diff --git a/src/netif/ppp/polarssl/md4.c b/src/netif/ppp/polarssl/md4.c
index 8de2eb96..64d77665 100644
--- a/src/netif/ppp/polarssl/md4.c
+++ b/src/netif/ppp/polarssl/md4.c
@@ -44,6 +44,8 @@
 
 #include "netif/ppp/polarssl/md4.h"
 
+#include <string.h>
+
 /*
  * 32-bit integer manipulation macros (little endian)
  */
diff --git a/src/netif/ppp/polarssl/sha1.c b/src/netif/ppp/polarssl/sha1.c
index 78fe46ef..89d4991a 100644
--- a/src/netif/ppp/polarssl/sha1.c
+++ b/src/netif/ppp/polarssl/sha1.c
@@ -43,6 +43,8 @@
 
 #include "netif/ppp/polarssl/sha1.h"
 
+#include <string.h>
+
 /*
  * 32-bit integer manipulation macros (big endian)
  */