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156 Web Application Penetration Testing Test: 408 Request Time-out telnet 80 GET / HTTP/1.1 - Wait X seconds – (Depending on the target server, 21 seconds for Apache by default) Result: HTTP/1.1 408 Request Time-out Date: Fri, 07 Dec 2013 00:58:33 GMT Server: Apache/2.2.22 (Ubuntu) PHP/5.3.10-1ubuntu3.9 with Suhosin-Patch Vary: Accept-Encoding Content-Length: 298 Connection: close Content-Type: text/html; charset=iso-8859-1 ... 408 Request Time-out ... Apache/2.2.22 (Ubuntu) PHP/5.3.10-1ubuntu3.9 with Suhosin-Patch at Port 80 ... Test: 501 Method Not Implemented telnet 80 RENAME /index.html HTTP/1.1 Host: Result: HTTP/1.1 501 Method Not Implemented Date: Fri, 08 Dec 2013 09:59:32 GMT Server: Apache/2.2.22 (Ubuntu) PHP/5.3.10-1ubuntu3.9 with Suhosin-Patch Allow: GET, HEAD, POST, OPTIONS Vary: Accept-Encoding Content-Length: 299 Connection: close Content-Type: text/html; charset=iso-8859-1 ... 501 Method Not Implemented ... Apache/2.2.22 (Ubuntu) PHP/5.3.10-1ubuntu3.9 with Suhosin-Patch at Port 80 ... Test: Enumeration of directories by using access denied error messages: http:// Result: Directory Listing Denied This Virtual Directory does not allow contents to be listed. Tools • ErrorMint - http://sourceforge.net/projects/errormint/ • ZAP Proxy - https://www.owasp.org/index.php/OWASP_Zed_ Attack_Proxy_Project References • [RFC2616] Hypertext Transfer Protocol -- HTTP/1.1 • [ErrorDocument] Apache ErrorDocument Directive • [AllowOverride] Apache AllowOverride Directive • [ServerTokens] Apache ServerTokens Directive • [ServerSignature] Apache ServerSignature Directive Remediation Error Handling in IIS and ASP .net ASP .net is a common framework from Microsoft used for developing web applications. IIS is one of the commonly used web servers. Errors occur in all applications, developers try to trap most errors but it is almost impossible to cover each and every exception (it is however possible to configure the web server to suppress detailed error messages from being returned to the user). IIS uses a set of custom error pages generally found in c:\winnt\ help\iishelp\common to display errors like ‘404 page not found’ to the user. These default pages can be changed and custom errors can be configured for IIS server. When IIS receives a request for an aspx page, the request is passed on to the dot net framework. There are various ways by which errors can be handled in dot net framework. Errors are handled at three places in ASP .net: • Inside Web.config customErrors section • Inside global.asax Application_Error Sub • At the the aspx or associated codebehind page in the Page_Error sub Handling errors using web.config mode=”On” will turn on custom errors. mode=RemoteOnly will show custom errors to the remote web application users. A user accessing the server locally will be presented with the complete stack trace and custom errors will not be shown to him. All the errors, except those explicitly specified, will cause a redirection to the resource specified by defaultRedirect, i.e., myerrorpagedefault.aspx. A status code 404 will be handled by myerrorpagefor404.aspx.
157 Web Application Penetration Testing Handling errors in Global.asax When an error occurs, the Application_Error sub is called. A developer can write code for error handling/page redirection in this sub. Private Sub Application_Error (ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Error End Sub Handling errors in Page_Error sub This is similar to application error. Private Sub Page_Error (ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Error End Sub Error hierarchy in ASP .net Page_Error sub will be processed first, followed by global.asax Application_Error sub, and, finally, customErrors section in web. config file. Information Gathering on web applications with server-side technology is quite difficult, but the information discovered can be useful for the correct execution of an attempted exploit (for example, SQL injection or Cross Site Scripting (XSS) attacks) and can reduce false positives. How to test for ASP.net and IIS Error Handling Fire up your browser and type a random page name http:\\www.mywebserver.com\anyrandomname.asp If the server returns The page cannot be found Internet Information Services it means that IIS custom errors are not configured. Please note the .asp extension. Also test for .net custom errors. Type a random page name with aspx extension in your browser http:\\www.mywebserver.com\anyrandomname.aspx If the server returns Server Error in ‘/’ Application. --------------------------------------------------------------- ----------------- The resource cannot be found. Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name custom errors for .net are not configured. Error Handling in Apache Apache is a common HTTP server for serving HTML and PHP web pages. By default, Apache shows the server version, products installed and OS system in the HTTP error responses. Responses to the errors can be configured and customized globally, per site or per directory in the apache2.conf using the Error- Document directive [2] ErrorDocument 404 “Customized Not Found error message” ErrorDocument 403 /myerrorpagefor403.html ErrorDocument 501 http:/www.externaldomain.com/errorpagefor501.html Site administrators are able to manage their own errors using .htaccess file if the global directive AllowOverride is configured properly in apache2.conf [3] The information shown by Apache in the HTTP errors can also be configured using the directives ServerTokens [4] and ServerSignature [5] at apache2.conf configuration file. “ServerSignature Off” (On by default) removes the server information from the error responses, while ServerTokens [ProductOnly|Major|Minor|Minimal|OS|Full] (Full by default) defines what information has to be shown in the error pages. Error Handling in Tomcat Tomcat is a HTTP server to host JSP and Java Servlet applications. By default, Tomcat shows the server version in the HTTP error responses. Customization of the error responses can be configured in the configuration file web.xml. 404 /myerrorpagefor404.html Testing for Stack Traces (OTG-ERR-002) Summary Stack traces are not vulnerabilities by themselves, but they often reveal information that is interesting to an attacker. Attackers attempt to generate these stack traces by tampering with the input to the web application with malformed HTTP requests and other input data. If the application responds with stack traces that are not managed it could reveal information useful to attackers. This information could then be used in further attacks. Providing debugging information as a result of operations that generate errors is considered a bad practice due to multiple reasons. For example,
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