There is a "feature" inherent in some web servers, such as Apache 1.3.x or MS IIS, that carries mild security implications that could allow web server attacks to go unnoticed. The problem relates to "allowable" REQUEST_METHODs when a dynamic resource, such as a CGI script is requested. Essentially _any_ (except for HEAD, TRACE and OPTIONS) REQUEST_METHOD can be used - even methods not defined in the HTTP protocol. Consider the following requests which all return the requested resource. GET /cgi-bin/environ.cgi HTTP/0.9 Azx5T8uHTRuDL /cgi-bin/environ.cgi HTTP/1.0 Even Control characters are allowed. Consider the following: ^H^H^H^H^H^H^H^H^H lots of these ^H^H /cgi-bin/environ.cgi HTTP/1.1 An attacker could issue this request in an attempt to hide their movements. When this request is logged in the access log and viewed using cat or more the above will appear with the IP address removed. # cat /var/log/httpd/access_log or # more /var/log/httpd/access_log reveals 10.20.20.1 - - [05/Jan/1999:18:00:00 GMT] "GET / HTTP/1.0" 200 1098 /cgi-bin/environ.cgi HTTP/1.1" 200 2034 10.20.20.2 -- [05/Jan/1999:18:01:00 GMT] "GET /index.html HTTP/0.9" 200 1098 Using a method similar to this it is possible for an attacker to make it appear as if the attack came from another IP address or completely remove the whole entry by placing certain control characters in the QUERY_STRING, too. This "hiding" works because the control characters are interpreted when piped to STDOUT and the ^H being the back space removes, from the screen at least, the IP address and date and time stamp. You could use the vi editor the view the "real" contents of the access log. This was tested on Apache 1.3.3 on RedHat 5.2 and Apache 1.2.7 on RedHat 5.0. Also affected is Microsoft's Internet Information Server 2, 3 and 4 but in the NT environment this is less of a problem because the log files are generally viewd in Notepad and not using the "type" command, which incidently will interpret the control characters. As I said it's only a mild problem most likely, really, to effect those that don't use a text editor to browse log files. Cheers, David Litchfield
This archive was generated by hypermail 2b30 : Fri Apr 13 2001 - 14:27:32 PDT