DOC: developing filters without DoS

DEVELOP

@@ -41,8 +41,91 @@ Filters
   example.com/example.org used for DNS names
 * Ensure ./fail2ban-regex testcases/files/logs/{samplelog} config/filter.d/{filter}.conf
   has matches for EVERY regex
-* Ensure regexs end with a $ and are restrictive as possible. E.g. not .* if
- [0-9]+ is sufficient
+* Ensure regexs start with a ^ and are restrictive as possible. E.g. not .* if
+  \d+ is sufficient
+* Use the functionality of regexs http://docs.python.org/2/library/re.html
+* Take a look at the source code of the application. You may see optional or
+  extra log messages, or parts there of, that need to form part of your regex.
+
+If you only have a basic knowledge of regular repressions read
+http://docs.python.org/2/library/re.html first.
+
+Filter Security
+---------------
+
+Poor filter regular expressions are suseptable to DoS attacks.
+
+When a remote user has the ability to introduce text that will match the 
+filter regex such that the inserted text matches the <HOST> part they have the
+ability to deny any host they choose.
+
+So the <HOST> part must be anchored on text generated by the application and not
+the user. Ideally this should anchor to the beginning and end of the log line
+however as more applications log at the beginning than the end, achoring the 
+beginning is more important.
+
+When creating a regex that extends back to the begining remember the date part
+has been removed within fail2ban so theres no need to match that. If the format
+is like '<date...> error 1.2.3.4 is evil' then you will need to match the < at
+the start so here the regex would start like '^<> <HOST> is evil$'.
+
+Some applications log spaces at the end. If you're not sure add \s*$ as the
+end part of the regex.
+
+Examples of poor filters
+------------------------
+
+1. Too restrictive
+
+We find a log message:
+
+    Apr-07-13 07:08:36 Invalid command fial2ban from 1.2.3.4
+
+We make a failregex
+
+    ^Invalid command \S+ from <HOST>
+
+Now think evil. The user does the command 'blah from 1.2.3.44'
+
+The program diliently logs:
+
+    Apr-07-13 07:08:36 Invalid command blah from 1.2.3.44 from 1.2.3.4
+
+And fail2ban matches 1.2.3.44 as the IP that it ban. A DoS attack was successful.
+
+The fix here is that the command can be anything so .* is approprate.
+
+    ^Invalid command .* from <HOST>
+
+Here the .* will match until the end of the string. Then realise it has more to
+match, i.e. "from <HOST>" and go back until it find this. Then it will ban
+1.2.3.4 correctly. Since the <HOST> is always at the end, end the regex witha $
+
+    ^Invalid command .* from <HOST>$
+
+Note if we'd just had the expression:
+
+    ^Invalid command \S+ from <HOST>$
+
+Then provided the user put a space in their command they would have never been
+banned.
+
+2. Applicaiton generates two identical log messages with different meanings
+
+If the application generates the following two messages under different
+circmstances:
+
+    client <IP>: authentication failed
+    client <USER>: authentication failed
+
+
+Then its obvious that a regex of "^client <HOST>: authentication
+failed$" will still cause problems if the user can trigger the second
+log message with a <USER> of 123.1.1.1.
+
+Here there's nothing to do except request/change the application so it logs
+messages differently.
+
 
 Code Testing
 ============