Sophos Firewall WAF Policy Crashing System

Hi John,

To me it sounds like Wordpress is using some file transfer method that is not part of the core HTTP standard, e.g. something like WebDAV which WAF doesn't support. If that is the case, then that is known to cause increased memory usage in WAF, which can starve other processes on the system, leading to the crash you see.

You can try to check if you can configure Wordpress to not use any non-standard protocol. If you can do this, that should fix the problem.

Alternatively you can try to define an exception for the site path route the file is uploaded to and skip all CTF checks for that site path route. This is not guaranteed to work, but might reduce the memory footprint of WAF just enough to avoid a crash.

Best regards,

Attila Kovacs

I'll look into this as well, thank you for the suggestion.  Even if this ends up being the issue, I still wouldn't expect the system to flat out crash.  A more graceful failure would probably be better.  I'll report back.

Yes, this is a logline from WAF. It basically means it received a request that is bigger than the 1GB limit allowed by ModSecurity. It is configurable, but 1GB is the maximum. It sounds like Wordpress is trying to transfer the whole file in a single HTTP request. However, this request should have simply been rejected by WAF with a 413 response and shouldn't cause a crash.

Just guessing, but it might be that Wordpress tried a standard file upload in a single request first, but got rejected by WAF, which in turn triggered some non-standard chunked file upload from Wordpress causing the crash.

It would be good to see a HAR capture from the client side to see the actual message flow between the browser and WAF, and also the full reverseproxy.log capturing all logs when the issue happens. If you are willing to do further testing on this and send the logs to me, I can check them.

Thanks for your reply.  I ran a few more tests last night and am seeing the same behavior with both 600MB and 300MB files though.  While still large, they seem to be within this limit... and I agree, either way it shouldn't cause the firewall to seize up.

I should be able to capture some of this today, and will report back.

Ok, just ran another test... this time with a 600MB MP4.  Here's the setup now:

• 600MB file
• Exception applied for the /wp-admin path for all policy rules.
• Within 5 seconds the server became unresponsive.  I let it go for about 5 minutes before finally killing it.
• During this time my SSH connection failed and I could not even log into the SOPHOS console directly via the Proxmox UI.  Completely unresponsive every way I normally connect to it.
• Before the test I deleted the /log/reverseproxy.log.
• I restarted the daemon by making a rule change.
• The reverseproxy log logged a couple entries before it looks like logging stopped... presumably due to the failure.
• I noticed in Proxmox the CPU of the VM going to about 40% and just staying there.
• I killed the machine around 11:39, and normal startup numbers are shown.

I'll PM you the reverse proxy logs.

Well, Sophos flagged my account as SPAM/malicious when I tried to PM you the logs... so I had to deal with that .  I think I'm unlocked now and the PM went through.  Let me know what else I can provide.

Sorry for the delay in response... SOPHOS somehow flagged me as SPAM/abusive and I had to go through the appeal process.  .  You should have received the PM now.  Thanks for taking the time.

can you do a atop logging? 

So log atop into a log file (not /tmp) and then we check this via atop.

https://unix.stackexchange.com/questions/276069/how-to-configure-atop-sampling-interval-log-path

atop -w /var/atop.log should be fine 

Then you can read it with atop -r /var/atop.log

I'll work on this and get back to you.  In the meantime, I just found this in reverseproxy.log:

statuscode="403" reason="WAF Anomaly" extra="Multipart parser detected a possible unmatched

[Wed Feb 01 01:44:01.774797 2023] [security2:error] [pid 3121:tid 140183714203392] [client 192.168.0.71:47162] [client 192.168.0.71] ModSecurity: Access denied with code 403 (phase 2). Match of "eq 0" against "MULTIPA
RT_UNMATCHED_BOUNDARY" required. [file "/usr/apache/conf/waf/modsecurity.conf"] [line "14"] [id "200004"] [msg "Multipart parser detected a possible unmatched boundary."] [hostname "<removed>"] [uri "/wp-a
dmin/async-upload.php"] [unique_id "Y9nD3V6YG94qmui-cq4hggAAAJU"], referer: <removed>/.../upload.php

Currently looking through this:  community.sophos.com/.../false-positive-which-can-t-be-skipped

Upon researching this appears to be a mod_security message, though not sure what causes it, or why it would freeze the system.

I have ignored filter id 200004, and so far every video I've tried (up to 650MB) is working fine.  It completes and the server does not freeze or crash.  I want to keep testing this through tomorrow, but so far this is a suitable workaround for me.

I'd still suggest a defect for the Sophos engineers to look at though.  I'm fine ignoring a filter, but I still worry that I may do something else which brings down the firewall (that shouldn't).  It seems like this has the potential to be critically impactful for others.  Fortunately for me it was just my home impacted.  Happy to provide whatever information necessary to help.

Thank you all for your help and time in troubleshooting the issue.

Interesting, rule 200004 is not part of the OWASP CRS, but comes with ModSecurity itself. It is supposed to catch attacks trying to prevent a WAF from parsing the request correctly and thus allowing a bypass. I found several issues reported regarding multipart handling in ModSecurity and they still have bugs open regarding it, so it seems likely that you have ran into this.

Unfortunately I don't see a fix for it that we could pick up with our next ModSecurity update, so you should keep the rule disabled for now. However, I suggest to only skip the rule on /wp-admin and any other site path where you upload files, just to not open up your site for a potential real attack.

I saw all those bugs too.  Lots of suggestions out there for disabling it too, but I didn't want to go down that path. 

I've added /wp-admin to the path specific routing exceptions list... but that entry does not allow me the ability to specify a filter to exclude.  How is this done?

I saw all those bugs too.  Lots of suggestions out there for disabling it too, but I didn't want to go down that path. 

I've added /wp-admin to the path specific routing exceptions list... but that entry does not allow me the ability to specify a filter to exclude.  How is this done?

Yes, unfortunately the UI doesn't allow you to skip a rule on site path level, you can only do that in the protection profile, which is applied to the whole WAF rule.

What you can do is block access to /wp-admin from any network in the original WAF rule, and create a second rule that only provides access to /wp-admin with another protection policy that has the problematic rule skipped.

This also allows you to have better control over who can access wp-admin. I guess the best practice would be to totally lock down this path from the Internet and only allow access to it from your internal network or from specific IPs, that way no attacker can access it. But this is just tweaking you can think about, not related to the original problem.

I tried this earlier, but it would not allow me to create a separate rule listening on the same domain.  Nonetheless, you've given me options to look into and think about.  I like your suggestion about only allowing /wp-admin internally.  That's the only location I ever need to access this path from.  I'll look into that.

You've been a great help, thanks Attila!

Yeah, you cannot use the same domain twice, but you can potentially do a subdomain, e.g. you have your site in mydomain.com, then you can have the admin interface covered by admin.mydomain.com. The only trick is to have something like a wildcard certificate for *.mydomain.com to cover both with a single certificate. Or have two separate certificates for the exact domains, whatever suits your needs the best.

I've got a wildcard, so no issues there.  Gonna give this a go now.  Thanks!

I don't think the product has the ability to map a subdomain to a path yet.  Not seeing a way to do this.  So for now I've just locked /wp-admin down to internal subnets and have removed the filter exclusion.  I figure once a month when I upload content I'll just apply the exception, upload, then remove it again.