Category:OWASP ModSecurity Core Rule Set Project

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ModSecurity™ is a web application firewall engine that provides very little protection on its own. In order to become useful, ModSecurity™ must be configured with rules. In order to enable users to take full advantage of ModSecurity™ out of the box, Trustwave's SpiderLabs is providing a free certified rule set for ModSecurity™ 2.x. Unlike intrusion detection and prevention systems, which rely on signatures specific to known vulnerabilities, the Core Rules provide generic protection from unknown vulnerabilities often found in web applications, which are in most cases custom coded. The Core Rules are heavily commented to allow it to be used as a step-by-step deployment guide for ModSecurity™.

Core Rules Content

In order to provide generic web applications protection, the Core Rules use the following techniques:

  • HTTP Protection - detecting violations of the HTTP protocol and a locally defined usage policy.
  • Real-time Blacklist Lookups - utilizes 3rd Party IP Reputation
  • Web-based Malware Detection - identifies malicious web content by check against the Google Safe Browsing API.
  • HTTP Denial of Service Protections - defense against HTTP Flooding and Slow HTTP DoS Attacks.
  • Common Web Attacks Protection - detecting common web application security attack.
  • Automation Detection - Detecting bots, crawlers, scanners and other surface malicious activity.
  • Integration with AV Scanning for File Uploads - detects malicious files uploaded through the web application.
  • Tracking Sensitive Data - Tracks Credit Card usage and blocks leakages.
  • Trojan Protection - Detecting access to Trojans horses.
  • Identification of Application Defects - alerts on application misconfigurations.
  • Error Detection and Hiding - Disguising error messages sent by the server.
ModSecurity Logo 2011.JPG
SpiderLabs Logo 2011.JPG

Let's talk here

Asvs-bulb.jpgModSecurity Communities

Further development of ModSecurity and the Core Rule Set occurs through mailing list discussions and occasional workshops, and suggestions for improvement are welcome. For more information, please contact us.

<paypal>ModSecurity Core Rule Set Project</paypal>

Want to help?

Asvs-waiting.JPGCRS Development

The CRS project is always on the lookout for volunteers who are interested in contributing. We need help in the following areas:

  • Documentation of the CRS
  • New Detection Methods
  • Updates to existing rules

Related resources

Asvs-satellite.jpgOWASP Resources


SVN Repository is here:

Sync with SVN:

svn co crs

CRS Releases are signed by Ryan Barnett. These public keys are available via most PGP key server mirrors.

Manual Downloading: You can always download the latest CRS version here -

Automated Downloading: Use the script in the CRS /util directory

  1. Get a list of what the repository contains:
$ ./ -r -l
modsecurity-crs {
  1. Get the latest stable version of "modsecurity-crs":
$ ./ -r -prules -Smodsecurity-crs
Fetching: modsecurity-crs/ ...
$ ls -R rules


Bug Tracker

JIRA Ticket System:


ModSecurity CRS Demonstration/Smoketest page:


Quick Start

Core Rule Set Quick Setup


To activate the rules for your web server installation:

 1) The modsecurity_crs_10_config.conf includes management rules and directives
    that can control important CRS functions. Pay attention to
    the SecRuleEngine setting (On by default) and that the SecDefaultAction
    directive is set to "pass".  The 49 inbound blocking and 59 outbound blocking
    rules files use the "block" action which
    inherits this setting.  The effectively means that you can toggle the
    SecDefaultAction setting to decide if you would like to deny on an
    anomaly scoring/correlation match.
    Update the PARANOID_MODE variable setting if you want to become more 
    aggressive in your detection.  Caution - this will cause more false positives.
    Should also update the appropriate anomaly scoring levels that will be propagated
    to the inbound/outbound blocking files.
    Update the TX policy settings for allowed Request Methods, File Extensions, etc...
 2) Add the following line to your httpd.conf (assuming
    you've placed the rule files into conf/modsecurity_crs/):
       <IfModule security2_module>
               Include conf/modsecurity_crs/*.conf
               Include conf/modsecurity_crs/base_rules/*.conf
 3) Restart web server.
 4) Make sure your web sites are still running fine.
 5) Simulate an attack against the web server. Then check
    the attack was correctly logged in the Apache error log,
    ModSecurity debug log (if you enabled it) and ModSecurity
    audit log (if you enabled it).


Each ModSecurity Rule in the CRS has an individual rule description page based on the following template file:


The ModSecurity Core Rule Set is provided to you under the terms and conditions of GPL version 2

This directory contains the files for Core ModSecurity Rule Set The rules are compatible with ModSecurity 2.5 (as of version 1.4.3)

Using ModSecurity requires rules. In order to enable users to take full advantage of ModSecurity immediately, Trustwave is providing a free Core rule set. Unlike intrusion detection and prevention systems which rely on signature specific to known vulnerabilities, the Core Rule Set provides generic protection from unknown vulnerabilities often found in web application that are in most cases custom coded. This is what we call "Attack Payload Detection."

Keep in mind that a predefined rule set is only part of the work required to protect your web site. We strongly urge you to consult Ivan Ristic's book, "ModSecurity Handbook" and the ModSecurity Reference Manual - The CRS is heavily commented to allow it to be used as a step-by-step deployment guide for ModSecurity.

For more information refer to the OWASP Core Rule Set Project page at

Core Rules Mail-list - Suscribe here: Archive:

CRS < 2.0 - Self-Contained Rules

Older (<2.0) CRS used individual, “self-contained” actions in rules

- If a rule triggered, it would either deny or pass and log
- No intelligence was shared between rules

Not optimal from a rules management perspective (handling false positives/exceptions)

- Editing the regex could blow it up
- Typical method was to copy/paste rules into custom rules files and then edit rule logic
  and disable core rule ID.
- Heavily customized rules were less likely to be updated by the user

Not optimal from a security perspective

- Not every site had the same risk tolerance
- Lower severity alerts were largely ignored
- Individual low severity alerts are not important but several low severity events
  in the same transaction are.

Rules - Detection and Management

Rules logic has changed by decoupling the inspection/detection from the blocking functionality

- Rules log.pass and set transactional variables (tx) to track anomaly scores and to 
  store meta-data about the rule match
- This TX rule match data can be used by other 3rd party rules (converter Emerging Threats
  Snort web attack rules) to more accurately correlate identified attacks with their
  attack vector locations.
- TX data of previous strong rule matches can also be used to conditionally apply weaker signatures
  that normally would have a high fasle positive rate.
- Rules also increase anomaly scores for both the attack category and global score which allows
  users to set a threshold that is appropriate for them.
- This also allows several low severity events to trigger alerts while individual ones are suppressed.
- Exceptions may be handled by either increasing the overall anomaly score threshold, or
  by adding rules to a local custom exceptions file where TX data of previous rule matches
  may be inspected and anomaly scores re-adjusted based on the false positive criteria.

User can now globally update which variables to inspect and the anomaly score settings in the modsecurity_crs_10_config.conf file.

- PARANOID_MODE setting which will apply rules to locations that have a higher false positive rate
- INBOUND_ANOMALY_SCORE setting will be populated in the inbound blocking file and if a transaction
  score at the end of phase:2 is equal to or greater than this number, it will be denied.
- OUTBOUND_ANOMALY_SCORE setting will be populated in the outbound blocking file and it a transaction
  score at the end of phase:4 is equal to or greater than this number, it will be denied.

The CRS rules themselves are configured with the pass action, which allows all the rules to be processed and for the proposed anomaly scoring/collaborative detection concept to work. The inbound/outbound anomaly score levels may be set in the modsecurity_crs_10_config.conf file. These scores will be evaluated in the modsecurity_crs_49_inbound_blocking.conf and modsecurity_crs_59_outbound_blocking.conf files.

One of the top feedback items we have heard is that the CRS events in the Apache error_log file were very chatty. This was due to each rule triggering its own error_log entry. What most people wanted was for 1 correlated event to be generated that would give the user a higher level determination as to what the event category was.

To that end- each CRS rule will generate an audit log event Message entry but they will not log to the error_log on their own. These rules are now considered basic or reference events and may be reviewed in the audit log if the user wants to see what individual events contributed to the overall anomaly score and event designation.

After the transaction has completed (in the logging phase), the rules in the base_rules/modsecurity_crs_60_correlation.conf file will conduct further post-processing by analyzing any inbound events with any outbound events in order to provide a more intelligent/priority correlated event.

- Was there an inbound attack?
- Was there an HTTP Status Code Error (4xx/5xx level)?
- Was there an application information leak?

If an inbound attack was detected and either an outbound application status code error or infolead was detected, then the overall event severity is raised -

- 0: Emergency - is generated from correlation where there is an inbound attack and
  an outbound leakage.
- 1: Alert - is generated from correlation where there is an inbound attack and an
  outbound application level error.

In order to provide generic web applications protection, the Core Rule Set uses the following techniques:

HTTP Protocol Validation and Protection

Detecting violations of the HTTP protocol and a locally defined usage policy. This first line of protection ensures that all abnormal HTTP requests are detected. This line of defense eliminates a large number of automated and non targeted attacks as well as protects the web server itself.


Protocol vulnerabilities such as Response Splitting, Request Smuggling, Premature URL ending

- Content length only for non GET/HEAD methods 
- Non ASCII characters or encoding in headers
- Valid use of headers (for example, content length is numerical)
- Proxy Access


Attack requests are different due to automation

- Missing headers such as Host, Accept, User-Agent
- Host is an IP address (common worm propagation method)


Policy is usually application specific

- Some restrictions can usually be applied generically
- White lists can be build for specific environments
  Limitations on Sizes
- Request size, Upload size
- # of parameters, length of parameter


Items that can be allowed or restricted

- Methods - Allow or restrict WebDAV, block abused methods such as CONNECT, TRACE or DEBUG 
- File extensions – backup files, database files, ini files
- Content-Types (and to some extent other headers)

Automation Detection

Automated clients are both a security risk and a commercial risk. Automated crawlers collect information from your site, consume bandwidth and might also search for vulnerabilities on the web site. Automation detection is especially useful for generic detection of comments spam.

Detecting bots, crawlers, scanners and other surface malicious activity. Not aimed against targeted attacks, but against general malicious internet activity

- Offloads a lot of cyberspace junk & noise
- Effective against comment spam
- Reduce event count


Detection of Malicious Robots

- Unique request attributes: User-Agent header, URL, Headers
- RBL Check of IP addresses
- Detection of security scanners
- Blocking can confuse security testing software (WAFW00f)


This rules file is only relevant if you are concerned about comment SPAM attacks. The rules file will run an RBL check against the source IP address at SPAMHAUS and will cache the response for 1 day. If the client sends subsequent requests, it will be denied without having to re-run an RBL check.

This file will also look for comment SPAM posting attacks which submit URL links.

Common Web Attacks Protection

Common Web Attacks Protection Rules on the second level address the common web application security attack methods. These are the issues that can appear in any web application. Some of the issues addressed are:

- SQL Injection
- Cross-Site Scripting (XSS)
- OS Command execution
- Remote code inclusion
- LDAP Injection
- SSI Injection
- Information leak
- Buffer overflows
- File disclosure


- OS command injection and remote command access
- Remote file inclusion
- Session Fixation


The rules in this file are considered BETA quality as they have not been rigorously tested. They attempt to address advanced attacks such as HTTP Parameter Pollution or use new rule features or techniques.


This rules file attempts to identify all directory traversal variations. It is prone to a high level of false positives so set PARANOID_MODE if you want to run these rules.


- SQL injection and blind SQL injection


- Cross site scripting (XSS)



SpiderLabs received authorization from PHPIDS ( to convert their rules and include them in the CRS

- Thanks to Mario Heiderich

Converted version of PHPIDS Converter.php functionality. These rules look for common evasion tactics.

Converted version of PHPIDS default_filters.xml data.

These rules are *not* PHP-specifc and apply to *all* web platforms

- Filters are heavily tested by the community and updated frequently
- ~70 regular expression rules to detect common attack payloads
- SQL Injection



Due to the high number of rules and the possible impact on performance, these rules have been placed in the optional_rules directory.

SpiderLabs received authorization from ET to convert their Snort rules and include them in the CRS

Converted the following ET Snort rule files

- emerging-web_server.rules
- emerging-web_specific_apps.rules 

Identifying attacks against known web vulnerabilities does have value

- Raised threat level
- If done correctly, lessens false positives

The issue to overcome is that the PCRE RegExs used in the rules are pretty poor. What we want to do is to combine the *what* of our generic attack payload detection (attack payloads) with the *where* (attack vector - URL + Parameter Name) of the ET known vuln data. The approach we took was to have most of the ET rules look for the attack vector data and then simply check all saved TX data for a corresponding attack vector match.

Trojan Protection

ModSecurity Core Rule Set detects access to back doors installed on a web server. This feature is very important in a hosting environment when some of this backdoors may be uploaded in a legitimate way and used maliciously. In addition the Core Rule Set includes a hook for adding an Anti-Virus program such as ClamAV for checking file uploads.


- Check uploading of http backdoor page
- Access detection
- Known signatures (x_key header)
- Generic file management output (gid, uid, drwx, c:\)


If all fails, the Core Rule Set will detect errors sent by the web server. Detecting and blocking errors prevents attackers from collecting reconnaissance information about the web application and also server as a last line of defense in case an attack was not detected eariler.


- HTTP Error Response Status Codes
- SQL Information Leakage
- Stack Dumps
- Source Code Leakage

Request Header Tagging

This concept is similar to anti-SPAM SMTP apps that will add additional mime headers to emails providing the SPAM detection analysis information. The CRS is attempting to mimic this concept at the HTTP layer by adding additional request headers that provide insight into any ModSecurity events that may have triggered during processing. The advantage of this approach is that it allows a WAF to be in a detection-only mode while still providing attack data to the destination application server. The recieving app server may then inspect the WAF request headers and make a determination whether or not to process the transaction. This concept is valuable in distributed web environments and hosting architectures where a determination to block may only be appropriate at the destination app server.


This rules file will take all of the TX attack variable data and populate Apache ENV variables that Apache can then use to add X-WAF-Event request header data to the request.

Example showing the consolidated X-WAF-Events and X-WAF-Score data -

GET /path/to/foo.php?test=1%27%20or%20%272%27=%272%27;-- HTTP/1.1
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20091109 Ubuntu/9.10 (karmic) Firefox/3.5.5
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
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
X-WAF-Events: TX: / 999935-Detects common comment types-WEB_ATTACK/INJECTION-ARGS:test, TX:999923-Detects JavaScript location/document property access and window access obfuscation-WEB_ATTACK/INJECTION-REQUEST_URI_RAW, TX:950001- WEB_ATTACK/SQL_INJECTION-ARGS:test
X-WAF-Score: Total=48; sqli=2; xss=
Connection: Keep-Alive

Presentations and Whitepapers

Current CRS v2 File:OWASP ModSecurity Core Rule Set.ppt presented at AppSec DC 2009.

Ofer Shezaf's presentation and whitepaper on the Core Rule Set v1 presented at 6th OWASP AppSec conference in Milan, Italy, in May 2007

Related Projects

ModSecurity-Open Source Web Application Firewall
OWASP Securing WebGoat using ModSecurity
OWASP AppSensor Project

Latest News and Mail List

Current Stable Version CRS 2.1.2

Version 2.1.2 - 02/17/2011

- Added experimental real-time application profiling ruleset.
- Added experimental Lua script for profiling the # of page scripts, iframes, etc..
  which will help to identify successful XSS attacks and planting of malware links.
- Added new CSRF detection rule which will trigger if a subsequent request comes too
  quickly (need to use the Ignore Static Content rules).
Bug Fixes:
- Added missing " in the skipAfter SecAction in the CC Detection rule set

Version 2.1.0 - 12/29/2010


- Added Experimental Lua Converter script to normalize payloads. Based on
  PHPIDS Converter code and it used with the advanced filters conf file. 
- Changed the name of PHPIDS converted rules to Advanced Filters
- Added Ignore Static Content (Performance enhancement) rule set
- Added XML Enabler (Web Services) rule set which will parse XML data
- Added Authorized Vulnerability Scanning (AVS) Whitelist rule set 
- Added Denial of Service (DoS) Protection rule set
- Added Slow HTTP DoS (Connection Consumption) Protection rule set
- Added Brute Force Attack Protection rule set
- Added Session Hijacking Detection rule set
- Added Username Tracking rule set
- Added Authentication Tracking rule set
- Added Anti-Virus Scanning of File Attachments rule set
- Added AV Scanning program to /util directory
- Added Credit Card Usage Tracking/Leakage Prevention rule set
- Added experimental CC Track/PAN Leakage Prevention rule set
- Added an experimental_rules directory to hold new BETA rules
- Moved the local exceptions conf file back into base_rules dirctory however
  it has a ".example" extension to prevent overwriting customized versions
  when upgrading
- Separated out HTTP Parameter Pollution and Restricted Character Anomaly Detection rules to
  the experimental_rules directory
- Adding the REQUEST_HEADERS:User-Agent macro data to the initcol in 10 config file, which will
  help to make collections a bit more unique

Version 2.0.8 - 08/27/2010


- Updated the PHPIDS filters
- Updated the SQL Injection filters to detect boolean attacks (1<2, foo == bar, etc..)
- Updated the SQL Injection filters to account for different quotes
- Added UTF-8 encoding validation support to the modsecurity_crs_10_config.conf file
- Added Rule ID 950109 to detect multiple URL encodings
- Added two experimental rules to detect anomalous use of special characters

Bug Fixes:

- Fixed Encoding Detection RegEx (950107 and 950108)
- Fixed script to better handle whitespace
- Fixed missing pass action bug in modsecurity_crs_21_protocol_anomalies.conf
- Fixed the anomaly scoring in the modsecurity_crs_41_phpids_filters.conf file
- Updated XSS rule id 958001 to improve the .cookie regex to reduce false postives  

Version 2.0.7 - 06/4/2010


- Added CSRF Protection Ruleset which will use Content Injection to add javascript to
  specific outbound data and then validate the csrf token on subsequent requests.
- Added new Application Defect Ruleset which will identify/fix missing HTTPOnly cookie
- Added Experimental XSS/Missing Output Escaping Ruleset which looks for user supplied
  data being echoed back to user unchanged.
- Added script and configuration file to allow users to automatically
  download CRS rules from the CRS rules repository. 
- Added new SQLi keyword for ciel() and reverse() functions.
- Updated the PHPIDS filters

Bug Fixes:

- Fixed false positives for Request Header Name matching in the 30 file by 
  adding boundary characters.  
- Added missing pass actions to @pmFromFile prequalifier rules
- Added backslash to SQLi regex
- Fixed hard coded anomaly score in PHPIDS filter file 
- Fixed restricted_extension false positive by adding boundary characters 

Version 2.0.6 - 02/26/2010

Bug Fixes:

- Added missing transformation functions to SQLi rules.
- Fixed duplicate rule IDs.
- Fixed typo in @pmFromFile in the Comment SPAM rules
- Added macro expansion to Restricted Headers rule
- Fixed misspelled SecMarker
- Fixed missing chain action in Content-Type header check
- Update phpids filters to use pass action instead of block

Version 2.0.5 - 02/01/2010


- Removed previous 10 config files as they may conflict with local customized Mod configs.
- Added a new 10 config file that allows the user to globally set TX variables to turn on/off
  PARANOID_MODE inspection, set anomaly score levels and http policies.
  Must have ModSecurity 2.5.12 to use the macro expansion in numeric operators.
- Added Rule Logic and Reference links to rules descriptions.
- Added Rule IDs to all rules.
- Added tag data mapping to new OWASP Top 10 and AppSensor Projects, WASC Threat Classification
- Removed Apache limit directives from the 23 file
- Added macro expansion to 23 file checks.
- Added @pmFromFile check to 35 bad robots file
- Added malicious UA strings to 35 bad robots check
- Created an experimental rules file
- Updated HTTP Parameter Pollution (HPP) rule logic to concat data into a TX variable for inspection
- Removed TX inspections for generic attacks and reverted to standard ARGS inspection
- Updated the variable list for standard inspections (ARGS|ARGS_NAMES|XML:/*) and moved the other
- Moved converted ET Snort rules to the /optional_rules directory
- Created a new Header Tagging ruleset (optional_rules) that will add matched rule data to the
  request headers.
- Updated Inbound blocking conf file to use macro expansion from the 10 config file settings
- Added separate anomaly scores for inbound, outbound and total to be evaluated for blocking.
- Updated the regex logic in the (1=1) rule to factor in quotes and other logical operators.
- Updated the SPAMMER RBL check rules logic to only check once per IP/Day.
- Added new outbound malware link detection rules.

Bug Fixes:

- Removed Non-numeric Rule IDs
- Updated the variable list on SQLi rules.
- Fixed outbound @pmFromFile action from allow to skipAfter to allow for outbound anomaly scoring
  and blocking

Version 2.0.4 - 11/30/2009


- Updated converted PHPIDS signatures (
- Updated PHPIDS rules logic to first search for payloads in ARGS and then if there is no match found
  then search more generically in request_body|request_uri_raw
- Updated PHPIDS rules logic to only set TX variables and to not log.  This allows for more clean
  exceptions in the 48 file which can then expire/delete false positive TX matches and adjust the
  anomaly scores.  These rules will then inspect for any TX variables in phase:5 and create appropriate
  alerts for any variable matches that exist.

Bug Fixes:

- Added Anomaly Score check to the 60 correlation file to recheck the anomaly score at the end of
  phase:4 which would allow for blocking based on information leakage issues.

Project Mail List
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Contributors, Users and Adopters

Project Leader

Ryan Barnett

Project Contributors

Josh Zlatin
Brian Rectanus
Roberto Salgado

The Core Rule Set (CRS) project is sponsored by:

SpiderLabs Logo 2010.JPG

Project Users

WASC Distributed Open Proxy Honeypot Project uses the Core Rule Set -

Akamai's WAF Service is based on a previous version of the Core Rule Set -

Project About

What does this OWASP project offer you?
What releases are available for this project?
what is this project?
Name: OWASP ModSecurity Core Rule Set Project (home page)
Purpose: ModSecurity is an Apache web server module that provides a web application firewall engine. The ModSecurity Rules Language engine is extrememly flexible and robust and has been referred to as the "Swiss Army Knife of web application firewalls." While this is certainly true, it doesn't do much implicitly on its own and requires rules to tell it what to do. In order to enable users to take full advantage of ModSecurity out of the box, we have developed the Core Rule Set (CRS) which provides critical protections against attacks across most every web architecture.

Unlike intrusion detection and prevention systems, which rely on signatures specific to known vulnerabilities, the CRS is based on generic rules which focus on attack payload identification in order to provide protection from zero day and unknown vulnerabilities often found in web applications, which are in most cases custom coded.

License: Apache Software License v2 (ASLv2)
who is working on this project?
Project Leader(s):
how can you learn more?
Project Pamphlet: Not Yet Created
Project Presentation: View
Mailing list: Mailing List Archives
Project Roadmap: View
Main links:
Key Contacts
  • Contact the GPC to report a problem or concern about this project or to update information.
current release
ModSecurity 2.2.8 - 06/30/2013 - (download)
Release description: == Version 2.2.8 - 06/30/2013 ==

Security Fixes:


  • Updatd the /util directory structure
  • Added scripts to check Rule ID duplicates
  • Added script to remove v2.7 actions so older ModSecurity rules will work
  • Added new PHP rule (958977) to detect PHP exploits (Plesk 0-day from king cope)

Bug Fixes:

  • fix 950901 - word boundary added
  • fix regex error
  • Updated the Regex in 981244 to include word boundaries
  • Problem with Regression Test (Invalid use of backslash) - Rule 960911 - Test2
  • ModSecurity: No action id present within the rule - ignore_static.conf
  • "Bad robots" rule blocks all Java applets on Windows XP machines
  • duplicated rules id 981173
Rating: Projects/OWASP ModSecurity Core Rule Set Project/GPC/Assessment/ModSecurity 2.2.8
last reviewed release
ModSecurity 2.0.6 - 2010-02-26 - (download)
Release description: ModSecurity is a web application firewall that can work either embedded or as a reverse proxy. It provides protection from a range of attacks against web applications and allows for HTTP traffic monitoring, logging and real-time analysis.
Rating: Greenlight.pngGreenlight.pngGreenlight.png Stable Release - Assessment Details

other releases

The CRS is an open source rule set licensed under ASLv2. ModSecurity Core Rule Set works with ModSecurity 2.5 and above.