[Owasp-modsecurity-core-rule-set] rule bypass

Ivan Ristic ivan.ristic at gmail.com
Sun Jan 17 10:50:01 EST 2010


On Sun, Jan 17, 2010 at 3:43 PM, Chris Datfung <chris.datfung at gmail.com> wrote:
>> >
>> > Hi Ivan,
>> > I'll try the SecRuleRemoveByMsg. As an aside, is there a reason that
>> > ruleRemoveById is a ctl option but ruleRemoveByMsg is not?
>>
>> Yes, there is. SecRuleRemoveBy(Id|Msg) operate at configure-time and
>> affect the configuration that will be used as a starting point for all
>> requests. The ctl:ruleRemoveById action is executed on per-transaction
>> basis and can only affect the transaction in which it executes.
>>
>
> Hi Ivan,
>
> I still don't understand why you can't have a ctl:ruleRemoveByMsg action
> that is executed on a per-transaction basis.

You most certainly can. In fact, that's the only way to conditionally
remove a rule (which sounds like what you're after).


>> > Ideally, in this
>> > case, I'd like to create a single rule that first matches the effected
>> > parameter and then removes the rule based on the message. I guess I
>> > could
>> > still do that by chaining two rules together.
>>
>> Yes, that sounds likely.
>
> I've been playing around with this and have hit a dead end. I created the
> following rule:
>
> SecRule REQUEST_URI "script.cfm"
> phase:1,t:none,t:urlDecode,t:lowercase,t:normalisePath,chain
> SecRule &FILES:Filename "@gt 0" chain
> SecRuleRemoveByMsg "Attempted multipart\/form-data bypass"

SecRuleRemoveByMsg is also not a rule and cannot be chained. It's a
configuration directive. You should be using the ctl: action in your
second rule.


> but whenever that rule is uncommented I get the following error:
>
> Syntax error on line 21 of
> /opt/modsecurity/etc/crs/base_rules/modsecurity_crs_20_protocol_violations.conf:
> ModSecurity: Execution phases can only be specified by chain starter rules.

What's on line 21?

-- 
Ivan Ristic
ModSecurity Handbook [https://www.feistyduck.com]
SSL Labs [https://www.ssllabs.com/ssldb/]


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