Siem Query

SIEM Query Builder

Performance discipline: a correct query that does not return in reasonable time is operationally unusable. A lot of SOC time is lost in queries that scan unnecessarily much data. The second half of this skill is performance discipline, not just syntax.

When to use

This skill is the tooling substrate underneath detection-engineer (rules), log-triage (incident investigation), threat-hunt (proactive), and ioc-hunter (enrichment queries).

Triggers on:

• A question like "write an SPL for X", "translate this KQL into EQL", "why is my query slow", "which index for this data", "set up a summary index".
• Cross-platform migration or a multi-platform organization where the same detection logic must exist in both.
• Performance tuning of existing queries that are too slow for real-time alerting.
• Setting up data models (Splunk CIM, Sentinel ASIM/Watchlist, Elastic ECS) for a consistent schema across sources.

When NOT (handoff)

• Detection-rule design and lifecycle → detection-engineer, alert-tuning. This skill provides query building blocks; those skills handle the lifecycle.
• Triage of live alerts/events → log-triage. This skill provides the query; that one analyses the result.
• IOC-feed management and threat-intel enrichment → ioc-hunter.
• Threat-hunt hypothesis design → threat-hunt (command).
• Forensic depth → forensics-assist.
• Log-pipeline engineering (collection, parsing, enrichment) → ops team. This skill works with data as the SIEM presents it.

Approach

Six phases. Phase 3 (cross-translation) and phase 4 (performance) are where SIEM time really sits.

1. Tool and context

Three mainstream platforms in most organizations:

• Splunk Enterprise / Cloud — SPL (Search Processing Language). Pipe-style, function-rich, powerful on mass data with an indexer/search-head architecture.
• Microsoft Sentinel + Microsoft Defender XDR — KQL (Kusto Query Language). Modern, fast, strong join and aggregation syntax. Sentinel = log-source side, Defender XDR = endpoint side; shared query language.
• Elastic Security — EQL (Event Query Language) for sequence detection, KQL/Lucene for search filters, ESQL as the newer unified language. Lots of tooling around the Elastic Common Schema (ECS).

Other stacks: IBM QRadar (AQL), Sumo Logic, Chronicle (YARA-L), Devo. Concepts overlap, syntax differs.

For each query task, the first question: which platform is the target? Which retention window? Which data model? Which time zone in the events?

2. Query-construction patterns

The canonical shapes per platform:

Splunk SPL:

index=cloudtrail eventName=ConsoleLogin
  | where 'additionalEventData.MFAUsed'="No"
  | stats count by userIdentity.userName, sourceIPAddress
  | where count > 5

Pipeline mindset: filter first (index=, sourcetype=), then transform (stats, eval, lookup), then present (table, chart).

Powerful features: tstats (data-model acceleration), transaction (event correlation), dedup, streamstats (running aggregates), lookup (CSV/KV-store enrichment), metasearch (metadata only, faster).

Sentinel/Defender KQL:

SigninLogs
| where ResultType == 0 and AppDisplayName == "Office 365"
| where ConditionalAccessStatus == "success"
| summarize SignInCount = count() by UserPrincipalName, IPAddress, bin(TimeGenerated, 1h)
| where SignInCount > 10

Pipeline here too (|-style). Strengths: summarize with bin() for time bucketing, join kind= for different join types, let for variables, materialize for caching.

Multi-table join: union, join, lookup. ASIM (Advanced SIEM Information Model) for cross-source normalization.

Elastic EQL (sequence-aware):

sequence by host.id with maxspan=5m
  [process where event.action == "fork" and process.name == "bash"]
  [network where event.action == "connection_attempted" and destination.port == 4444]

EQL is unique in its out-of-the-box support for event sequences. For ad-hoc searching, KQL/Lucene/ESQL is more practical.

Consistent ECS field names: event.action, process.name, host.id, user.name. Migration of non-ECS data is a separate task.

3. Cross-translation patterns

The three languages are semantically similar but differ in syntax. Pattern per category:

sigma-cli with the right backend (see detection-engineer) automates much of this. The reviewer role remains: check edge cases (field-name mapping, semantic difference between wildcard and contains).

4. Performance tuning

A correct-but-slow query is operationally not usable. Top categories:

• Index/scope filtering first. SPL: index= directly. KQL: table-specific (SigninLogs, not union *). Elastic: tighten the index pattern.
• Time-range discipline. Default last 24h, not "all time". Every extension deliberate.
• Avoid eager field extraction: SPL extract on mass data is expensive — use regex-extracted fields at index time where possible.
• Subsearches to a minimum. SPL subsearches run a separate search and have a default 10k-result limit. KQL let with materialize is often more performant.
• Use data models (Splunk CIM, Sentinel ASIM, ECS). A tstats against an accelerated CIM data model is orders of magnitude faster than equivalent search-time extraction.
• Summary indexes / scheduled saved searches for recurring aggregations. Do not recompute the same hourly count from scratch — roll up once per hour.
• Cross-cluster search (CCS) in Splunk and Elastic: deliberate about whether you want to span clusters. Latency cost.
• JOIN cost: in KQL join, order matters; smaller table on the left. Elastic's enrich pipeline at index time is often better than a query-time join.
• Result cardinality: a query that returns 10M rows in order to show 100 has a forgotten filter.

Profiling help:

• Splunk: the Search Job Inspector shows per-stage time. tstats vs raw-search comparison.
• Sentinel/Defender: query-takes-long warnings + set notruncation=true debug, evaluate execute_query_stats(...).
• Elastic: profile API for the query-execution plan.

5. Library management

Well-built queries are reusable. Discipline to keep them that way:

• Detection-as-code repo with Sigma rules + transpiled queries + saved searches in version control. Not just in the SIEM UI.
• Macros / functions / saved searches: SPL macros, KQL functions (function keyword), Elastic painless scripts. Reuse per common sub-pattern (define "what is a suspicious LSASS access" once, use it in N rules).
• Naming convention: prefix pattern for saved searches (sec_, hunt_, dashboard_) so the purpose is immediately clear.
• Documentation per query: purpose, owner, last-tested date, FP notes. In the commit message or comment block.

6. Verification-loop

Layer 1: scope (query covers all relevant fields + time window, no unintended scope creep through wildcard table selection?), assumptions (data model up to date, schema fields exist?), gaps (FP path checked, not just the TP path?). Layer 2: function names against current platform docs (KQL and SPL add and deprecate functions), no invented table names or field syntax, time-bucket syntax correct, performance claims supported by profiling output.

Output

Query package — <goal>
Platform target: <Splunk SPL | Sentinel KQL | Defender KQL | Elastic EQL/KQL | multi>
Data source(s):  <indexes / tables / index patterns>
Time window:     <default + customizable>

Query (primary):
  <full query text>

Translations (multi-platform):
  Splunk SPL: <...>
  KQL:        <...>
  EQL:        <...>

Performance profile:
  Test run:        <events scanned, runtime>
  Bottleneck:      <field extraction / subsearch / join / etc>
  Optimization applied: <data model / summary / index filter>

Edge cases / FPs:
  - <known FP pattern, how to filter>
  - <field-name issues when translating between platforms>

Library integration:
  Saved as:                <macro/function/saved-search name>
  Detection-as-code path:  <repo path>

Handoffs:
  detection-engineer:  <if query becomes the basis for a rule>
  log-triage:          <if query is for incident investigation>
  threat-hunt:         <hunt context>

Verification-loop: ...

References

• Splunk Search Reference — https://docs.splunk.com/Documentation/Splunk/latest/SearchReference/. Full SPL reference.
• Splunk Common Information Model (CIM) — https://docs.splunk.com/Documentation/CIM/latest/User/Overview. Cross-source data model.
• Microsoft KQL quick reference — https://learn.microsoft.com/en-us/azure/data-explorer/kql-quick-reference.
• Microsoft Sentinel ASIM — https://learn.microsoft.com/en-us/azure/sentinel/normalization. Cross-source normalization for Sentinel.
• Microsoft Defender XDR Hunting — https://learn.microsoft.com/en-us/defender-xdr/advanced-hunting-overview.
• Elastic Security Reference — https://www.elastic.co/guide/en/security/current/.
• Elastic Common Schema (ECS) — https://www.elastic.co/guide/en/ecs/current/index.html. Field-naming standard.
• Elastic ESQL — https://www.elastic.co/guide/en/elasticsearch/reference/current/esql.html. Newer unified query language.
• Sigma project — https://github.com/SigmaHQ/sigma. Source of truth for cross-platform rules.
• uncoder.io — https://uncoder.io/. Browser tool for cross-platform query translation; always verify the output.

Categories

• blue