Test Application Platform Configuration (OTG-CONFIG-002)

Brief Summary
Proper configuration of the single elements that make up an application architecture is important in order to prevent mistakes that might compromise the security of the whole architecture.

Description of the issue
Configuration review and testing is a critical task in creating and maintaining such an architecture since many different systems will be usually provided with generic configurations which might not be suited to the task they will perform on the specific site they're installed on. While the typical web and application servers installation will spot a lot of functionalities (like application examples, documentation, test pages) what is not essential to and should be removed before deployment to avoid post-install exploitation.

Sample/known files and directories
Many web servers and application servers provide, in a default installation, sample application and files that are provided for the benefit of the developer and in order to test that the server is working properly right after installation. However, many default web server applications have been later known to be vulnerable. This was the case, for example, for CVE-1999-0449 (Denial of Service in IIS when the Exair sample site had been installed), CAN-2002-1744 (Directory traversal vulnerability in CodeBrws.asp in Microsoft IIS 5.0), CAN-2002-1630 (Use of sendmail.jsp in Oracle 9iAS), or CAN-2003-1172 (Directory traversal in the view-source sample in Apache’s Cocoon).

CGI scanners include a detailed list of known files and directory samples that are provided by different web or application servers and might be a fast way to determine if these files are present. However, the only way to be really sure is to do a full review of the contents of the web server and/or application server and determination of whether they are related to the application itself or not.

Comment review
It is very common, and even recommended, for programmers to include detailed comments on their source code in order to allow for other programmers to better understand why a given decision was taken in coding a given function. Programmers usually do it too when developing large web-based applications. However, comments included inline in HTML code might reveal a potential attacker internal information that should not be available to them. Sometimes, even source code is commented out since a functionality is no longer required, but this comment is leaked out to the HTML pages returned to the users unintentionally.

Comment review should be done in order to determine if any information is being leaked through comments. This review can only be thoroughly done through an analysis of the web server static and dynamic content and through file searches. It can be useful, however, to browse the site either in an automatic or guided fashion and store all the content retrieved. This retrieved content can then be searched in order to analyse the HTML comments available, if any, in the code.

Configuration review
The web server or application server configuration takes an important role in protecting the contents of the site and it must be carefully reviewed in order to spot common configuration mistakes. Obviously, the recommended configuration varies depending on the site policy, and the functionality that should be provided by the server software. In most cases, however, configuration guidelines (either provided by the software vendor or external parties) should be followed in order to determine if the server has been properly secured. It is impossible to generically say how a server should be configured, however, some common guidelines should be taken into account:


 * Only enable server modules (ISAPI extensions in the IIS case) that are needed for the application. This reduces the attack surface since the server is reduced in size and complexity as software modules are disabled. It also prevents vulnerabilities that might appear in the vendor software affect the site if they are only present in modules that have been already disabled.
 * Handle server errors (40x or 50x) with custom made pages instead of with the default web server pages. Specifically make sure that any application errors will not be returned to the end-user and that no code is leaked through these since it will help an attacker. It is actually very common to forget this point since developers do need this information in pre-production environments.
 * Make sure that the server software runs with minimised privileges in the operating system. This prevents an error in the server software from directly compromising the whole system. Although an attacker could elevate privileges once running code as the web server.
 * Make sure the server software logs properly both legitimate access and errors.
 * Make sure that the server is configured to properly handle overloads and prevent Denial of Service attacks. Ensure that the server has been performance tuned properly.

Logging
Logging is an important asset of the security of an application architecture since it can be used to detect flaws in applications (users constantly trying to retrieve a file that does not really exist) as well as sustained attacks from rogue users. Logs are typically properly generated by web and other server software but it is not so common to find applications that properly log their actions to a log and, when they do, they main intention of the application logs is to produce debugging output that could be used by the programmer to analyse a particular error.

In both cases (server and application logs) several issues should be tested and analysed based on the log contents:


 * 1) Do the logs contain sensitive information?
 * 2) Are the logs stored in a dedicated server?
 * 3) Can log usage generate a Denial of Service condition?
 * 4) How are they rotated? Are logs kept for the sufficient time?
 * 5) How are logs reviewed? Can administrators use these reviews to detect targeted attacks?
 * 6) How are log backups preserved?
 * 7) Is the data being logged data validated (min/max length, chars etc) prior to being logged?

Sensitive information in logs

Some applications might, for example use GET requests to forward form data which will be viewable in the server logs. This means that server logs might contain sensitive information (such as usernames as passwords, or bank account details). This sensitive information can be misused by an attacker if logs were to be obtained by an attacker, for example, through administrative interfaces or known web server vulnerabilities or misconfiguration (like the well-known server-status misconfiguration in Apache-based HTTP servers ).

Also, in some jurisdictions, storing some sensitive information in log files, such as personal data, might oblige the enterprise to apply the data protection laws that they would apply to their back-end databases to log files too. And failure to do so, even unknowingly, might carry penalties under the data protection laws that apply.

Log location
Typically, servers will generate local logs of their actions and errors, consuming disk of the system the server is running on. However, if the server is compromised, its logs can be wiped out by the intruder to clean up all the traces of its attack and methods. If this were to happen the system administrator would have no knowledge of how the attack occurred or what the attack source was located. Actually, most attacker toolkits include a log zapper that is capable to clean up any logs that hold a given information (like the IP address of the attacker) and are routinely used in attacker’s system-level rootkits.

Consequently, it is wiser to keep logs in a separate location and not in the web server itself. This also makes it easier to aggregate logs from different sources that refer to the same application (such as those of a web server farm) and it also makes it easier to do log analysis (which can be CPU intensive) without affecting the server itself.

Log storage
Logs can introduce a Denial of Service condition if they are not properly stored. Obviously, any attacker with sufficient resources, could be able to, unless detected and blocked, to produce a sufficient number of requests that would fill up the allocated space to log files. However, if the server is not properly configured, the log files will be stored in the same disk partition as the one used for the operating system software or the application itself. This means that, if the disk were to be filled up, the operating system or the application might fail because they are unable to write on disk.

Typically, in UNIX systems logs will be located in /var (although some server installations might reside in /opt or /usr/local) and it is thus important to make sure that the directories that logs are stored at are in a separate partition. In some cases, and in order to prevent the system logs to be affected, the log directory of the server software itself (such as /var/log/apache in the Apache web server) should be stored in a dedicated partition.

This is not to say that logs should be allowed to grow to fill up the filesystem they reside in. Growth of server logs should be monitored in order to detect this condition since it may be indicative of an attack.

Testing this condition is as easy as, and as dangerous in production environments, as firing off a sufficient and sustained number of requests to see if these requests are logged and, if so, if there is a possibility to fill up the log partition through these requests. In some environments where QUERY_STRING parameters are also logged regardless of whether they are produced through GET or POST requests, big queries can be simulated that will fill up the logs faster since, typically, a single request will cause only a small amount of data to be logged: date and time, source IP address, URI request, and server result.

Log rotation
Most servers (but few custom applications) will rotate logs in order to prevent them from filling up the filesystem they reside on. The assumption when rotating logs is that the information in them is only necessary for a limited amount of time.

This feature should be tested in order to ensure that:


 * Logs are kept for the time defined in the security policy, not more and not less.
 * Logs are compressed once rotated (this is a convenience, since it will mean that more logs will be stored for the same available disk space)
 * Filesystem permission of rotated log files are the same (or stricter) that those of the log files itself. For example, web servers will need to write to the logs they use but they don’t actually need to write to rotated logs which means that the permissions of the files can be changed upon rotation to preventing the web server process from modifying these.

Some servers might rotate logs when they reach a given size. If this happens, it must be ensured that an attacker cannot force logs to rotate in order to hide its tracks.

Log review
Review of logs can be used for more that extraction of usage statistics of files in the web servers (which is typically what most log-based application will focus on) but also to determine if attacks take place at the web server.

In order to analyse web server attacks the error log files of the server need to be analysed. Review should concentrate on:


 * 40x (not found) error messages, a large amount of these from the same source might be indicative of a CGI scanner tool being used against the web server
 * 50x (server error) messages. These can be an indication of an attacker abusing parts of the application which fail unexpectedly. For example, the first phases of a SQL injection attack will produce these error message when the SQL query is not properly constructed and its execution fails on the backend database.

Log statistics or analysis should not be generated, nor stored, in the same server that produces the logs. Otherwise, an attacker might, through a web server vulnerability or improper configuration, gain access to them and retrieve similar information as the one that would be disclosed by log files themselves.