Invalid TCP header flags

[kyle.r.maxwell () verizon com]

We're seeing occasional TCP traffic with FIN-RST-ACK or FIN-PSH-RST-ACK set
in the header. The strange part is that it's always set for port 110 (this
is in fact a legitimate POP server). The traffic is observed inside the
firewall; I don't have an IDS sensor outside.

Could this just be port scanning, OS fingerprinting, a broken stack, or
something else? I've googled around but haven't found too much useful info,
other than to see that other folks have seen similar stuff.

--
Kyle Maxwell
InfoSec Engineer
Global Security Operations Center
Verizon International Security
Office  - 972-929-1287
Hotline - 972-929-1290
kyle.r.maxwell () verizon com



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