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Revision as of 20:17, 13 May 2014
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== Brief Summary ==
== Brief Summary ==
−
File extensions are commonly used in web servers to easily determine which technologies
/
languages
/
plugins must be used to
fulfil
the web request.
+
File extensions are commonly used in web servers to easily determine which technologies
,
languages
and
plugins must be used to
fulfill
the web request
. While this behavior is consistent with RFCs and Web Standards, using standard file extensions provides the penetration tester useful information about the underlying technologies used in a web appliance and greatly simplifies the task of determining the attack scenario to be used on particular technologies. In addition, mis-configuration of web servers could easily reveal confidential information about access credentials
.
−
While this behavior is consistent with RFCs and Web Standards, using standard file extensions provides the pentester useful information about the underlying technologies used in a web appliance and greatly simplifies the task of determining the attack scenario to be used on particular technologies.
−
In addition
,
misconfiguration in web servers could easily reveal confidential information about access credentials
.
+
Extension checking is often used to validate files to be uploaded
,
which can lead to unexpected results because the content is not what is expected, or because of unexpected OS file name handling
.
−
Extension checking is often used to validate files to be uploaded, which can lead to unexpected results because if the content is not what is expected, or because of unexpected OS filename handling.
==Description of the Issue==
==Description of the Issue==
−
Determining how web servers handle requests corresponding to files having different extensions may help
us to understand
web server
behaviour
depending on the kind of files
we try to access
. For example, it can help
us
understand which file extensions are returned as text
/
plain versus those
which
cause execution on the server side. The latter are indicative of technologies
/
languages
/
plugins
which
are used by web servers or application servers, and may provide additional insight on how the web application is engineered. For example, a “.pl” extension is usually associated with server-side Perl support
(though
the file extension alone may be deceptive and not fully conclusive
; for
example, Perl server-side resources might be renamed to conceal the fact that they are indeed Perl related
)
. See
also
next section on “web server components” for more on identifying server side technologies and components.
+
Determining how web servers handle requests corresponding to files having different extensions may help
in understanding
web server
behavior
depending on the kind of files
that are accessed
. For example, it can help
to
understand which file extensions are returned as text
or
plain versus those
that
cause execution on the server side. The latter are indicative of technologies
,
languages
or
plugins
that
are used by web servers or application servers, and may provide additional insight on how the web application is engineered. For example, a “.pl” extension is usually associated with server-side Perl support
. However,
the file extension alone may be deceptive and not fully conclusive
. For
example, Perl server-side resources might be renamed to conceal the fact that they are indeed Perl related. See
the
next section on “web server components” for more on identifying server side technologies and components.
+
==Black Box testing and example - forced browsing==
==Black Box testing and example - forced browsing==
−
Submit http[s] requests involving different file extensions and verify how they are handled.
These verifications
should be on a per web directory basis.
+
Submit http[s] requests involving different file extensions and verify how they are handled.
The verification
should be on a per web directory basis.
Verify directories that allow script execution. Web server directories can be identified by vulnerability scanners, which look for the presence of well-known directories. In addition, mirroring the web site structure allows the tester to reconstruct the tree of web directories served by the application.
+
−
Verify directories which allow script execution
.
Web server directories can
be
identified by vulnerability scanners
,
which look for
the
presence
of
well-known directories
. In
addition, mirroring
the web
site structure allows us to reconstruct
the
tree
of web
directories served by the application
.
+
If the web application architecture is load-balanced, it is important to assess all of the web servers
.
This may or may not
be
easy
,
depending on
the
configuration
of
the balancing infrastructure
. In
an infrastructure with redundant components there may be slight variations in
the
configuration of individual
web
or application servers. This may happen if
the
web architecture employs heterogeneous technologies (think of a set
of
IIS and Apache
web
servers in a load-balancing configuration, which may introduce slight asymmetric behavior between them, and possibly different vulnerabilities)
.
−
If the web application architecture is load-balanced, it is important to assess all of the web servers. This may or may not be easy, depending on the configuration of the balancing infrastructure. In an infrastructure with redundant components there may be slight variations in the configuration of individual web / application servers; this may happen, for example, if the web architecture employs heterogeneous technologies (think of a set of IIS and Apache web servers in a load-balancing configuration, which may introduce slight asymmetric behaviour between themselves, and possibly different vulnerabilities).
'''Example:''
'''Example:''
−
We have
identified the existence of a file named connection.inc. Trying to access it directly gives back its contents, which are:
+
The tester has
identified the existence of a file named connection.inc. Trying to access it directly gives back its contents, which are:
<pre>
<pre>
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</pre>
</pre>
−
We determine the existence of a MySQL DBMS back end, and the (weak) credentials used by the web application to access it. This example (which occurred in a real assessment) shows how dangerous can be the access to some kind of files.
−
The following file extensions should
NEVER
be returned by a web server, since they are related to files which may contain sensitive information
,
or to files for which there is no reason to be served.
+
The tester determines the existence of a MySQL DBMS back end, and the (weak) credentials used by the web application to access it.
+
+
+
The following file extensions should
never
be returned by a web server, since they are related to files which may contain sensitive information or to files for which there is no reason to be served.
* .asa
* .asa
* .inc
* .inc
+
The following file extensions are related to files which, when accessed, are either displayed or downloaded by the browser. Therefore, files with these extensions must be checked to verify that they are indeed supposed to be served (and are not leftovers), and that they do not contain sensitive information.
The following file extensions are related to files which, when accessed, are either displayed or downloaded by the browser. Therefore, files with these extensions must be checked to verify that they are indeed supposed to be served (and are not leftovers), and that they do not contain sensitive information.
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* .doc, .rtf, .xls, .ppt, ...: Office documents
* .doc, .rtf, .xls, .ppt, ...: Office documents
* .bak, .old and other extensions indicative of backup files (for example: ~ for Emacs backup files)
* .bak, .old and other extensions indicative of backup files (for example: ~ for Emacs backup files)
+
The list given above details only a few examples, since file extensions are too many to be comprehensively treated here. Refer to http://filext.com/ for a more thorough database of extensions.
The list given above details only a few examples, since file extensions are too many to be comprehensively treated here. Refer to http://filext.com/ for a more thorough database of extensions.
−
To
sum it up, in order to
identify files having a given extensions
,
a mix of techniques can be employed
, including:
Vulnerability Scanners, spidering and mirroring tools, manually inspecting the application (this overcomes limitations in automatic spidering), querying search engines (see [[Testing: Spidering and googling]]). See also [[Testing for Old, Backup and Unreferenced Files (OWASP-CM-006)|Testing for Old, Backup and Unreferenced Files]] which deals with the security issues related to "forgotten" files.
+
+
To identify files having a given extensions a mix of techniques can be employed
. THese techniques can include
Vulnerability Scanners, spidering and mirroring tools, manually inspecting the application (this overcomes limitations in automatic spidering), querying search engines (see [[Testing: Spidering and googling]]). See also [[Testing for Old, Backup and Unreferenced Files (OWASP-CM-006)|Testing for Old, Backup and Unreferenced Files]] which deals with the security issues related to "forgotten" files.
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SHELL~1.PHP will be expanded and returned by the OS shell, then processed by the PHP ISAPI handler
SHELL~1.PHP will be expanded and returned by the OS shell, then processed by the PHP ISAPI handler
</pre>
</pre>
+
==Gray Box testing and example==
==Gray Box testing and example==
−
Performing white box testing against file extensions handling amounts to checking the configurations of web
server(s)/
application
server(s)
taking part in the web application architecture, and verifying how they are instructed to serve different file extensions.
+
Performing white box testing against file extensions handling amounts to checking the configurations of web
servers or
application
servers
taking part in the web application architecture, and verifying how they are instructed to serve different file extensions
.
+
+
+
If the web application relies on a load-balanced, heterogeneous infrastructure, determine whether this may introduce different behavior
.
−
If the web application relies on a load-balanced, heterogeneous infrastructure, determine whether this may introduce different behaviour.
==References==
==References==
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'''Tools'''<br>
'''Tools'''<br>
−
Vulnerability scanners, such as Nessus and Nikto check for the existence of well-known web directories. They may allow
as well
to download the web site structure, which is helpful when trying to determine the configuration of web directories and how individual file extensions are served. Other tools that can be used for this purpose include:
+
Vulnerability scanners, such as Nessus and Nikto check for the existence of well-known web directories. They may allow
the tester
to download the web site structure, which is helpful when trying to determine the configuration of web directories and how individual file extensions are served. Other tools that can be used for this purpose include:
* wget - http://www.gnu.org/software/wget
* wget - http://www.gnu.org/software/wget
* curl - http://curl.haxx.se
* curl - http://curl.haxx.se
* google for “web mirroring tools”.
* google for “web mirroring tools”.