diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index 1f3fe8dd6..d2fd400e0 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -386,6 +386,7 @@ enum lock_label {
 	PIPES_LOCK,
 	START_LOCK,
 	TLSKEYS_REF_LOCK,
+	AUTH_LOCK,
 	LOCK_LABELS
 };
 struct lock_stat {
@@ -501,6 +502,7 @@ static inline const char *lock_label(enum lock_label label)
 	case PIPES_LOCK:           return "PIPES";
 	case START_LOCK:           return "START";
 	case TLSKEYS_REF_LOCK:     return "TLSKEYS_REF";
+	case AUTH_LOCK:            return "AUTH";
 	case LOCK_LABELS:          break; /* keep compiler happy */
 	};
 	/* only way to come here is consecutive to an internal bug */
diff --git a/src/auth.c b/src/auth.c
index 3dd3a1de0..599a3abbc 100644
--- a/src/auth.c
+++ b/src/auth.c
@@ -28,6 +28,7 @@
 #include <types/global.h>
 #include <common/config.h>
 #include <common/errors.h>
+#include <common/hathreads.h>
 
 #include <proto/acl.h>
 #include <proto/log.h>
@@ -37,6 +38,10 @@
 
 struct userlist *userlist = NULL;    /* list of all existing userlists */
 
+#ifdef CONFIG_HAP_CRYPT
+__decl_hathreads(static HA_SPINLOCK_T auth_lock);
+#endif
+
 /* find targets for selected gropus. The function returns pointer to
  * the userlist struct ot NULL if name is NULL/empty or unresolvable.
  */
@@ -245,7 +250,9 @@ check_user(struct userlist *ul, const char *user, const char *pass)
 
 	if (!(u->flags & AU_O_INSECURE)) {
 #ifdef CONFIG_HAP_CRYPT
+		HA_SPIN_LOCK(AUTH_LOCK, &auth_lock);
 		ep = crypt(pass, u->pass);
+		HA_SPIN_UNLOCK(AUTH_LOCK, &auth_lock);
 #else
 		return 0;
 #endif