This article applies to:
Question:
- How can I address false positives?
- How do I set up exception handling?
Procedure:
There are many methods of implementing Exception Handling in ModSecurity to address false positive alerts. This article outlines an approach which will dynamically modify the variable TARGET listing of a rule ID without editing the SecRule directly. This is achieved by using the ctl:ruleRemoveTargetById action.
For example, consider the following ModSecurity alert message:
[Sun Jun 30 13:45:43 2013] [error] [client 192.168.11.143] ModSecurity: Access denied with code 403 (phase 2).
Pattern match "(^[\"'\\xc2\\xb4\\xe2\\x80\\x99\\xe2\\x80\\x98;]+|[\\"'\xc2\xb4\xe2\x80\x99\xe2\x80\x98;]+$)" at ARGS:DocCopyList. [file "/etc/modsecurity/activated_rules/modsecurity_crs_41_sql_injection_attacks.conf"] [line "66"]
[id "981318"][rev "2"] [msg "SQL Injection Attack: Common Injection Testing Detected"] [data "Matched Data: \xb4 found within ARGS:DocCopyList: \xd9\x8a\xd8\xb4\xd9\x8a\xd8\xb4\xd8\xb3\xd9\x8a\xd8\xb4"]
[severity "CRITICAL"] [ver "OWASP_CRS/2.2.7"] [maturity "9"] [accuracy "8"]
[tag "OWASP_CRS/WEB_ATTACK/SQL_INJECTION"] [tag "WASCTC/WASC-19"]
[tag "OWASP_TOP_10/A1"] [tag "OWASP_AppSensor/CIE1"] [tag "PCI/6.5.2"]
[hostname "192.168.11.60"] [uri "/DMS-V2/doc_out/doc_out_new.php"] [unique_id "UdAMV8CoCzwAAHRHMhYAAAAB"]
If this alert is deemed a false positive then you need to modify the rules.
Rather than disabling this rule globally, or for the URL, we can instead modify the TARGET list to remove inspection of the ARGS:DocCopyList parameter by adding the following new rule it to a modsecurity_crs_00_custom.conf file:
SecRule REQUEST_FILENAME "@streq /DMS-V2/doc_out/doc_out_new.php" "id:1,phase:1,t:none,nolog,pass,ctl:ruleRemoveTargetById=981318;ARGS:DocCopyList"
This rule would conditionally remove inspection of ARGS:DocCopyList for rule ID 981318 for the current transaction if the client was requesting that webpage. Make sure that the modsecurity_crs_00_custom.conf file is activated in the web server configurations so that it is called up BEFORE the normal ModSecurity rules.
Notes:
For general information about ModSecurity see the Reference Manual (one source for this manual is in the Related Links section below).
