Anomaly Scoring Mode

OWASP CRS version 3.x allows users to quickly switch between Traditional and Anomaly Scoring detection modes. The default starting with CRS 3.x is Anomaly Scoring mode. Within the csr-setup.conf.example file there are two settings to control which mode your CRS instance will work in. Within this file, you can also control the following related CRS items:

  • Anomaly Scoring Severity Levels
  • Anomaly Scoring Threshold Levels (Blocking)
  • Enable/Disable Blocking
  • Choose the default logging actions
  • and much more!

Traditional Detection Mode

Traditional Detection Mode (or IDS/IPS mode) is the old default operating mode. This is the most basic operating mode where all of the rules are “self-contained”. In this mode there is no intelligence is shared between rules and each rule has no information about any previous rule matches. That is to say, 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 modifying the SecDefaultAction directive in the csr-setup.conf file to use a disruptive action other than the default ‘pass’, such as deny:

# Default (Anamoly Mode)
SecDefaultAction "phase:2,pass,log"
# Updated To Enable Traditional Mode
SecDefaultAction "phase:2,deny,status:403,log"

Pros and Cons of Traditional Detection Mode


  • The functionality of this mode is much easier for a new user to understand.
  • Better performance (lower latency/resources) as the first disruptive match will stop further processing.


  • Not all rules that could have triggered will be logged, only the first
  • Not every site has the same risk tolerance
  • Lower severity alerts may not trigger traditional mode
  • Single low severity alerts may not be deemed critical enough to block, but multiple lower severity alerts in aggregate could be

Anamoly Scoring Mode

Within anamoly scoring mode we are implementing the concept of Collaborative Detection and Delayed Blocking. This is to say 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) for later logging.

Configuring Anomaly Scoring Mode

The default mode in CRS 3.x is Anomaly Scoring mode, you can verify this is your mode by checking that the SecDefaultAction line in the csr-setup.conf file usees the pass action:

SecDefaultAction "phase:2,pass,log"

In this 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:

  "msg:'SQL Injection Attack Detected via LibInjection',\
   logdata:'Matched Data: %{TX.0} found within %{MATCHED_VAR_NAME}: %{MATCHED_VAR}',\

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. Below is a snippet from the above rule (id:942100) where that occurs:


This adds a variable amount, tx.critical_anomaly_score, to the current anomaly scores. The user can configure what each score represents from within the csr-setup.conf file and these scores will be propagated out for use in the rules by using macro expansion. The following is an excerpt from csr-setup.conf where that configuration is set:

# -=[ 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.
setvar:tx.critical_anomaly_score=5, \
setvar:tx.error_anomaly_score=4, \
setvar:tx.warning_anomaly_score=3, \

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:

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".

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 various different anomaly scoring thresholds to set for both specific vulnerability types and generic requests/response levels. These will be evaluated in two different files. Inbound request are evaluated at the end of phase:2 in the rules/REQUEST-49-BLOCKING-EVALUATION.conf file and outbound responses are evaluated at the end of phase:4 in the rules/RESPONSE-59-BLOCKING-EVALUATION.conf file. The thresholds are configured in the csr-setup.conf file.

SecAction \

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 Anomaly Scoring mode, if you want to allow blocking, you should set the SecRueEngine to On and then uncomment the following SecAction in the csr-setup.conf file. Note: this is done by default in CRS 3.x:

SecAction \

When this rule is enabled, The rule within the rules/REQUEST-49-BLOCKING-EVALUATION.conf and rules/RESPONSE-59-BLOCKING-EVALUATION.conf files will evaluate the anomaly scores at the end of the request/response phases and will block the request if it exceeds a given anomaly threshold. An example of such a rule is as follows:

SecRule TX:ANOMALY_SCORE "@ge %{tx.inbound_anomaly_score_threshold}" \
        "msg:'Inbound Anomaly Score Exceeded (Total Score: %{TX.ANOMALY_SCORE}, Last Matched Message: %{tx.msg}',\
                SecRule TX:ANOMALY_SCORE_BLOCKING "@streq on" chain
                        SecRule TX:/^\d+\-/ "(.*)"

Notice that there is an explicit deny within this rule. This explitly listed disruptive action will override the default action of pass (within anomaly mode)and block the transaction. If you would like a different action to occur you would set it within the two BLOCKING-EVALUATION files.

Pros and Cons of Anomaly Scoring Detection Mode


  • 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 local custom exceptions file


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