diff --git a/cmake/targets/BuildMimalloc.cmake b/cmake/targets/BuildMimalloc.cmake
index 874891765f..c503920a2a 100644
--- a/cmake/targets/BuildMimalloc.cmake
+++ b/cmake/targets/BuildMimalloc.cmake
@@ -69,8 +69,18 @@ if(ENABLE_VALGRIND)
   list(APPEND MIMALLOC_CMAKE_ARGS -DMI_VALGRIND=ON)
 endif()
 
-# Enable SIMD optimizations when not building for baseline (older CPUs)
-if(NOT ENABLE_BASELINE)
+# Enable architecture-specific optimizations when not building for baseline.
+# On Linux aarch64, upstream mimalloc force-enables MI_OPT_ARCH which adds
+# -march=armv8.1-a (LSE atomics). This crashes on ARMv8.0 CPUs
+# (Cortex-A53, Raspberry Pi 4, AWS a1 instances). Use MI_NO_OPT_ARCH
+# to prevent that, but keep SIMD enabled. -moutline-atomics for runtime
+# dispatch to LSE/LL-SC. macOS arm64 always has LSE (Apple Silicon) so
+# MI_OPT_ARCH is safe there.
+if(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64|arm64|ARM64|AARCH64" AND NOT APPLE)
+  list(APPEND MIMALLOC_CMAKE_ARGS -DMI_NO_OPT_ARCH=ON)
+  list(APPEND MIMALLOC_CMAKE_ARGS -DMI_OPT_SIMD=ON)
+  list(APPEND MIMALLOC_CMAKE_ARGS "-DCMAKE_C_FLAGS=-moutline-atomics")
+elseif(NOT ENABLE_BASELINE)
   list(APPEND MIMALLOC_CMAKE_ARGS -DMI_OPT_ARCH=ON)
   list(APPEND MIMALLOC_CMAKE_ARGS -DMI_OPT_SIMD=ON)
 endif()
diff --git a/cmake/tools/SetupWebKit.cmake b/cmake/tools/SetupWebKit.cmake
index 48146eb5fa..9763f3cf38 100644
--- a/cmake/tools/SetupWebKit.cmake
+++ b/cmake/tools/SetupWebKit.cmake
@@ -2,7 +2,7 @@ option(WEBKIT_VERSION "The version of WebKit to use")
 option(WEBKIT_LOCAL "If a local version of WebKit should be used instead of downloading")
 
 if(NOT WEBKIT_VERSION)
-  set(WEBKIT_VERSION 596e48e22e3a1090e5b802744a7938088b1ea860)
+  set(WEBKIT_VERSION 515344bc5d65aa2d4f9ff277b5fb944f0e051dcd)
 endif()
 
 # Use preview build URL for Windows ARM64 until the fix is merged to main