diff --git a/.cirrus.yml b/.cirrus.yml
index 5d2e3b6a73..0d7d7d60f4 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -73,8 +73,8 @@ gcc_task:
        USE_CONFIG: no
     - environment:
        USE_CONFIG: yes
-       CFLAGS: -std=c99 -pedantic -Wformat=2 -fsanitize=address
-       LDFLAGS: -fsanitize=address
+       CFLAGS: -std=c99 -pedantic -fsanitize=address,undefined -DHTS_ALLOW_UNALIGNED=0
+       LDFLAGS: -fsanitize=address,undefined
        USE_LIBDEFLATE: yes
 
   install_script: |
@@ -109,8 +109,9 @@ ubuntu_task:
        DO_UNTRACKED_FILE_CHECK: yes
     - environment:
        USE_CONFIG: yes
-       CFLAGS: -g -Wall -O3
+       CFLAGS: -std=c99 -pedantic -Wformat=2 -g -Wall -O3
        USE_LIBDEFLATE: yes
+       UBSAN_OPTIONS: print_stacktrace=1:halt_on_error=1
 
   # NB: we could consider building a docker image with these
   # preinstalled and specifying that instead, to speed up testing.