Cisco Secure Access Control System (ACS) 5.5 is and no longer officially available for download from Cisco. Support for this version ended on April 30, 2018
Password Recovery: For legacy systems where the admin password is lost and the original recovery ISO is missing, some administrators attempt to use newer ISOs (like version 5.8), though compatibility is not guaranteed . cisco acs 55 iso download updated
: Cisco has removed ACS 5.x software from its official download portal because it is now considered obsolete. Security Risk Cisco Secure Access Control System (ACS) 5
For labs and archival research only, if you cannot access the original Cisco media, some reputable network engineering forums archive "abandonware" ISOs. These include: GUI login failures
Some of the new features in Cisco ACS 5.5 include: