Certificate questions

Zeek
1

Greetings Zeek community,

I’m very new to Zeek, but really like what I’m see so far. I need some help or perhaps a bit of education though. I have it setup in a Security Onion VM.

I see a lot of messages about SSL including “unable to get local issuer certificate”, which I understand COULD be self-signed certs.

I also see many, many SSL::Invalid_Server_Cert notices in Kibana. Many others say "SSL certificate validation failed with (self signed certificate in certificate chain).

These would all be of interest, however they ALL point back to very legitimate sources like Apple and Microsoft. I find it hard to believe that these major companies have problems with that many certificates and servers. Could this really be the case???

I could find very little information on Google regarding this. One article said something about Zeek not being able to match them up with root cert servers or something like that.

Is it possible that Zeek is missing something like a list of root CAs or something? Is this just garbage caused by something else. This will leave me scratching my head until I come back on Monday. I appreciate the help.

Jay Fluharty
Network Analyst
NS Wheeling-Nisshin Inc.
PO Box 635
Follansbee, WV 26037
jayf@wheeling-nisshin.com
1-304-527-4819

2

Excellent question.

The reason you see those errors is the lack of the Root CA in Zeek’s certificate store.

Zeek, by default, uses Mozilla certificate store - the same one your Firefox uses. Try going to one of these pages, like https://slscr.update.microsoft.com in FF and you will see certificate errors as well. You will not see them in Edge. Why’s that?

For Microsoft, those certificates chain to a CA that has the root CA certificate present in the windows certificate store, but nowhere else. For Apple, the situation is similar - these root CA certificates are present on the system level but no where else.

Since those are for services not accessed by general public, but things like iCloud and software updates, these have never been submitted to us for inclusion into Mozilla root CA program - and hence never landed in Zeek’s land.

An example right here here

subject
CN=slscr.update.microsoft.com,OU=DSP,O=Microsoft,L=Redmond,ST=WA,C=US

issued by
CN=Microsoft ECC Update Secure Server CA 2.1,O=Microsoft Corporation,L=Redmond,ST=Washington,C=US

issued by
CN=Microsoft ECC Product Root Certificate Authority 2018,O=Microsoft Corporation,L=Redmond,ST=Washington,C=US

Present in MS root store

There is a fix for that - you have to fetch those certificates with tools like openssl or the latest Firefox (it’s got this nice thing where you can download the full chain), transform them into Zeek’s scripts and include.

I think Justin wrote a nice script for that.

https://gist.github.com/JustinAzoff/7a1b92c976a2fa6e8601

mkdir tmp && cd tmp

openssl s_client -host slscr.update.microsoft.com -port 443 -showcerts < /dev/null | sed -n ‘/BEGIN/,/END/p’ | openssl x509 -outform DER > o.der

(do that for each CA - ignore the “verify error”)

python …/gen_certs.py . cacert.zeek

And then you can @load the cacert.zeek in a script or in a local.zeek

3

Thanks for the answer. I need a bit of clarification though.

Your instructions said, openssl s_client -host slscr.update.microsoft.com -port 443 -showcerts < /dev/null | sed -n ‘/BEGIN/,/END/p’ | openssl x509 -outform DER > o.der
(do that for each CA - ignore the “verify error”)’

Wouldn’t fetching each certificate overwrite “o.der”. Should that be a “>>”? Or do need to modify “o.der” for each certificate I fetch, then do a “python …/gen_certs.py . cacert.zeek”? Would that grab all the .der files? Do I need to make a seperate, unique “cacert.zeek” to put in my “@load” statement.

Also, I put “@/path/to/cacert.zeek” file in my “/opt/bro/share/bro/site/local.bro” file. I assume that’s where the statement goes?

Sorry for all the questions, but I’ve not found this information elsewhere. It really is appreciated.

Jay Fluharty
Network Analyst
NS Wheeling-Nisshin Inc.
PO Box 635
Follansbee, WV 26037
jayf@wheeling-nisshin.com
1-304-527-4819

Inactive hide details for Michał Purzyński ---11/17/2019 05:45:24 AM---Excellent question. The reason you see those errors isMichał Purzyński —11/17/2019 05:45:24 AM—Excellent question. The reason you see those errors is the lack of the Root CA in Zeek’s

4

Answers inline. Keep asking, this is tricky to get right.

Thanks for the answer. I need a bit of clarification though.

Your instructions said, openssl s_client -host slscr.update.microsoft.com -port 443 -showcerts < /dev/null | sed -n ‘/BEGIN/,/END/p’ | openssl x509 -outform DER > o.der
(do that for each CA - ignore the “verify error”)’

Wouldn’t fetching each certificate overwrite “o.der”. Should that be a “>>”?

It will and that’s why you want to redirect output for each server you’re fetching certificate chain from, to a new file.
gen_certs.py can then consume entire directory full of “der” files and combined that into a hex representation for a Zeek script.

Or do need to modify “o.der” for each certificate I fetch, then do a “python …/gen_certs.py . cacert.zeek”? Would that grab all the .der files? Do I need to make a seperate, unique “cacert.zeek” to put in my “@load” statement.

So I just tested it a bit more and I need to modify my original instructions

openssl s_client -host init.push.apple.com -port 443 -showcerts < /dev/null

For each server inspect the output carefully - you want to grab the “CA” and that’s usually the last one. Ignore the sed, etc here, it won’t, unfortunately, work as I wanted it to.

For example, Apple returns

CONNECTED(00000005)

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