ISM.DLL in IIS 4.0 and 5.0 allows remote attackers to read file contents by requesting the file and appending a large number of encoded spaces (%20) and terminated with a .htr extension, aka the ".HTR File Fragment Reading" or "File Fragment Reading via .HTR" vulnerability.
https://exchange.xforce.ibmcloud.com/vulnerabilities/4448
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2000/ms00-031