GEN:SID 1:496
Message ATTACK RESPONSES directory listing
Summary This event is generated by the successful completion of a directory listing operation. This may be indicative of post-compromise behavior indicating the use of a Windows command shell for listing directory contents.
Impact Serious. An attacker may have the ability to execute commands remotely
Detailed Information This event is generated when a standard Windows command for listing directories is executed. The string "Directory of" is typically shown in front of the directory listing on Windows NT/2000/XP.  

Seeing this response in HTTP traffic indicates that an attacker may have been able to spawn a shell bound to a web port and has successfully executed at least one command to list the contents of a directory directory. Note that the source address of this event is actually
the victim and not that of the attacker.
Affected Systems  
Attack Scenarios An attacker gains an access to a Windows web server via IIS vulnerability and manages to start a cmd.exe shell. He then proceeds to look for interesting files on the compromised server via the "dir" command.
Ease of Attack Simple. This post-attack behavior can accompany different attacks.
Corrective Action Investigate the web server for signs of compromise.

Look for other IDS events involving the same IP addresses.
Additional References