diff --git a/auth-options.c b/auth-options.c
index 514221882..ca92f7a73 100644
--- a/auth-options.c
+++ b/auth-options.c
@@ -21,6 +21,7 @@
 
 #include <netdb.h>
 #include <pwd.h>
+#include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdarg.h>
diff --git a/hmac.c b/hmac.c
index 1c879640c..e90b294fb 100644
--- a/hmac.c
+++ b/hmac.c
@@ -18,6 +18,8 @@
 #include "includes.h"
 
 #include <sys/types.h>
+
+#include <stdlib.h>
 #include <string.h>
 
 #include "sshbuf.h"
diff --git a/krl.c b/krl.c
index bb960882f..f36ba68ac 100644
--- a/krl.c
+++ b/krl.c
@@ -25,6 +25,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
+#include <stdlib.h>
 #include <string.h>
 #include <time.h>
 #include <unistd.h>
diff --git a/mac.c b/mac.c
index 9a504e892..eab453a41 100644
--- a/mac.c
+++ b/mac.c
@@ -27,6 +27,7 @@
 
 #include <sys/types.h>
 
+#include <stdlib.h>
 #include <string.h>
 #include <stdio.h>