diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4fc023abd..37879946f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -17,6 +17,11 @@ env:
   AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
   AWS_REGION: us-east-1
 
+# cancel in-progress builds after a new commit
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+  
 jobs:
   linux-compat-use-openssl:
     runs-on: ubuntu-22.04 # latest
@@ -128,12 +133,15 @@ jobs:
 
   linux-shared-libs:
     runs-on: ubuntu-22.04 # latest
+    strategy:
+      matrix:
+        compiler: [gcc-4.8, gcc-11] # oldest, latest
     steps:
         # We can't use the `uses: docker://image` version yet, GitHub lacks authentication for actions -> packages
     - name: Build ${{ env.PACKAGE_NAME }}
       run: |
         aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
-        ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --cmake-extra=-DBUILD_SHARED_LIBS=ON
+        ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --compiler=${{ matrix.compiler }} --cmake-extra=-DBUILD_SHARED_LIBS=ON
 
   linux-openssl-static:
     runs-on: ubuntu-22.04 # latest
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 560ed5259..9f062caa6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -83,8 +83,8 @@ if(BUILD_DEPS)
     add_subdirectory(crt/aws-c-common)
 
     if(UNIX AND NOT APPLE AND NOT BYO_CRYPTO)
-        include(AwsPrebuildDependency)
         if(NOT USE_OPENSSL)
+            include(AwsPrebuildDependency)
 
             set(AWSLC_CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
 
@@ -116,14 +116,8 @@ if(BUILD_DEPS)
             )
         endif()
 
-        # prebuild s2n-tls.
-        aws_prebuild_dependency(
-            DEPENDENCY_NAME S2N
-            SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/crt/s2n
-            CMAKE_ARGUMENTS
-                -DUNSAFE_TREAT_WARNINGS_AS_ERRORS=OFF
-                -DBUILD_TESTING=OFF
-        )
+        set(UNSAFE_TREAT_WARNINGS_AS_ERRORS OFF CACHE BOOL "Disable warnings-as-errors when building S2N")
+        add_subdirectory(crt/s2n)
     endif()
 
     add_subdirectory(crt/aws-c-sdkutils)
@@ -302,7 +296,13 @@ endif()
 
 set_target_properties(${PROJECT_NAME} PROPERTIES LINKER_LANGUAGE CXX)
 set_target_properties(${PROJECT_NAME} PROPERTIES CXX_STANDARD ${CMAKE_CXX_STANDARD})
-set_target_properties(${PROJECT_NAME} PROPERTIES CXX_VISIBILITY_PRESET hidden VISIBILITY_INLINES_HIDDEN ON)
+
+# Hide symbols by default
+# Except for ancient GCC, because it leads to crashes in shared-lib builds
+# see: https://github.com/awslabs/aws-crt-cpp/pull/675
+if(NOT (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0"))
+    set_target_properties(${PROJECT_NAME} PROPERTIES CXX_VISIBILITY_PRESET hidden VISIBILITY_INLINES_HIDDEN ON)
+endif()
 
 aws_prepare_symbol_visibility_args(${PROJECT_NAME} "AWS_CRT_CPP")
 
@@ -348,7 +348,7 @@ target_link_libraries(${PROJECT_NAME} PUBLIC ${DEP_AWS_LIBS})
 
 install(FILES ${AWS_CRT_HEADERS} DESTINATION "include/aws/crt" COMPONENT Development)
 install(FILES ${AWS_CRT_AUTH_HEADERS} DESTINATION "include/aws/crt/auth" COMPONENT Development)
-install(FILES ${AWS_CRT_CHECKSUM_HEADERS} DESTINATION "include/aws/crt/crypto" COMPONENT Development)
+install(FILES ${AWS_CRT_CHECKSUM_HEADERS} DESTINATION "include/aws/crt/checksum" COMPONENT Development)
 install(FILES ${AWS_CRT_CRYPTO_HEADERS} DESTINATION "include/aws/crt/crypto" COMPONENT Development)
 install(FILES ${AWS_CRT_IO_HEADERS} DESTINATION "include/aws/crt/io" COMPONENT Development)
 install(FILES ${AWS_CRT_IOT_HEADERS} DESTINATION "include/aws/iot" COMPONENT Development)
diff --git a/VERSION b/VERSION
index ae6dd4e20..5540b6e00 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.29.0
+0.29.3
diff --git a/crt/aws-c-io b/crt/aws-c-io
index fe93d0afc..e247ef89c 160000
--- a/crt/aws-c-io
+++ b/crt/aws-c-io
@@ -1 +1 @@
-Subproject commit fe93d0afcc1cede32ac9569abd8669ed011b1b8c
+Subproject commit e247ef89cdf170cadcb4c665fb0146e82cd6e135
diff --git a/crt/s2n b/crt/s2n
index ffe0bf42d..9f4baecc7 160000
--- a/crt/s2n
+++ b/crt/s2n
@@ -1 +1 @@
-Subproject commit ffe0bf42da8f139eff8fd2237f47fbde40b478fb
+Subproject commit 9f4baecc753d6fe01c13e4f422d2e327c64d06b8