henriquescastilho Analyzing Indicators of Compromise
When to Use
Use this skill when:
- A phishing email or alert generates IOCs (URLs, IP addresses, file hashes) requiring rapid triage
- Automated feeds deliver bulk IOCs that need confidence scoring before ingestion into blocking controls
- An incident investigation requires contextual enrichment of observed network artifacts
Do not use this skill in isolation for high-stakes blocking decisions — always combine automated enrichment with analyst judgment, especially for shared infrastructure (CDNs, cloud providers).
Prerequisites
- VirusTotal API key (free or Enterprise) for multi-AV and sandbox lookup
- AbuseIPDB API key for IP reputation checks
- MISP instance or TIP for cross-referencing against known campaigns
- Python with
requestsandvt-pylibraries, or SOAR platform with pre-built connectors
Workflow
Step 1: Normalize and Classify IOC Types
Before enriching, classify each IOC:
- IPv4/IPv6 address: Check if RFC 1918 private (skip external enrichment), validate format
- Domain/FQDN: Defang for safe handling (
evil[.]com), extract registered domain via tldextract - URL: Extract domain + path separately; check for redirectors
- File hash: Identify hash type (MD5/SHA-1/SHA-256); prefer SHA-256 for uniqueness
- Email address: Split into domain (check MX/DMARC) and local part for pattern analysis
Defang IOCs in documentation (replace . with [.] and :// with [://]) to prevent accidental clicks.
Step 2: Multi-Source Enrichment
VirusTotal (file hash, URL, IP, domain):
import vt
client = vt.Client("YOUR_VT_API_KEY")
# File hash lookup
file_obj = client.get_object(f"/files/{sha256_hash}")
detections = file_obj.last_analysis_stats
print(f"Malicious: {detections['malicious']}/{sum(detections.values())}")
# Domain analysis
domain_obj = client.get_object(f"/domains/{domain}")
print(domain_obj.last_analysis_stats)
print(domain_obj.reputation)
client.close()
AbuseIPDB (IP addresses):
import requests
response = requests.get(
"https://api.abuseipdb.com/api/v2/check",
headers={"Key": "YOUR_KEY", "Accept": "application/json"},
params={"ipAddress": "1.2.3.4", "maxAgeInDays": 90}
)
data = response.json()["data"]
print(f"Confidence: {data['abuseConfidenceScore']}%, Reports: {data['totalReports']}")
MalwareBazaar (file hashes):
response = requests.post(
"https://mb-api.abuse.ch/api/v1/",
data={"query": "get_info", "hash": sha256_hash}
)
result = response.json()
if result["query_status"] == "ok":
print(result["data"][0]["tags"], result["data"][0]["signature"])
Step 3: Contextualize with Campaign Attribution
Query MISP for existing events matching the IOC:
from pymisp import PyMISP
misp = PyMISP("https://misp.example.com", "API_KEY")
results = misp.search(value="evil-domain.com", type_attribute="domain")
for event in results:
print(event["Event"]["info"], event["Event"]["threat_level_id"])
Check Shodan for IP context (hosting provider, open ports, banners) to identify if the IP belongs to bulletproof hosting or a legitimate cloud provider (false positive risk).
Step 4: Assign Confidence Score and Disposition
Apply a tiered decision framework:
- Block (High Confidence ≥ 70%): ≥15 AV detections on VT, AbuseIPDB score ≥70, matches known malware family or campaign
- Monitor/Alert (Medium 40–69%): 5–14 AV detections, moderate AbuseIPDB score, no campaign attribution
- Whitelist/Investigate (Low <40%): ≤4 AV detections, no abuse reports, legitimate service (Google, Cloudflare CDN IPs)
- False Positive: Legitimate business service incorrectly flagged; document and exclude from future alerts
Step 5: Document and Distribute
Record findings in TIP/MISP with:
- All enrichment data collected (timestamps, source, score)
- Disposition decision and rationale
- Blocking actions taken (firewall, proxy, DNS sinkhole)
- Related incident ticket number
Export to STIX indicator object with confidence field set appropriately.
Key Concepts
| Term | Definition |
|---|---|
| IOC | Indicator of Compromise — observable network or host artifact indicating potential compromise |
| Enrichment | Process of adding contextual data to a raw IOC from multiple intelligence sources |
| Defanging | Modifying IOCs (replacing . with [.]) to prevent accidental activation in documentation |
| False Positive Rate | Percentage of benign artifacts incorrectly flagged as malicious; critical for tuning block thresholds |
| Sinkhole | DNS server redirecting malicious domain lookups to a benign IP for detection without blocking traffic entirely |
| TTL | Time-to-live for an IOC in blocking controls; IP indicators should expire after 30 days, domains after 90 days |
Tools & Systems
- VirusTotal: Multi-engine malware scanner and threat intelligence platform with 70+ AV engines, sandbox reports, and community comments
- AbuseIPDB: Community-maintained IP reputation database with 90-day abuse report history
- MalwareBazaar (abuse.ch): Free malware hash repository with YARA rule associations and malware family tagging
- URLScan.io: Free URL analysis service that captures screenshots, DOM, and network requests for phishing URL triage
- Shodan: Internet-wide scan data providing hosting provider, open ports, and banner information for IP enrichment
Common Pitfalls
- Blocking shared infrastructure: CDN IPs (Cloudflare 104.21.x.x, AWS CloudFront) may legitimately host malicious content but blocking the IP disrupts thousands of legitimate sites.
- VT score obsession: Low VT detection count does not mean benign — zero-day malware and custom APT tools often score 0 initially. Check sandbox behavior, MISP, and passive DNS.
- Missing defanging: Pasting live IOCs in emails or Confluence docs can trigger automated URL scanners or phishing tools.
- No expiration policy: IOCs without TTLs accumulate in blocklists indefinitely, generating false positives as infrastructure is repurposed by legitimate users.
- Over-relying on single source: VirusTotal aggregates AV opinions — all may be wrong or lag behind emerging malware. Use 3+ independent sources for high-stakes decisions.