Breaking SHA1-HMAC

[feedworks]

Code:

$ cat dict.txt
abcd
$ cat m0160.txt # masked
8717XXX45b7cXXX66d9XXX55c6e5b9eXXXXc46c:1234
$ ./hashcat-cli64.bin --quiet -m 160 m0160.txt dict.txt
8717XXX45b7cXXX66d9XXX55c6e5b9eXXXXc46c:1234:abcd

What should I do if I only know the hash? Is it possible to break? I know the hash is generated using SHA1 HMAC and length of the key and length of the plain text.

Thanks for your reply.