Ioc Hunter

IOC Hunter

Source discipline and TLP respect: IOCs come with a share policy (TLP-RED/AMBER/GREEN/CLEAR). Forwarding to parties the feed contract does not allow is a breach of trust and, in some contracts, of license. Sharing where permitted (intra-industry ISAC, CSIRT-NL, sector PAC) is a net-positive habit.

When to use

IOCs (Indicators of Compromise) are the tactical layer of threat intel: hashes, IPs, domains, URLs, mutexes, certificate fingerprints, JA3/JA4 strings. This skill helps manage feeds, dedup and confidence-score them, plug them into SIEM/EDR, and retro-hunt.

Triggers on:

• A question like "add this IOC feed to our stack", "is this hash known", "retro-hunt the last 30 days against these IOCs", "how do we score IOC confidence", "set up a MISP instance".
• A handoff from detection-engineer (rule needs IOC input), log-triage or the threat-hunt command (enrichment of findings), malware-triage (extracted IOCs to be integrated).
• A new APT-campaign publication whose IOCs need processing.
• A periodic (quarterly) feed-hygiene review: which feeds deliver value, which do not.

When NOT (handoff)

• Writing a detection rule that consumes the IOC → detection-engineer. This skill provides the IOC input.
• Triage of the alert that fires from an IOC match → log-triage.
• A threat-hunt session itself → the threat-hunt command.
• Malware-sample analysis that produces IOCs → malware-triage.
• Forensic confirmation of IOC impact → forensics-assist.
• Strategic CTI (actor profiles, geopolitical context) is out of scope here — that is separate CTI work, not in this skill.
• Vulnerability feed (CVEs) → cve-triage.

Approach

Six phases. Phase 2 (confidence scoring) and phase 5 (retro-hunt) are where IOC work delivers operational value.

1. Feed curation

Which feeds deserve your SIEM bandwidth? Not all of them.

Source categories:

• Government / national CERT: CISA AIS, NCSC-NL, ENISA feeds, BSI (DE), ANSSI (FR). High-trust, sometimes delayed.
• ISAC / sector PAC: FI-ISAC NL for financial, MS-ISAC for government. Sector-specific relevance.
• Commercial CTI: Mandiant, Recorded Future, CrowdStrike Intelligence, Mandiant Advantage, IntSights/Rapid7, Group-IB. High quality, high price.
• Open-source/community: AlienVault OTX, abuse.ch (URLhaus, MalwareBazaar, ThreatFox), Spamhaus, Emerging Threats. Mixed quality, free or low-cost.
• Vendor-specific threat intel: Microsoft Security Graph, Cisco Talos, Palo Alto Unit 42 — often baked into the product license.
• Internal sources: your own incidents produce the highest-trust IOCs for your organization. Categorize as "internal" with a separate TTL.

Curation criteria:

• Coverage: does the feed cover sectors/regions that touch you?
• Freshness: how quickly after an incident does an IOC appear in the feed?
• Validity: what % of IOCs are still true-positive after 30/60/90 days?
• False-positive history: known noisy feeds (e.g. NRD feeds that flag every newly registered domain) need a separate cadence.
• Format: STIX 2.1 / OpenIOC / MISP event format / CSV. STIX is the de facto standard.

A feed that is not used for matching or retro-hunt does not deserve its ingest slot. Periodically downscale.

2. Deduplication and confidence scoring

One IP can show up in 5 feeds. A hash with 12 sources is not 12× more valuable than one with 1 source — but somewhat more. Confidence model:

• Source-reputation score per feed (kept internally based on TP-rate history). High-trust source = higher weight.
• Age: the older an IOC, the less relevant. Default decay: confidence halves after 30 days; after 90 days it is a retirement candidate unless there is APT context. Some IOC types age faster (IPs > domains > hashes for sustained malware).
• Sightings: number of times reported by different sources. Boosts confidence positively, not linearly.
• TLP: not so much confidence as a sharing restriction. TLP-RED = specific audience only, TLP-AMBER = limited external, TLP-GREEN = community, TLP-CLEAR = open. Sharing discipline: never upgrade TLP without source consent.
• Type-specific:
  • Hashes (SHA-256 preferred): highest precision, long life provided the hash does not change. Low false-positive risk.
  • IPs: short-lived in cloud context (CDN-shared, ephemeral cloud resources). Drop confidence quickly, retire after 14–30 days unless confirmed APT infrastructure.
  • Domains: medium life; sinkholing and takedowns accelerate retirement. Watch out for wildcard-feed noise.
  • URLs: short-lived, exploit-kit rotation. Retire fast.
  • JA3/JA4 fingerprints: TLS-handshake fingerprint, sustained as long as the malware author keeps the same TLS stack.
  • Email addresses / sender domains: medium life, BEC context.
  • Mutexes: implementation detail, sustained.
  • Yara rules: a pattern, not an IOC, but a functionally related layer — see malware-triage.

Dedup implementation: MISP/OpenCTI do this natively when events are merged on the same attribute. A custom stack needs its own normalization (canonical form of IPs, lowercased domains, hash-format normalization).

3. Storage and sharing layer

• MISP (open-source) — the de facto standard for threat-sharing platforms in the EU. Event-based, multi-tenant, sharing policy per event/attribute. Strong in the NL/EU community via FI-ISAC and CSIRT-NL.
• OpenCTI (open-source) — more modern, knowledge-graph oriented, better visualization. Imports STIX 2.1 natively.
• Anomali ThreatStream, EclecticIQ, Recorded Future — commercial alternatives.
• Commercial-platform choice is usually contractual with a CTI feed vendor. Open-source self-hosted (MISP) is feasible for medium-size orgs.

Sharing direction:

• Inbound: integrate feeds via MISP-sync, STIX/TAXII feeds, OpenCTI connectors, or API pulls.
• Outbound: what do you generate yourself and share back? Internal IOCs from incidents, thinly confirmed detections. NL FI-ISAC and CSIRT-NL accept contributions. Anonymize where TLP policy requires.

4. Enrichment pipeline to SIEM and EDR

An IOC that does not match anywhere is dead weight. Pipeline to make them active:

• SIEM ingestion: feeds into lookup tables (Splunk lookup files, Sentinel watchlists, Elastic enrich policies). Fresh feed = fresh lookup. Default refresh cadence hourly to daily depending on feed update tempo.
• EDR ingestion: native IOC modules in Microsoft Defender, CrowdStrike Falcon, SentinelOne, Carbon Black accept IOCs as a blocklist or detection trigger. Differentiate detect-only vs. auto-block per type.
• Network tooling: firewall blocklists (IP/domain), DNS-RPZ (recursive policy zone) for domain blocking, proxy/SWG categorization.
• Email gateway: sender domains, hashes for attachments. Microsoft 365 Defender, Proofpoint, Mimecast.

Enrichment discipline: not every feed in every layer. High-confidence actor feeds → block action; lower-confidence feeds → detect-only with a SIEM alert. Aggressive blocking on weak feeds is operational disruption.

5. Retro-hunt and lifecycle

• Retro-hunt on new IOCs: with each new high-confidence feed update, sweep the last N days of logs (typically 30–90 days, depending on retention and compute budget). With siem-query as the building block.
• Hits triage: a retro-hit is not a confirmed compromise — it can be benign historical traffic. Path to confirmation via log-triage (was the actor suspicious?) and forensics-assist (was there a follow-on action?).
• IOC lifecycle:
  • Active: in scoring, in lookup tables.
  • Aging: confidence falling, in detect-only mode.
  • Retired: out of active detection. Keep in archive for later retro-hunt.
  • Re-promote: an old IOC reappears in a new feed or incident → re-activated with fresh confidence.
• Expiry discipline: feed IOCs have a default TTL from the feed policy. Internal IOCs use a manual expiry (default 90 days, revisable).

6. Verification-loop

Layer 1: scope (feed curation up to date, no feeds nobody reads anymore?), assumptions (IOC source-reputation scoring based on data, not gut feel?), gaps (TLP policy respected on sharing-out, retro-hunt cadence on new high-conf feeds?). Layer 2: feed sources alive (multiple CTI vendors merge or shut down — verify URLs and API status), TLP classification correct (TLP 2.0 since 2022, the old TLP-AMBER+STRICT no longer exists as such), STIX version consistent (STIX 2.1 default in 2024+).

Output

Two modes: feed management (in advance, living) and a hunt report (after a retro-hunt or investigation).

Feed-management mode:

IOC feed inventory
Date: YYYY-MM-DD | Reviewer: <name>

Active feeds:
  Name | Category | Format | Update cadence | Source reputation | TLP policy

Per feed:
  Coverage:           <regions, sectors>
  Freshness:          <average delay between incident and publication>
  Validity rate:      <% TPs after 30/60/90 days>
  TP-rate trend:      <stable / falling / rising>
  Decision:           <retain / downgrade / replace / drop>

Sharing-out:
  Internal IOCs generated: <N>
  Shared via: <FI-ISAC NL / MISP-sync / CSIRT-NL>
  TLP classification: <per IOC>

Coverage gaps:
  <which threat clusters not covered by current feeds>

Hunt report (after retro-hunt on a new IOC set):

IOC retro-hunt — <feed update / campaign name>
Window: last N days | Date: YYYY-MM-DD

Input IOCs:
  Hashes: N | IPs: N | Domains: N | URLs: N | JA3/JA4: N

Hits per type:
  Hash:    N (true-positive: M, false-positive: K, pending triage: L)
  IP:      N (...)
  Domain:  N (...)
  URL:     N (...)

Confirmed sightings:
  - <IOC + match context + system + timestamp + handoff>

Pending triage:
  - <IOC + reason for uncertainty + next step>

Handoffs:
  ir-runbook:         <if confirmed compromise>
  log-triage:         <pending-triage depth>
  detection-engineer: <if IOCs basis for a new rule>
  forensics-assist:   <if post-incident investigation needed>

Verification-loop: ...

References

• MISP — https://www.misp-project.org/. De facto threat-sharing platform, open-source, widely used in EU/NL.
• OpenCTI — https://www.opencti.io/. Modern threat-intel platform with a knowledge graph.
• STIX 2.1 — https://oasis-open.github.io/cti-documentation/. Structured Threat Information Expression standard.
• TAXII 2.1 — https://oasis-open.github.io/cti-documentation/taxii/intro. Transport mechanism for STIX.
• TLP 2.0 — https://www.first.org/tlp/. Traffic Light Protocol, FIRST-maintained.
• abuse.ch (URLhaus, MalwareBazaar, ThreatFox) — https://abuse.ch/. Open-source IOC feeds.
• AlienVault OTX — https://otx.alienvault.com/. Community pulse-based feeds.
• CISA AIS — https://www.cisa.gov/topics/cyber-threats-and-advisories/information-sharing/automated-indicator-sharing-ais.
• NCSC-NL Threat Intel — https://www.ncsc.nl/.
• MITRE ATT&CK — https://attack.mitre.org/. For mapping IOCs to TTPs.

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