Support #3511
closedmodify regexes not being applied reliably
Added by J Cliff Armstrong over 4 years ago. Updated over 4 years ago.
Description
from the documentation (https://suricata-update.readthedocs.io/en/latest/update.html#modifying-rules):
Example converting all drop rules with noalert back to alert:
re:. "^drop(.*)noalert(.*)" "alert\\1noalert\\2"
This fails to work on the following rule (leaves it unmodified):
drop tcp any any -> $HOME_NET 3389 (msg:"ET DOS Microsoft Remote Desktop (RDP) Syn then Reset 30 Second DoS Attempt"; flags:R; flow:to_server; flowbits:isset,ms.rdp.synack; flowbits:isnotset,ms.rdp.established; flowbits:unset,ms.rdp.synack; reference:cve,2012-0152; classtype:attempted-dos; noalert; sid:2014384; rev:8; metadata:created_at 2012_03_13, updated_at 2012_03_13;)
I've tested this regex (https://regex101.com/r/rqBaSC/3) and it should be applied to this rule, but is not. It is not only this rule that is effected. I have not been able to identify why this rule or any other is being exempted from modification.
Files
| modify.conf (5.92 KB) modify.conf | /etc/suricata/modify.conf | J Cliff Armstrong, 03/09/2020 10:43 PM |
Updated by J Cliff Armstrong over 4 years ago
Appologies, I am very very tired. suricata-update version is `suricata-update version 1.1.0 (rev: 63493db)`
Updated by Kenneth Kolano over 4 years ago
This is likely due to the flowbits.
Flowbit rules are re-enabled late in the update process. To disable these rules one apparently must purge the flowbits clause from the rules in addition to disabling them. Refer...
https://redmine.openinfosecfoundation.org/issues/3102
Updated by J Cliff Armstrong over 4 years ago
- File modify.conf modify.conf added
There seems to be a misunderstanding. I am not attempting to disable any rules. I'm using `/etc/suricata/modify.conf` to change the rules of certain classtypes from alert to reject/drop (suricata is operating in IPS mode). Then the rule above is last in the file and is used to change rules with the `noalert` keyword back from reject/drop to alert. It's unclear from the documentation what regex capabilities are available (I assume whatever python provides) but I felt using read-aheads would be unnecessarily computationally expensive/prone to error and simply reverting such rules at the end a better choice.
I've attached my current modify.conf as an example. I've removed the `^` anchor from the modify rules so that commented suricata rules will be affected as well without "enabling" them. Then, if they are enabled elsewhere, they should be set to the correct action. Regardless, I have tried with the modify rule exactly as stated in the documentation and it does not correctly apply the change from reject/drop back to alert in all cases.
Additionally, I've recompiled suricata to the latest stable version and so now have suricata-update 1.1.1... the issue remains.
Updated by Kenneth Kolano over 4 years ago
You may need to revise the rev # as well?
Updated by J Cliff Armstrong over 4 years ago
Kenneth Kolano wrote in #note-4:
You may need to revise the rev # as well?
wolferz@tiphares ~ $ sudo suricata-update -V suricata-update version 1.1.1
:-/ no revision listed.
Updated by Jason Ish over 4 years ago
The problem here is that the original rule is commented out: "#alert tcp any any ...". Your modifications that turn this into a drop don't consider the enabled state of the rule, so its converted to "#drop ...". Your revertion does consider the commented out, so it never gets converted back.
The flowbit resolution comes up and sees this rule is required by another enabled rule, and turns it on, but in its drop state.
My suggestion is to keep your conversion to drop rules to only those that are enabled. For example:
re:classtype:\s*attempted-dos "^alert(.*)" "drop\\1" # medium Attempted Denial of Ser vice
Which should resolve this issue.
Updated by Jason Ish over 4 years ago
- Tracker changed from Bug to Support
Changing tracker to support for now.
Updated by J Cliff Armstrong over 4 years ago
Jason Ish wrote in #note-6:
Your revertion does consider the commented out, so it never gets converted back.
No, it doesn't. Check the modify.conf file I attached. As I explicitly stated in my previous note, I removed the `^` anchor from all modify rules. All of them. The problem remains. Putting it back on all modify rules also doesn't solve the problem (even if this rule is fixed, others aren't).
Updated by Jason Ish over 4 years ago
J Cliff Armstrong wrote in #note-8:
Jason Ish wrote in #note-6:
Your revertion does consider the commented out, so it never gets converted back.
No, it doesn't. Check the modify.conf file I attached. As I explicitly stated in my previous note, I removed the `^` anchor from all modify rules. All of them. The problem remains. Putting it back on all modify rules also doesn't solve the problem (even if this rule is fixed, others aren't).
Oops. Typo on my part. Should have set does not consider the commented out.
I see the problem now, 2014384 does not have noalert set, and is disabled by default. So what is happening is:
- Rule is modified from "# alert ..." to "# drop ..."
- At this point is does not have noalert set, so the regex to turn it back into an alert will not match.
- This rule is now enabled as part of flowbit resolution, which is what enables it and adds noalert.
Open for discussion is if a drop rule is enabled as part of flowbit resolution, should it also be converted to alert?
For now, I'd suggest your modify.conf match and convert rules that are "^alert", so you don't change disabled rules.
If you are having issues with another rule, please point it out specifically.
Updated by J Cliff Armstrong over 4 years ago
Ah, I see.
What order are the suricata-update configuration files processed in? I can't find it in the documentation. As long as enable.conf is processed before modify.conf then I can do as you suggest without issue. Otherwise I'm going to have to start using modify.conf to enable rules as well to ensure they get enabled first.
Updated by Jason Ish over 4 years ago
J Cliff Armstrong wrote in #note-10:
Ah, I see.
What order are the suricata-update configuration files processed in? I can't find it in the documentation. As long as enable.conf is processed before modify.conf then I can do as you suggest without issue. Otherwise I'm going to have to start using modify.conf to enable rules as well to ensure they get enabled first.
Yeah, this doesn't appear to be documented. We'll have to do so. The order is:
- disable.conf
- enable.conf
- drop.conf
- modify.conf
Updated by Shivani Bhardwaj over 4 years ago
- Related to Documentation #3535: Document the order in which configuration is applied to rules added
Updated by Shivani Bhardwaj over 4 years ago
Hi J Cliff Armstrong!
Does Jason's suggestion help you with your issue? I have added a ticket to update the documentation regarding the same. Please let me know if this works for you and if this issue can be closed. Thanks for reporting!
Updated by J Cliff Armstrong over 4 years ago
Apologies, I've had the last few days overturned by Covid-19 related issues.
Yes, it seems the issue is resolved, now that I'm aware of the addition of `noalert;` to rules which are auto-enabled for flowbits it is also clear why I must anchor the regexes to the beginning of the line. I'd suggest this is added to the documentation as well, assuming I didn't simply miss it.
Updated by Shivani Bhardwaj over 4 years ago
Please expect this in the documentation in the next major release. Closing the issue for now. In case you come across anything similar again, please feel free to open a new issue.
Stay safe!