Muffett joined Sun Microsystems in 1992, working initially as a systems administrator. He rose through the ranks to become the principal engineer for security, a position which he held until he was retrenched, with many others, in 2009[1] (shortly before Oracle acquired Sun). While at Sun he was one of the researchers who worked on the factorization of the 512 bit RSA Challenge Number; RSA-155 was successfully factorized in August 1999.[2] He created Crack, the original password cracker for Unix, and for the CrackLib password-integrity testing library.
In 2015, Muffett was named as one of the top six influential security thinkers by SC Magazine.[3]