Bug #5799
closeddetect: sigs using DETECT_SM_LIST_PMATCH can break other signatures
Description
With the following smb pcap and two simple rules, we were able to reproduce a severe bug in 6.0.9 and 7.0 Beta/Master.
The following two rules trigger the bug:
```
alert ssh $HOME_NET any -> any any (msg:"pcre without content"; pcre:"/rabbit/B"; sid:1; rev:1;)
alert smb $HOME_NET any -> any any (msg:"smb share content"; smb.share; content:"C"; sid:2; rev:1;)
```
The second rule for smb would match and trigger 4 alerts on itself, but as soon as the first rule is parsed and registered as well, the second rule won't match anymore.
If you add a `content` match to the first rule it works again and generates 4 alerts:
```
alert ssh $HOME_NET any -> any any (msg:"pcre with content"; content:"r"; pcre:"/rabbit/B"; sid:1; rev:1;)
alert smb $HOME_NET any -> any any (msg:"smb share content"; smb.share; content:"C"; sid:2; rev:1;)
```
It seems to be related to `DETECT_SM_LIST_PMATCH` somehow overwriting parts that are relevant for the second rule.
The impact could be quite relevant, once we added such a dummy rule we did some tests and for example within a malicious pcap used against the ET ruleset, the alerts went down from around 890 to 850 so a rule that doesn't even match can have an impact on the amount of valid alerts.
Using the `pcre` is just on example, other keywords that use `DETECT_SM_LIST_PMATCH` also have the potential to break other rules.
Files
Updated by Andreas Herz over 1 year ago
There is a more verbose pcap from https://www.malware-traffic-analysis.net/2019/07/05/2019-07-05-Ursnif-with-Trickbot-and-IcedID.pcap.zip which is public and I was able to create another scenario where a valid signature is not matching anymore:
alert ssh $HOME_NET any -> any any (msg:"pcre without content and no match"; pcre:"/rabbit/"; sid:1; rev:1;) alert krb5 $HOME_NET any -> any any (msg:"krb match"; krb5_cname; content:"marlo"; sid:3; rev:1;)
If both rules are included, 0 alerts, once the first rule is commented the second rule triggers 6 alerts. So it's not only related to smb but in that case also has an impact on kerberos.
The interesting case on this one is, that adding a content match to the first rule doesn't help.
Updated by Eric Leblond over 1 year ago
I fear I've missed something but the attached patch is fixing the issue.
In the C code, we just set it:
grep -r STREAM_FLUSH src/ src/detect.c: flow_flags &=~ STREAM_FLUSH; src/detect.c: TRACE_SID_TXS(s->id, tx, "FLUSH? %s", (flow_flags & STREAM_FLUSH)?"true":"false");
In rust, it is just defined:
grep -r STREAM_FLUSH rust/src/ rust/src/core.rs:pub const STREAM_FLUSH: u8 = 0x80;
So this is unused in the code. Maybe we have a conflict there. But at least it should not be set there because it is not used anywhere.
Updated by Jason Ish about 1 year ago
Possibly related: https://redmine.openinfosecfoundation.org/issues/4759
Updated by Jason Ish about 1 year ago
- Related to Bug #4759: TCP DNS query not found when tls filter is active added
Updated by Andreas Herz about 1 year ago
Jason Ish wrote in #note-6:
I assume this
smb.pcapis public?
yep I extracted this flow from the MTA public pcap
Updated by Jason Ish about 1 year ago
- Status changed from New to Closed
Updated by Victor Julien about 1 year ago
- Subject changed from Signatures using DETECT_SM_LIST_PMATCH could break other signatures on detection to detect: sigs using DETECT_SM_LIST_PMATCH can break other signatures
- Priority changed from High to Normal
- Label Needs backport to 6.0 added