ModSecurity™ 2.x allows rules to be placed in one of the following five phases:
Request headers (REQUEST_HEADERS)
Request body (REQUEST_BODY)
Response headers (RESPONSE_HEADERS)
Response body (RESPONSE_BODY)
Below is a diagram of the standard Apache Request Cycle. In the diagram, the 5 ModSecurity™ processing phases are shown.
In order to select the phase a rule executes during, use the phase
action either directly in the rule or in using the
SecDefaultAction "log,pass,phase:2" SecRule REQUEST_HEADERS:Host "!^$" "deny,phase:1"
Keep in mind that rules are executed according to phases, so even
if two rules are adjacent in a configuration file, but are set to
execute in different phases, they would not happen one after the other.
The order of rules in the configuration file is important only within
the rules of each phase. This is especially important when using the
LOGGING phase is special. It is executed at
the end of each transaction no matter what happened in the previous
phases. This means it will be processed even if the request was
intercepted or the
allow action was used to pass the
Rules in this phase are processed immediately after Apache completes reading the request headers (post-read-request phase). At this point the request body has not been read yet, meaning not all request arguments are available. Rules should be placed in this phase if you need to have them run early (before Apache does something with the request), to do something before the request body has been read, determine whether or not the request body should be buffered, or decide how you want the request body to be processed (e.g. whether to parse it as XML or not).
Rules in this phase can not leverage Apache scope directives (Directory, Location, LocationMatch, etc...) as the post-read-request hook does not have this information yet. The exception here is the VirtualHost directive. If you want to use ModSecurity™ rules inside Apache locations, then they should run in Phase 2. Refer to the Apache Request Cycle/ModSecurity™ Processing Phases diagram.
This is the general-purpose input analysis phase. Most of the application-oriented rules should go here. In this phase you are guaranteed to have received the request arguments (provided the request body has been read). ModSecurity™ supports three encoding types for the request body phase:
application/x-www-form-urlencoded - used to
transfer form data
multipart/form-data - used for file
text/xml - used for passing XML data
Other encodings are not used by most web applications.
This phase takes place just before response headers are sent back to the client. Run here if you want to observe the response before that happens, and if you want to use the response headers to determine if you want to buffer the response body. Note that some response status codes (such as 404) are handled earlier in the request cycle by Apache and my not be able to be triggered as expected. Additionally, there are some response headers that are added by Apache at a later hook (such as Date, Server and Connection) that we would not be able to trigger on or sanitize. This should work appropriately in a proxy setup or within phase:5 (logging).
This is the general-purpose output analysis phase. At this point you can run rules against the response body (provided it was buffered, of course). This is the phase where you would want to inspect the outbound HTML for information disclosure, error messages or failed authentication text.
This phase is run just before logging takes place. The rules placed into this phase can only affect how the logging is performed. This phase can be used to inspect the error messages logged by Apache. You cannot deny/block connections in this phase as it is too late. This phase also allows for inspection of other response headers that weren't available during phase:3 or phase:4. Note that you must be careful not to inherit a disruptive action into a rule in this phase as this is a configuration error in ModSecurity™ 2.5.0 and later versions.