The guestbook CGI program in ICQ Web Front service for ICQ 2000a, 99b, and others allows remote attackers to cause a denial of service via a URL with a long name parameter.
ICQ99 ICQ web server build 1701 with "Active Homepage" enabled generates allows remote attackers to determine the existence of files on the server by comparing server responses when a file exists ("404 Forbidden") versus when a file does not exist ("404 not found").
Win32 ICQ 98a 1.30, and possibly other versions, does not display the entire portion of long filenames, which could allow attackers to send an executable file with a long name that contains so many spaces that the .exe extension is not displayed, which could make the user believe that the file is safe to open from the client.
ICQ 98 beta on Windows NT leaks the internal IP address of a client in the TCP data segment of an ICQ packet instead of the public address (e.g. through NAT), which provides remote attackers with potentially sensitive information about the client or the internal network configuration.