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Bugtraq archives for 1st quarter (Jan-Mar) 1999: HTTP REQUEST_METHOD flaw


mnemonix (mnemonixGLOBALNET.CO.UK)
Wed, 6 Jan 1999 13:16:07 -0000

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


 # more /var/log/httpd/access_log

reveals - - [05/Jan/1999:18:00:00 GMT] "GET / HTTP/1.0" 200 1098
/cgi-bin/environ.cgi HTTP/1.1" 200 2034 -- [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.

David Litchfield