passwd: Authentication token manipulation error

passwd: Authentication token manipulation error on Linux 

[muthu@testserver ~]$ passwd

Changing password for user muthu.

Changing password for muthu

(current) UNIX password:

passwd: Authentication token manipulation error

[muthu@testserver ~]$ ls -lrt /etc/passwd

-rw-r--r-- 1 root root 2499 Oct 17 05:17 /etc/passwd
[muthu@testserver ~]$ ls -lrt /etc/shadow
-r-------- 1 root root 1899 Oct 17 05:15 /etc/shadow
[muthu@testserver ~]$ cat /etc/shadow
cat: /etc/shadow: Permission denied
[muthu@testserver ~]$ su -
Password:
[root@testserver ~]# chmod -R 755 /etc/shadow /etc/passwd
[root@testserver ~]# su - muthu

Password:
[muthu@testserver ~]$ passwd
Changing password for user muthu.
Changing password for muthu
(current) UNIX password:
New UNIX password:

Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[muthu@testserver ~]# ls -lr /etc/shadow /etc/passwd
-rwxr-xr-x 1 root root 1920 Oct 22 01:42 /etc/shadow
-rwxr-xr-x 1 root root 2499 Oct 17 05:17 /etc/passwd
[muthu@testserver ~]$