diff -up shadow-4.1.5.1/src/usermod.c.passwd shadow-4.1.5.1/src/usermod.c
--- shadow-4.1.5.1/src/usermod.c.passwd	2015-12-17 14:05:47.959743073 +0100
+++ shadow-4.1.5.1/src/usermod.c	2015-12-18 12:42:28.290405529 +0100
@@ -360,14 +360,17 @@ static char *new_pw_passwd (char *pw_pas
 		strcat (buf, pw_pass);
 		pw_pass = buf;
 	} else if (Uflg && pw_pass[0] == '!') {
-		char *s;
+		char *s = pw_pass;
 
-		if (pw_pass[1] == '\0') {
+		while ('!' == *s)
+			++s;
+
+		if (*s == '\0') {
 			fprintf (stderr,
 			         _("%s: unlocking the user's password would result in a passwordless account.\n"
 			           "You should set a password with usermod -p to unlock this user's password.\n"),
 			         Prog);
-			return pw_pass;
+			return NULL;
 		}
 
 #ifdef WITH_AUDIT
@@ -376,12 +379,15 @@ static char *new_pw_passwd (char *pw_pas
 		              user_newname, (unsigned int) user_newid, 1);
 #endif
 		SYSLOG ((LOG_INFO, "unlock user '%s' password", user_newname));
-		s = pw_pass;
-		while ('\0' != *s) {
-			*s = *(s + 1);
-			s++;
-		}
+		memmove (pw_pass, s, strlen (s) + 1);
 	} else if (pflg) {
+		if (strchr (user_pass, ':') != NULL) {
+			fprintf (stderr,
+			         _("%s: The password field cannot contain a colon character.\n"),
+			         Prog);
+			return NULL;
+
+		}
 #ifdef WITH_AUDIT
 		audit_logger (AUDIT_USER_CHAUTHTOK, Prog,
 		              "updating-password",
@@ -430,6 +436,8 @@ static void new_pwent (struct passwd *pw
 	if (   (!is_shadow_pwd)
 	    || (strcmp (pwent->pw_passwd, SHADOW_PASSWD_STRING) != 0)) {
 		pwent->pw_passwd = new_pw_passwd (pwent->pw_passwd);
+		if (pwent->pw_passwd == NULL)
+			fail_exit (E_PW_UPDATE);
 	}
 
 	if (uflg) {
@@ -544,6 +552,8 @@ static void new_spent (struct spwd *spen
 	 *  + aging has been requested
 	 */
 	spent->sp_pwdp = new_pw_passwd (spent->sp_pwdp);
+	if (spent->sp_pwdp == NULL)
+		fail_exit(E_PW_UPDATE);
 
 	if (pflg) {
 		spent->sp_lstchg = (long) time ((time_t *) 0) / SCALE;