.target.cpu == 'amd64'
-        uses: msys2/setup-msys2@v2
+      - name: Restore llvm-mingw (Windows) from cache
+        if: runner.os == 'Windows'
+        id: windows-mingw-cache
+        uses: actions/cache@v3
         with:
-          path-type: inherit
-          install: >-
-            base-devel
-            git
-            mingw-w64-x86_64-toolchain
-            mingw-w64-x86_64-cmake
+          path: external/mingw-${{ matrix.target.cpu }}
+          key: 'mingw-llvm-17-${{ matrix.target.cpu }}'
+
+      - name: Install llvm-mingw dependency (Windows)
+        if: >
+          steps.windows-mingw-cache.outputs.cache-hit != 'true' &&
+          runner.os == 'Windows'
+        run: |
+          mkdir -p external
+          MINGW_BASE="https://github.com/mstorsjo/llvm-mingw/releases/download/20230905"
+          MINGW_URL="$MINGW_BASE/llvm-mingw-20230905-ucrt-x86_64.zip"
+          curl -L "$MINGW_URL" -o "external/mingw-${{ matrix.target.cpu }}.zip"
+          7z x -y "external/mingw-${{ matrix.target.cpu }}.zip" -oexternal/mingw-${{ matrix.target.cpu }}/
+          mv external/mingw-${{ matrix.target.cpu }}/**/* ./external/mingw-${{ matrix.target.cpu }}
 
       - name: Restore Nim DLLs dependencies (Windows) from cache
         if: runner.os == 'Windows'
@@ -108,6 +114,7 @@ jobs:
         if: >
           runner.os == 'Windows'
         run: |
+          echo '${{ github.workspace }}'"/external/mingw-${{ matrix.target.cpu }}/bin" >> $GITHUB_PATH
           echo "${{ github.workspace }}/external/dlls-${{ matrix.target.cpu }}" >> $GITHUB_PATH
           # for miniupnp that runs "wingenminiupnpcstrings.exe" from the current dir
           echo "." >> $GITHUB_PATH
diff --git a/Makefile b/Makefile
index 7b6edacff..990ce2e3a 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-# Copyright (c) 2019-2023 Status Research & Development GmbH. Licensed under
+# Copyright (c) 2019-2024 Status Research & Development GmbH. Licensed under
 # either of:
 # - Apache License, version 2.0
 # - MIT license
@@ -775,7 +775,6 @@ libnimbus_lc.a: | build deps
 		echo -e $(BUILD_END_MSG) "build/$@"
 
 # `-Wno-maybe-uninitialized` in Linux: https://github.com/nim-lang/Nim/issues/22246
-# `-fsanitize=undefined` in Windows: https://github.com/msys2/MINGW-packages/issues/3163
 # `-Wl,--stack,0x0000000000800000` in Windows: MinGW default of 2 MB leads to `SIGSEGV` in `___chkstk_ms` in Nim 2.0
 test_libnimbus_lc: libnimbus_lc.a
 	+ echo -e $(BUILD_MSG) "build/$@" && \
@@ -789,9 +788,9 @@ test_libnimbus_lc: libnimbus_lc.a
 			;; \
 		MINGW64_*) \
 			if (( $${WITH_UBSAN:-0} )); then \
-				echo "MINGW cannot find -lubsan." && exit 1; \
+				EXTRA_FLAGS+=('-fsanitize=undefined'); \
 			fi; \
-			gcc -D__DIR__="\"beacon_chain/libnimbus_lc\"" --std=c17 -Wall -Wextra -pedantic -Werror -pedantic-errors -flto -Wl,--stack,0x0000000000800000 -o build/test_libnimbus_lc -D_CRT_SECURE_NO_WARNINGS beacon_chain/libnimbus_lc/test_libnimbus_lc.c build/libnimbus_lc.a "$${EXTRA_FLAGS[@]}"; \
+			gcc -D__DIR__="\"beacon_chain/libnimbus_lc\"" --std=c17 -Wall -Wextra -pedantic -Werror -pedantic-errors -flto -Wno-nullability-extension -Wl,--stack,0x0000000000800000 -o build/test_libnimbus_lc -D_CRT_SECURE_NO_WARNINGS beacon_chain/libnimbus_lc/test_libnimbus_lc.c build/libnimbus_lc.a "$${EXTRA_FLAGS[@]}"; \
 			;; \
 		*) \
 			if (( $${WITH_UBSAN:-0} )); then \