Threat Hunting Exercise: Analyzing Firewall VPN Proxy Rotation Geofence Bypass Dataset

This hunt uses a simulated perimeter firewall and VPN gateway dataset (firewall_geofence_bypass_2022-10-20T080000.json) capturing T1090.002: Proxy (External Proxy) used to defeat T1621-adjacent geofencing controls, where an attacker rotates through a residential/commercial proxy or VPN exit-node pool specifically to circumvent a geolocation-based access control policy restricting logins to approved countries.

Step 1: Hypothesis Formation

Hypothesis: A single account authenticates successfully from a rapid succession of distinct source IPs, each geolocating to a different country consistent with commercial VPN/proxy exit-node ranges (rather than genuine travel), immediately following one or more failed attempts from IPs in a blocked geography - indicating deliberate proxy rotation to bypass geofencing rather than legitimate travel or a shared-NAT artifact. Indicators:

• Multiple successful authentications for one account within a short window, each from a different country/ASN, with impossible-travel velocity between them.
• Source IP ASN/organization fields matching known commercial VPN or residential-proxy providers.
• A pattern of failed authentication from a geofenced/blocked country immediately preceding successful authentication from an allowed country for the same account.
• No corresponding travel notification, expense report, or calendar entry supporting legitimate international travel.

Null Hypothesis: The user is a legitimate frequent traveler or uses a corporate-approved VPN service that happens to rotate exit nodes across countries. Invalidate by checking the account against the approved corporate VPN provider's known IP ranges and any submitted travel notifications.

Rationale: Geofencing is a common compensating control for accounts that cannot yet support strong MFA or for restricting access from high-risk regions; attackers with valid stolen credentials frequently route through proxy pools specifically to defeat this control, so detecting the rotation pattern itself - independent of any single IP's reputation - is essential.

Step 2: Data Sources and Scope

• Sources: VPN/SSO gateway authentication logs with geolocation and ASN enrichment; perimeter firewall connection logs; conditional-access policy logs.
• Scope: ~2022-10-20T08:00:00-09:30:00 UTC; Account: d.oyelaran@corp.com; Home country: United States; Geofence policy: allow US/CA/UK only.
• SIEM Queries (Splunk/ELK):
• index=sso_auth user="d.oyelaran@corp.com" | stats count, values(geo_country), values(src_ip) by _time
• Failed-then-success pattern: index=sso_auth user="d.oyelaran@corp.com" (result=failure geo_country=blocked_list) | append [search result=success geo_country=allowed_list] | sort _time
• ASN/proxy check: | lookup asn_reputation src_ip OUTPUT asn_org, is_known_vpn_proxy

Step 3: Key Findings

Parsed events (5 shown) confirm the account d.oyelaran@corp.com failed authentication twice from a geofenced country before succeeding four times within 75 minutes from four different countries, all resolving to known commercial VPN/proxy ASNs.

Validation:

• Timeline: blocked-geography failure, immediately followed by a rapid sequence of successful logins from three different allowed-region VPN exit points within 75 minutes - an impossible-travel, proxy-rotation pattern.
• False Positives: no corporate-approved VPN provider match, and no travel record supports any of the observed locations.
• Correlation: geofence-block precursor, impossible travel velocity, multiple distinct commercial VPN ASNs, and absence of travel justification jointly confirm deliberate geofence-bypass account abuse.

Step 4: Recommendations & Next Steps

• Immediate Response: Force-terminate all active sessions for d.oyelaran@corp.com, reset credentials and require re-enrollment of MFA, review all actions taken during each of the four sessions for data access or exfiltration, and temporarily require step-up verification for this account regardless of geography.
• Detection: Sigma-style rule: title: Geofence Bypass via Rotating VPN Proxy → selection1: result=failure AND geo_country IN (blocked_list) selection2: result=success AND is_known_vpn_proxy=true within 30m of selection1, count(distinct geo_country) >= 3 within 2h → condition: selection1 and selection2.
• Pro Tip: Layer impossible-travel velocity analysis on top of geofencing rather than relying on country allow-lists alone - geofencing is trivially defeated by any commercial VPN, but the physics of travel time between two locations is much harder for an attacker to fake convincingly, making velocity-based correlation a durable complement to location-based policy.

Hypothesis confirmed - an attacker with valid stolen credentials rotated through at least three different commercial VPN providers within 75 minutes to bypass the organization's geofencing policy after an initial blocked-country login attempt!