Mod Security Rules

Configuration

The basic modsecurity.conf looks like the following code:

<IfModule mod_security.c> # Turn the filtering engine On or Off SecFilterEngine On # The audit engine works independently and can be turned # On or Off on a per-server or per-directory basis SecAuditEngine RelevantOnly # Make sure that URL encoding is valid SecFilterCheckURLEncoding On # Unicode encoding check SecFilterCheckUnicodeEncoding On # Only allow bytes from this range SecFilterForceByteRange 1 255 # Cookie format checks. SecFilterCheckCookieFormat On # The name of the audit log file SecAuditLog logs/audit_log # Should mod_security inspect POST payloads SecFilterScanPOST On # Default action set SecFilterDefaultAction "deny,log,status:500" </IfModule>

Now, let’s look at some basic configuration directives:

• SecFilterEngine: When set to On (that is, SecFilterEngine On), it starts monitoring requests. It is Off (disabled) by default.


• SecFilterScanPOST: When On, enables scanning the request body/POST payload.


• SecFilterScanOutput: When On, enables scanning the response body also.

Similarly, to check URL encoding, you can use SecFilterCheckURLEncoding; to control request body buffering, use SecRequestBodyAccess; to control what happens once the response body limit is reached, use SecResponseBodyLimitAction; and to specify the response body buffering limit, use SecResponseBodyLimit.

The full list of configuration directives, their usage and syntax is at available on modsecurity.org.

Rules — the basics

The mod_security rule engine is where gathered data is checked for any malicious or particular content. Rules are directives in the configuration file that decide what to do with the data parsed by the configuration directives. The rule language is a vast topic; we’ll only discuss basic rule-writing syntax, and rule directives to secure Web applications from all the attacks we’ve discussed so far.

The main directive used to create rules is SecRule, whose syntax is as follows:

SecRule VARIABLES OPERATOR [ ACTIONS]

• VARIABLES: Specify which places to check in an HTTP transaction. mod_securitypreprocesses raw transaction data, making it easy for rules to focus on the logic of detection. Currently, variables are divided into request, server, and response variables, parsing flags and time variables. You can use multiple variables in a single rule with the | operator.


• OPERATORS: Specify a regular expression, pattern or keyword to be checked in the variable(s). There are four types of operators: string-matching, numerical, validation and miscellaneousoperators. Operators always begin with a @ character, and are always followed by a space.


• ACTIONS: Specify what to do if the rule evaluates to “true” — step on to another rule, display an error message, or any other task. Actions are divided into seven categories: disruptive,flow, metadata, variable, logging, special and miscellaneous actions.

Here is a simple example of a rule:

SecRule ARGS|REQUEST_HEADERS "@rx <script" id:101,msg: 'XSS Attack', severity:ERROR,deny,status:404

Here, ARGS and REQUEST_HEADERS are variables (request parameters and request headers, respectively); @rx is the operator used to match a pattern in the variables (here, this pattern is<script); id, msg, severity, deny and status are all actions to be performed if the pattern is matched. This rule is used to avoid XSS attacks by checking for a <script pattern in the request parameters and header, and generates an 'XSS Attack' message. The id:101 is given to the rule; it will deny any matching request with a 404 status response.

Let’s look at another example, for more clarity:

SecRule ARGS:username "@streq admin" chain,deny SecRule REMOTE_ADDR "!@streq 192.168.1.1"

This is an example of chaining two rules, and the transfer of control to another rule if the first rule holds true. The first rule checks for the string admin in the request’s username parameter. If found, the second rule will be activated, which denies all such requests that are not from the192.168.1.1 IP address. Thus, chaining rules help to create complex rules.

Now, writing filtering rules for each attack will be very cumbersome, and also prone to human error. Here, mod_security provides users with another directive, SecFilter. This looks for a keyword in the request. It will be applied to the first line of the request (the one that looks like GET /index.php?parameter=value HTTP/1.0). In case of POST requests, the body of the request will be searched too (provided request body buffering is enabled). All pattern matches are case-insensitive, by default. The syntax for SecFilter is SecFilter KEYWORD.

Rules against major attacks

Let’s look at some rules to prevent major attacks on Web applications.

SQL injection

Suppose you have an application that is vulnerable to SQL-injection attacks. An attacker could try to delete all records from a MySQL table, like this:

http://www.example.com/login.php?user=arpit';DELETE%20FROM%20users--

This can be prevented with the following directive:

SecFilter "delete[[:space:]]+from"

Whenever such a request is caught by the filter, something similar to the following code is logged to audit_log:

======================================== Request: 192.168.0.207 - - [04/Jul/2006:23:43:00 +1200] "GET /login.php?user=tom';DELETE%20FROM%20users-- HTTP/1.1" 500 1215 Handler: (null) ---------------------------------------- GET /login.php?user=arpit';DELETE%20FROM%20users-- HTTP/1.1 Host: 192.168.0.100 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4 Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive mod_security-message: Access denied with code 500. Pattern match "delete[[:space:]]+from" at THE_REQUEST mod_security-action: 500 HTTP/1.1 500 Internal Server Error Last-Modified: Fri, 21 Oct 2005 14:30:18 GMT ETag: "8238-4bf-833a5280" Accept-Ranges: bytes Content-Length: 1215 Connection: close Content-Type: text/html

In response to the attack, SecFilterDefaultAction is applied (the request is denied, logged, and the attacker gets a 500 error). If you want a different action to take place (like, redirect the request to a HTML page that can provide customised warning content), you can specify this in the rule, as follows:

SecFilter "delete[[:space:]]+from" log,redirect:http://example.com/invalid_request.html

To prevent more SQL injection attacks, we can add a few other directives like:

SecFilter "insert[[:space:]]+into" SecFilter "select.+from" SecFilter "drop[[:space:]]table" SecFilter create[[::space:]]+table SecFilter update.+set.+= SecFilter union.+select SecFilter or.+1[[:space:]]*= [[:space:]]1 SecFilter '.+-- SecFilter xp_enumdsn SecFilter xp_cmdshell SecFilter xp_regread SecFilter xp_regwrite SecFilter xp_regdeletekey

The last five are particularly used for MS SQL server-specific injection attacks.

The only problem with SecFilter is that it scans the whole request instead of particular fields. Here, SecFilterSelective is useful; it allows you to choose exactly what to search. The syntax is:

SecFilterSelective LOCATION KEYWORD [ACTIONS]

Here, LOCATION decides which area of the request to be filtered. Hence, for SQL injection, you can also use:

SecFilterSelective SCRIPT_FILENAME "login.php" chain SecFilterSelective ARG_user "!^[a-zA-Z0-9\.@!]{1,10}$"

The above code will validate the user parameter, and allow only the white-list of characters we have given. If for some reason you cannot take this approach, and must use a deny-what-is-badmethod, then at least remove single quotes ('), semicolons (;), dashes, hyphens (-), and parenthesis (()).

XSS attacks

For XSS attacks, we can use the following directives:

SecFilter "<(.|\n)+>" SecFilter "<[[:space:]]*script" SecFilter "<script" SecFilter "<.+>"

And also, some additional filters like:

SecFilterSelective THE_REQUEST "<[^>]*meta*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*style*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*script*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*iframe*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*object*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*img*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*applet*\"?[^>]*>" SecFilterSelective THE_REQUEST "<[^>]*form*\"?[^>]*>"

Though these filters will detect a large number of XSS attacks, they are not foolproof. Due to the multitude of different scripting languages, it is possible for an attacker to create many different methods for implementing an XSS attack that would bypass these filters. Hence, here it is advised that you also keep on adding your own filters.

To protect against an XSS attack done via PHP session cookies, you can use the following:

SecFilterSelective ARG_PHPSESSID "!^[0-9a-z]*$" SecFilterSelective COOKIE_PHPSESSID "!^[0-9a-z]*$"

Command execution attacks

For command execution attacks, you can use the following directives:

SecFilter /etc/password SecFilter /bin/ls

Here, the attacker may try to use a string like /bin/./sh to bypass the filter — butmod_security automatically reduces /./ to / and // to /, and also decodes URL-encoded characters. You can also use the white-list approach:

SecFilterSelective SCRIPT_FILENAME "directory.php" chain SecFilterSelective ARG_dir "!^[a-zA-Z/_-\.0-9]+$"

This chained rule-set will only allow letters, numbers, underscore, dash, forward slash, and period in the dir parameter. Filtering out command directory names is also a good option, and can be done as follows:

SecFilterSelective THE_REQUEST "/^(etc|bin|sbin|tmp|var|opt|dev|kernel)$/" SecFilterSelective ARGS "bin/"

Session fixation

During session fixation, in one of its phases, the attacker needs to somehow inject the desired session ID into the victim’s browser. We can mitigate these issues by implementing the following:

# Weaker XSS protection, but allows common HTML tags SecFilter "<[[:space:]]*script" # Prevent XSS attacks (HTML/Javascript injection) SecFilter "<.+>" # Block passing Cookie/Session IDs in the URL SecFilterSelective THE_REQUEST "(document\.cookie|Set-Cookie|SessionID=)"

Directory traversal attacks

For path/directory traversal attacks, the following directives are mostly used:

SecFilter "\.\./" SecFilterSelective SCRIPT_FILENAME "/scripts/foo.cgi" chain SecFilterSelective ARG_home "!^[a-zA-Z].{15,}\.txt"

The last two filters are chained, and will reject all parameters to the home argument that is a filename of more than 15 alpha characters, and that doesn’t have a .txt extension.

Similarly, you can prevent predictable resource location attacks also, and protect against sensitive file misuse, with two recommended solutions. First, remove files that are not intended for public viewing from all Web server-accessible directories. After this, you can create security filters to identify if someone probes for these files:

SecFilterSelective REQUEST_URI "^/(scripts|cgi-local|htbin|cgibin|cgis|win-cgi|cgi-win|bin)/" SecFilterSelective REQUEST_URI ".*\.(bak|old|orig|backup|c)$"

These two filters will deny access to both — unused, but commonly scanned for directories, and files with common backup extensions.

Web pages that are dynamically created by the directory-indexing function will have a title that starts with “Index of /”. We can use this as a signature, and add the following directives to catch and deny access to this data:

SecFilterScanOutput On SecFilterSelective OUTPUT "\<title\>Index of /"

Information leakage

Here, we are introduced to the OUTPUT filtering capabilities of mod_security, which you should enable by adding SecFilterScanOutput On in the configuration file. We can easily set up a filter to watch for common database error messages being sent to the client, and then generate a generic 500 status code instead of the verbose message:

SecFilterScanOutput On SecFilterSelective OUTPUT "An Error Has Occurred" status:500 SecFilterSelective OUTPUT "Fatal error:"

Output filtering can also be used to detect successful intrusions. These rules will monitor output, and detect typical keywords resulting from a command execution on the server.

SecFilterSelective OUTPUT "Volume Serial Number" SecFilterSelective OUTPUT "Command completed" SecFilterSelective OUTPUT "Bad command or filename" SecFilterSelective OUTPUT "file(s) copied" SecFilterSelective OUTPUT "Index of /cgi-bin/" SecFilterSelective OUTPUT ".*uid\=\("

Secure file uploads

mod_security is capable of intercepting files uploaded through POST requests and multi-part/form-data encoding through PUT requests. It will always upload files to a temporary directory. You can choose the directory using the SecUploadDir directive:

SecUploadDir /tmp

It is better to choose a private directory for file storage, somewhere that only the Web server user account is allowed access. Otherwise, other server users may be able to access files uploaded through the Web server. You can choose to execute an external script to verify a file before it is allowed to go through to the application. The SecUploadApproveScript directive enables this, like the following example:

SecUploadApproveScript /usr/local/apache/bin/upload_verify.pl

RFI attacks

RFI attacks are generally easy to detect, with something like the following directive:

SecRule ARGS “@rx (?i)^(f|ht)tps?://([^/])” msg:’Remote File Inclusion attack’ # To detect inclusions containing IP address SecRule ARGS "@rx (ht|f)tps?://([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])" msg:'Remote File Inclusion attack' #To detect inclusions containing PHP function ‘include()’ SecRule ARGS "@rx \binclude\s*\([\w|\s]*(ht|f)tps?://" "msg:'Remote File Inclusion'" # To detect inclusion ending with ‘?’ SecRule ARGS "@rx (ft|htt)ps?.*\?+$" msg:'Remote File Inclusion'

Miscellaneous security features

You can also block IP addresses by the following command:

SecFilterSelective "REMOTE_ADDR" "^192.168.1.1$"

If you have an input field URL in your comment form, and you want to scan the value of URL for the string c99, you do it as follows:

SecFilterSelective "ARG_url" "c99"

The following configuration helps fight HTTP fingerprinting, and accepts only valid protocol versions:

SecFilterSelective SERVER_PROTOCOL !^HTTP/(0\.9|1\.0|1\.1)$

The following configuration allows supported request methods only, and helps fight XST attacks:

SecFilterSelective REQUEST_METHOD !^(GET|HEAD|POST)$

Often during the reconnaissance phase, attackers look for the Web server identity and version. Web servers typically send their identity with every HTTP response, in the Server header. Apache is particularly helpful here; it not only sends its name and full version, by default, but also allows server modules to append their versions. Here, you can confuse the attackers by using something like:

SecServerSignature "Microsoft-IIS/5.0"

PHP code cannot be injected directly, but it may be possible to have code recorded on disk to be executed later, using an LFI attack. The following rule will detect such an injection attempt, but will ignore XML documents, which use similar syntax:

SecRule ARGS "@rx <\?(?!xml)"

Logging

There are three places where, depending on the configuration, you may find mod_securitylogging information:

• mod_security debug log: If enabled via the SecFilterDebugLevel andSecFilterDebugLog directives, it contains a large number of entries for every request processed. Each log entry is associated with a log level, which is a number from 0 (no messages at all) to 4 (maximum logging). You normally keep the debug log level at 0, and increase it only when you are debugging your rule set.


• Apache error log: Some of the messages from the debug log will make it into the Apache error log (even if you set mod_security debug log level to 0). These are the messages that require an administrator’s attention, such as information about requests being rejected.


• mod_security audit log: When audit logging is enabled (using the SecAuditEngine andSecAuditLog directives), mod_security can record each request (and its body, provided request body buffering is enabled) and the corresponding response headers.

Here is an example of an error message resulting from invalid content discovered in a cookie:

[Tue Jun 26 17:44:36 2011] [error] [client 127.0.0.1] mod_security: Access denied with code 500. Pattern match "!(^$|^[a-zA-Z0-9]+$)" at COOKIES_VALUES(sessionid) [hostname "127.0.0.1"] [uri "/test.php"] [unique_id 3434fvnij54jktynv45fC8QQQQAB]

The message indicates that the request was rejected (“Access denied”) with an HTTP 500response because the content of the cookie sessionid contained content that matched the pattern !(^$|^[a-zA-Z0-9]+$). (The pattern allows a cookie to be empty, but if it is not, it must consist only of one or more letters and digits.)

Note: I once again stress that neither LFY nor myself are responsible for the misuse of the information given here. Any attack techniques described here are meant to give you the knowledge that you need to protect your own infrastructure. Please use the tools and techniques sensibly.

This article has just scratched the surface of mod_security. For more details on rule writing and other important directives, please refer to ModSecurity Handbook by Ivan Ristic — a must-read book for anyone interested in this topic.

We will deal with other ways to secure Apache in the next article. Always remember: Know hacking, but no hacking.