diff --git a/CHANGES b/CHANGES
index cc153cb29d..9c3e14cdd4 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,8 @@
+6149.	[test]		As a workaround, include an OpenSSL header file before
+			including cmocka.h in the unit tests, because OpenSSL
+			3.1.0 uses __attribute__(malloc), conflicting with a
+			redefined malloc in cmocka.h. [GL #4000]
+
 6145.	[bug]		Fix a possible use-after-free bug in the
 			dns__catz_done_cb() function. [GL #3997]
 
diff --git a/tests/dns/dh_test.c b/tests/dns/dh_test.c
index 246de2197a..9358a8f61f 100644
--- a/tests/dns/dh_test.c
+++ b/tests/dns/dh_test.c
@@ -19,6 +19,13 @@
 #include <string.h>
 #include <unistd.h>
 
+/*
+ * As a workaround, include an OpenSSL header file before including cmocka.h,
+ * because OpenSSL 3.1.0 uses __attribute__(malloc), conflicting with a
+ * redefined malloc in cmocka.h.
+ */
+#include <openssl/err.h>
+
 #define UNIT_TESTING
 #include <cmocka.h>
 
diff --git a/tests/dns/dst_test.c b/tests/dns/dst_test.c
index f18bd2f359..0fecb908ea 100644
--- a/tests/dns/dst_test.c
+++ b/tests/dns/dst_test.c
@@ -19,6 +19,13 @@
 #include <stdlib.h>
 #include <unistd.h>
 
+/*
+ * As a workaround, include an OpenSSL header file before including cmocka.h,
+ * because OpenSSL 3.1.0 uses __attribute__(malloc), conflicting with a
+ * redefined malloc in cmocka.h.
+ */
+#include <openssl/err.h>
+
 #define UNIT_TESTING
 #include <cmocka.h>
 
diff --git a/tests/dns/rsa_test.c b/tests/dns/rsa_test.c
index 40652541f5..c814ab68d9 100644
--- a/tests/dns/rsa_test.c
+++ b/tests/dns/rsa_test.c
@@ -20,6 +20,13 @@
 #include <string.h>
 #include <unistd.h>
 
+/*
+ * As a workaround, include an OpenSSL header file before including cmocka.h,
+ * because OpenSSL 3.1.0 uses __attribute__(malloc), conflicting with a
+ * redefined malloc in cmocka.h.
+ */
+#include <openssl/err.h>
+
 #define UNIT_TESTING
 #include <cmocka.h>
 
diff --git a/tests/isc/doh_test.c b/tests/isc/doh_test.c
index ab74813cff..090fb8f318 100644
--- a/tests/isc/doh_test.c
+++ b/tests/isc/doh_test.c
@@ -22,6 +22,13 @@
 #include <unistd.h>
 #include <uv.h>
 
+/*
+ * As a workaround, include an OpenSSL header file before including cmocka.h,
+ * because OpenSSL 3.1.0 uses __attribute__(malloc), conflicting with a
+ * redefined malloc in cmocka.h.
+ */
+#include <openssl/err.h>
+
 #define UNIT_TESTING
 #include <cmocka.h>
 
diff --git a/tests/isc/hmac_test.c b/tests/isc/hmac_test.c
index ac567d6884..3d459d0a18 100644
--- a/tests/isc/hmac_test.c
+++ b/tests/isc/hmac_test.c
@@ -20,6 +20,13 @@
 #include <stdlib.h>
 #include <string.h>
 
+/*
+ * As a workaround, include an OpenSSL header file before including cmocka.h,
+ * because OpenSSL 3.1.0 uses __attribute__(malloc), conflicting with a
+ * redefined malloc in cmocka.h.
+ */
+#include <openssl/err.h>
+
 #define UNIT_TESTING
 #include <cmocka.h>
 
diff --git a/tests/isc/netmgr_test.c b/tests/isc/netmgr_test.c
index 1527c68112..b2745521f7 100644
--- a/tests/isc/netmgr_test.c
+++ b/tests/isc/netmgr_test.c
@@ -19,6 +19,13 @@
 #include <unistd.h>
 #include <uv.h>
 
+/*
+ * As a workaround, include an OpenSSL header file before including cmocka.h,
+ * because OpenSSL 3.1.0 uses __attribute__(malloc), conflicting with a
+ * redefined malloc in cmocka.h.
+ */
+#include <openssl/err.h>
+
 #define UNIT_TESTING
 #include <cmocka.h>