[Security Solution][Detections] Fixes Severity Override not matching …

…for Elastic Endpoint Security rule (#74317)

## Summary

Fixes an issue where the `Severity Override` would not match for the `Elastic Endpoint Security` rule. This is a temporary fix until we can provide more robust comparisons between the user provided `severityMapping.value` (string) and the `severityMapping.field`'s type.  

### Checklist

- [x] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios

x-pack/plugins/security_solution/server/lib/detection_engine/signals/__mocks__/es_results.ts

@@ -109,6 +109,24 @@ export const sampleDocNoSortId = (
   sort: [],
 });
 
+export const sampleDocSeverity = (
+  severity?: Array<string | number | null> | string | number | null
+): SignalSourceHit => ({
+  _index: 'myFakeSignalIndex',
+  _type: 'doc',
+  _score: 100,
+  _version: 1,
+  _id: sampleIdGuid,
+  _source: {
+    someKey: 'someValue',
+    '@timestamp': '2020-04-20T21:27:45+0000',
+    event: {
+      severity: severity ?? 100,
+    },
+  },
+  sort: [],
+});
+
 export const sampleEmptyDocSearchResults = (): SignalSearchResponse => ({
   took: 10,
   timed_out: false,

x-pack/plugins/security_solution/server/lib/detection_engine/signals/mappings/build_severity_from_mapping.test.ts

@@ -4,15 +4,15 @@
  * you may not use this file except in compliance with the Elastic License.
  */
 
-import { sampleDocNoSortId } from '../__mocks__/es_results';
+import { sampleDocNoSortId, sampleDocSeverity } from '../__mocks__/es_results';
 import { buildSeverityFromMapping } from './build_severity_from_mapping';
 
 describe('buildSeverityFromMapping', () => {
   beforeEach(() => {
     jest.clearAllMocks();
   });
 
-  test('severity defaults to provided if mapping is incomplete', () => {
+  test('severity defaults to provided if mapping is undefined', () => {
     const severity = buildSeverityFromMapping({
       doc: sampleDocNoSortId(),
       severity: 'low',
@@ -22,5 +22,45 @@ describe('buildSeverityFromMapping', () => {
     expect(severity).toEqual({ severity: 'low', severityMeta: {} });
   });
 
+  test('severity is overridden to highest matched mapping', () => {
+    const severity = buildSeverityFromMapping({
+      doc: sampleDocSeverity(23),
+      severity: 'low',
+      severityMapping: [
+        { field: 'event.severity', operator: 'equals', value: '23', severity: 'critical' },
+        { field: 'event.severity', operator: 'equals', value: '23', severity: 'low' },
+        { field: 'event.severity', operator: 'equals', value: '11', severity: 'critical' },
+        { field: 'event.severity', operator: 'equals', value: '23', severity: 'medium' },
+      ],
+    });
+
+    expect(severity).toEqual({
+      severity: 'critical',
+      severityMeta: {
+        severityOverrideField: 'event.severity',
+      },
+    });
+  });
+
+  test('severity is overridden when field is event.severity and source value is number', () => {
+    const severity = buildSeverityFromMapping({
+      doc: sampleDocSeverity(23),
+      severity: 'low',
+      severityMapping: [
+        { field: 'event.severity', operator: 'equals', value: '13', severity: 'low' },
+        { field: 'event.severity', operator: 'equals', value: '23', severity: 'medium' },
+        { field: 'event.severity', operator: 'equals', value: '33', severity: 'high' },
+        { field: 'event.severity', operator: 'equals', value: '43', severity: 'critical' },
+      ],
+    });
+
+    expect(severity).toEqual({
+      severity: 'medium',
+      severityMeta: {
+        severityOverrideField: 'event.severity',
+      },
+    });
+  });
+
   // TODO: Enhance...
 });

x-pack/plugins/security_solution/server/lib/detection_engine/signals/mappings/build_severity_from_mapping.ts

@@ -24,17 +24,38 @@ interface BuildSeverityFromMappingReturn {
   severityMeta: Meta; // TODO: Stricter types
 }
 
+const severitySortMapping = {
+  low: 0,
+  medium: 1,
+  high: 2,
+  critical: 3,
+};
+
 export const buildSeverityFromMapping = ({
   doc,
   severity,
   severityMapping,
 }: BuildSeverityFromMappingProps): BuildSeverityFromMappingReturn => {
   if (severityMapping != null && severityMapping.length > 0) {
     let severityMatch: SeverityMappingItem | undefined;
-    severityMapping.forEach((mapping) => {
-      // TODO: Expand by verifying fieldType from index via  doc._index
-      const mappedValue = get(mapping.field, doc._source);
-      if (mapping.value === mappedValue) {
+
+    // Sort the SeverityMapping from low to high, so last match (highest severity) is used
+    const severityMappingSorted = severityMapping.sort(
+      (a, b) => severitySortMapping[a.severity] - severitySortMapping[b.severity]
+    );
+
+    severityMappingSorted.forEach((mapping) => {
+      const docValue = get(mapping.field, doc._source);
+      // TODO: Expand by verifying fieldType from index via doc._index
+      // Till then, explicit parsing of event.severity (long) to number. If not ECS, this could be
+      // another datatype, but until we can lookup datatype we must assume number for the Elastic
+      // Endpoint Security rule to function correctly
+      let parsedMappingValue: string | number = mapping.value;
+      if (mapping.field === 'event.severity') {
+        parsedMappingValue = Math.floor(Number(parsedMappingValue));
+      }
+
+      if (parsedMappingValue === docValue) {
         severityMatch = { ...mapping };
       }
     });