Suricata protocol DCERPC cannot trigger alert when adding new rule

vjulien · July 31, 2024, 12:27pm

If you’re able to test things, please have a look at:

github.com/OISF/suricata

dcerpc: don't reuse completed tx

OISF:master ← victorjulien:dcerpc-detect-7187/v1

opened 12:18PM - 31 Jul 24 UTC

victorjulien

+6 -0

In the DCERPC over TCP pcap, logging and rule matching is disrupted by adding a …simple rule: alert tcp any any -> any any (flow:to_server,established; \ dce_iface:5d2b62aa-ee0a-4a95-91ae-b064fdb471fc; dce_opnum:1; \ dce_stub_data; content:"|42 77 4E 6F 64 65 49 50 2E 65 78 65 20|"; \ content:!"|00|"; within:100; distance:97; sid:1; rev:1; ) Works: alert + 3 dcerpc records. But when adding a trivial rule: alert tcp any any -> any any (flow:to_server,established; \ dce_iface:5d2b62aa-ee0a-4a95-91ae-b064fdb471fc; dce_opnum:1; \ dce_stub_data; content:"|42 77 4E 6F 64 65 49 50 2E 65 78 65 20|"; \ content:!"|00|"; within:100; distance:97; sid:1; rev:1; ) alert tcp any any -> any any (dsize:3; sid:2; rev:1; ) The alert for sid:1 disappears and also there is one dcerpc event less. In the single rule case we can aggressively free the transactions, as there is only an sgh in the toserver direction. This means that when we encounter the 2nd REQUEST, the first 2 transactions have already been processed and freed. So for the 2nd REQUEST we open a new TX and run inspection and logging on it. When the 2nd rule is added, it adds toclient sgh as well. This means that we will now slightly delay the freeing of the transactions. As a consequence we still have the TX for the first REQUEST when the 2nd REQUEST is parsed. This leads to the 2nd REQUEST re-using the TX. Since the TX is already marked as inspected, it means the toserver rule now no longer matches. Also we're not logging this TX correctly now. This commit fixes the issue by not "finding" a TX that as already been marked complete in the search direction. Bug #7187. SV_BRANCH=https://github.com/OISF/suricata-verify/pull/1995