NAT Overload and Clock Skew in an Outbound Transfer Timeline
At 02:14 UTC, a SIEM alert shows 4.8 GB outbound from 10.20.4.15 to 203.0.113.88:443. The firewall log is in UTC but its NTP sync failed and is 7 minutes slow; the proxy log is in local time (UTC-5) and records CONNECT requests to cdn-update.example. The DHCP server shows 10.20.4.15 leased to workstation FIN-042 from 2024-09-10 18:00 to 2024-09-11 18:00. FIN-042's Windows event logs show a logon by user jdoe at 20:58 local and a service installation at 21:03 local. The firewall uses NAT overload; the alert includes source port 51422. Proxy logs show the same source IP and port at 21:07 local with 4.7 GB uploaded. The company has offices in two time zones. Legal wants a precise timeline.
1. Build a normalized UTC timeline from these records, explaining each conversion.
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Convert proxy local time (UTC-5) to UTC: 21:07 local becomes 02:07 UTC. Convert Windows event times: 20:58 local becomes 01:58 UTC and 21:03 local becomes 02:03 UTC. The firewall clock is 7 minutes slow, so a recorded 02:14 means actual time is about 02:21 UTC. The normalized sequence is: 01:58 UTC logon by jdoe; 02:03 UTC service installation; 02:07 UTC proxy CONNECT from 10.20.4.15 source port 51422; about 02:21 UTC firewall session record showing 4.8 GB outbound. State that the firewall time is an estimate with a documented skew and that the proxy start time and firewall close time can describe the same long-lived session.
2. How do you decide whether the proxy and firewall events represent the same NAT session?
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NAT overload means the source IP alone is not enough, so match the source port 51422, destination 203.0.113.88:443, and time window. The proxy records 4.7 GB and the firewall 4.8 GB; the difference can come from protocol overhead, accounting methods, or when each log closes. The 14-minute gap is plausible if the proxy logs the CONNECT at session start and the firewall logs the session at close. Check firewall session table or flow logs, proxy event IDs with start and end times, and whether any other host used port 51422 in that window. The reasoning is to correlate independent fields and rule out port reuse before treating the records as one transfer.
3. What caveats should the report include about precision and attribution?
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The report should disclose the firewall NTP failure and the 7-minute correction, the proxy time zone conversion, and any daylight-saving ambiguity. It should state that NAT overload permits port reuse and that the match rests on IP, port, destination, and timing. The proxy may record only CONNECT metadata, not content, and TLS prevents confirming the payload from the proxy alone. The destination name cdn-update.example could be legitimate or masquerading. Attribution to jdoe is supported by the logon and service installation on FIN-042, but the timeline alone does not prove the user personally initiated the transfer. Legal gets a precise but qualified timeline with the assumptions exposed.