diff --git a/x-pack/plugins/cases/common/constants/index.ts b/x-pack/plugins/cases/common/constants/index.ts
index dcaf2bb0e6460..45125e4411f3a 100644
--- a/x-pack/plugins/cases/common/constants/index.ts
+++ b/x-pack/plugins/cases/common/constants/index.ts
@@ -127,6 +127,7 @@ export const MAX_DELETE_IDS_LENGTH = 100 as const;
 export const MAX_SUGGESTED_PROFILES = 10 as const;
 export const MAX_CASES_TO_UPDATE = 100 as const;
 export const MAX_BULK_CREATE_ATTACHMENTS = 100 as const;
+export const MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES = 100 as const;
 
 /**
  * Cases features
diff --git a/x-pack/plugins/cases/server/client/cases/mock.ts b/x-pack/plugins/cases/server/client/cases/mock.ts
index bee726524061a..ca96d20bad570 100644
--- a/x-pack/plugins/cases/server/client/cases/mock.ts
+++ b/x-pack/plugins/cases/server/client/cases/mock.ts
@@ -9,7 +9,7 @@ import type { CaseUserActionsDeprecatedResponse } from '../../../common/types/ap
 import { ConnectorTypes, UserActionActions } from '../../../common/types/domain';
 import type { Comment, CommentResponseAlertsType } from '../../../common/api';
 import { CommentType, ExternalReferenceStorageType } from '../../../common/api';
-import { SECURITY_SOLUTION_OWNER } from '../../../common/constants';
+import { FILE_ATTACHMENT_TYPE, SECURITY_SOLUTION_OWNER } from '../../../common/constants';
 
 export const updateUser = {
   updated_at: '2020-03-13T08:34:53.450Z',
@@ -228,6 +228,16 @@ export const commentPersistableState: Comment = {
   version: 'WzEsMV0=',
 };
 
+export const commentFileExternalReference: Comment = {
+  ...commentExternalReference,
+  externalReferenceAttachmentTypeId: FILE_ATTACHMENT_TYPE,
+  externalReferenceMetadata: { files: [{ name: '', extension: '', mimeType: '', created: '' }] },
+  externalReferenceStorage: {
+    type: ExternalReferenceStorageType.savedObject as const,
+    soType: 'file',
+  },
+};
+
 export const basicParams = {
   description: 'a description',
   title: 'a title',
diff --git a/x-pack/plugins/cases/server/common/limiter_checker/base_limiter.ts b/x-pack/plugins/cases/server/common/limiter_checker/base_limiter.ts
index cf9fcec712e61..f4e72554636ae 100644
--- a/x-pack/plugins/cases/server/common/limiter_checker/base_limiter.ts
+++ b/x-pack/plugins/cases/server/common/limiter_checker/base_limiter.ts
@@ -10,8 +10,8 @@ import type { Limiter } from './types';
 
 interface LimiterParams {
   limit: number;
-  attachmentType: CommentType;
-  field: string;
+  attachmentType: CommentType | CommentType[];
+  field?: string;
   attachmentNoun: string;
 }
 
diff --git a/x-pack/plugins/cases/server/common/limiter_checker/index.ts b/x-pack/plugins/cases/server/common/limiter_checker/index.ts
index 97928f2eeb356..1ef34d70e7e25 100644
--- a/x-pack/plugins/cases/server/common/limiter_checker/index.ts
+++ b/x-pack/plugins/cases/server/common/limiter_checker/index.ts
@@ -13,6 +13,7 @@ import type { AttachmentService } from '../../services';
 import type { Limiter } from './types';
 import { AlertLimiter } from './limiters/alerts';
 import { FileLimiter } from './limiters/files';
+import { PersistableStateAndExternalReferencesLimiter } from './limiters/persistable_state_and_external_references';
 
 export class AttachmentLimitChecker {
   private readonly limiters: Limiter[];
@@ -22,7 +23,11 @@ export class AttachmentLimitChecker {
     fileService: FileServiceStart,
     private readonly caseId: string
   ) {
-    this.limiters = [new AlertLimiter(attachmentService), new FileLimiter(fileService)];
+    this.limiters = [
+      new AlertLimiter(attachmentService),
+      new FileLimiter(fileService),
+      new PersistableStateAndExternalReferencesLimiter(attachmentService),
+    ];
   }
 
   public async validate(requests: CommentRequest[]) {
diff --git a/x-pack/plugins/cases/server/common/limiter_checker/limiters/persistable_state_and_external_references.test.ts b/x-pack/plugins/cases/server/common/limiter_checker/limiters/persistable_state_and_external_references.test.ts
new file mode 100644
index 0000000000000..df40e3841f1d7
--- /dev/null
+++ b/x-pack/plugins/cases/server/common/limiter_checker/limiters/persistable_state_and_external_references.test.ts
@@ -0,0 +1,85 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { createAttachmentServiceMock } from '../../../services/mocks';
+import { PersistableStateAndExternalReferencesLimiter } from './persistable_state_and_external_references';
+import {
+  createExternalReferenceRequests,
+  createFileRequests,
+  createPersistableStateRequests,
+  createUserRequests,
+} from '../test_utils';
+import { MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES } from '../../../../common/constants';
+
+describe('PersistableStateAndExternalReferencesLimiter', () => {
+  const caseId = 'test-id';
+  const attachmentService = createAttachmentServiceMock();
+  attachmentService.countPersistableStateAndExternalReferenceAttachments.mockResolvedValue(1);
+
+  const limiter = new PersistableStateAndExternalReferencesLimiter(attachmentService);
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  describe('public fields', () => {
+    it('sets the errorMessage to the 100 limit', () => {
+      expect(limiter.errorMessage).toMatchInlineSnapshot(
+        `"Case has reached the maximum allowed number (100) of attached persistable state and external reference attachments."`
+      );
+    });
+
+    it('sets the limit to 100', () => {
+      expect(limiter.limit).toBe(MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES);
+    });
+  });
+
+  describe('countOfItemsWithinCase', () => {
+    it('calls the attachment service with the right params', () => {
+      limiter.countOfItemsWithinCase(caseId);
+
+      expect(
+        attachmentService.countPersistableStateAndExternalReferenceAttachments
+      ).toHaveBeenCalledWith({ caseId });
+    });
+  });
+
+  describe('countOfItemsInRequest', () => {
+    it('returns 0 when passed an empty array', () => {
+      expect(limiter.countOfItemsInRequest([])).toBe(0);
+    });
+
+    it('returns 0 when the requests are not for persistable state attachments or external references', () => {
+      expect(limiter.countOfItemsInRequest(createUserRequests(2))).toBe(0);
+    });
+
+    it('counts persistable state attachments or external references correctly', () => {
+      expect(
+        limiter.countOfItemsInRequest([
+          createPersistableStateRequests(1)[0],
+          createExternalReferenceRequests(1)[0],
+          createUserRequests(1)[0],
+          createFileRequests({
+            numRequests: 1,
+            numFiles: 1,
+          })[0],
+        ])
+      ).toBe(2);
+    });
+
+    it('excludes fileAttachmentsRequests from the count', () => {
+      expect(
+        limiter.countOfItemsInRequest(
+          createFileRequests({
+            numRequests: 1,
+            numFiles: 1,
+          })
+        )
+      ).toBe(0);
+    });
+  });
+});
diff --git a/x-pack/plugins/cases/server/common/limiter_checker/limiters/persistable_state_and_external_references.ts b/x-pack/plugins/cases/server/common/limiter_checker/limiters/persistable_state_and_external_references.ts
new file mode 100644
index 0000000000000..b16baa919cffd
--- /dev/null
+++ b/x-pack/plugins/cases/server/common/limiter_checker/limiters/persistable_state_and_external_references.ts
@@ -0,0 +1,37 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import type { AttachmentService } from '../../../services';
+import { CommentType } from '../../../../common/api';
+import type { CommentRequest } from '../../../../common/api';
+import { MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES } from '../../../../common/constants';
+import { isFileAttachmentRequest, isPersistableStateOrExternalReference } from '../../utils';
+import { BaseLimiter } from '../base_limiter';
+
+export class PersistableStateAndExternalReferencesLimiter extends BaseLimiter {
+  constructor(private readonly attachmentService: AttachmentService) {
+    super({
+      limit: MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES,
+      attachmentType: [CommentType.persistableState, CommentType.externalReference],
+      attachmentNoun: 'persistable state and external reference attachments',
+    });
+  }
+
+  public async countOfItemsWithinCase(caseId: string): Promise<number> {
+    return this.attachmentService.countPersistableStateAndExternalReferenceAttachments({
+      caseId,
+    });
+  }
+
+  public countOfItemsInRequest(requests: CommentRequest[]): number {
+    const totalReferences = requests
+      .filter(isPersistableStateOrExternalReference)
+      .filter((request) => !isFileAttachmentRequest(request));
+
+    return totalReferences.length;
+  }
+}
diff --git a/x-pack/plugins/cases/server/common/limiter_checker/test_utils.ts b/x-pack/plugins/cases/server/common/limiter_checker/test_utils.ts
index f0c990e439bdf..fc9515c133069 100644
--- a/x-pack/plugins/cases/server/common/limiter_checker/test_utils.ts
+++ b/x-pack/plugins/cases/server/common/limiter_checker/test_utils.ts
@@ -11,6 +11,8 @@ import type {
   CommentRequestUserType,
   CommentRequestAlertType,
   FileAttachmentMetadata,
+  CommentRequestPersistableStateType,
+  CommentRequestExternalReferenceType,
 } from '../../../common/api';
 import type { FileAttachmentRequest } from '../types';
 
@@ -26,6 +28,37 @@ export const createUserRequests = (num: number): CommentRequestUserType[] => {
   return requests;
 };
 
+export const createPersistableStateRequests = (
+  num: number
+): CommentRequestPersistableStateType[] => {
+  return [...Array(num).keys()].map(() => {
+    return {
+      persistableStateAttachmentTypeId: '.test',
+      persistableStateAttachmentState: {},
+      type: CommentType.persistableState as const,
+      owner: 'test',
+    };
+  });
+};
+
+export const createExternalReferenceRequests = (
+  num: number
+): CommentRequestExternalReferenceType[] => {
+  return [...Array(num).keys()].map((value) => {
+    return {
+      type: CommentType.externalReference as const,
+      owner: 'test',
+      externalReferenceAttachmentTypeId: '.test',
+      externalReferenceId: 'so-id',
+      externalReferenceMetadata: {},
+      externalReferenceStorage: {
+        soType: `${value}`,
+        type: ExternalReferenceStorageType.savedObject,
+      },
+    };
+  });
+};
+
 export const createFileRequests = ({
   numRequests,
   numFiles,
diff --git a/x-pack/plugins/cases/server/common/models/case_with_comments.test.ts b/x-pack/plugins/cases/server/common/models/case_with_comments.test.ts
index 94b3ac3e3be3d..779ed8767a9b3 100644
--- a/x-pack/plugins/cases/server/common/models/case_with_comments.test.ts
+++ b/x-pack/plugins/cases/server/common/models/case_with_comments.test.ts
@@ -10,6 +10,12 @@ import type { SavedObject } from '@kbn/core-saved-objects-api-server';
 import { createCasesClientMockArgs } from '../../client/mocks';
 import { alertComment, comment, mockCaseComments, mockCases, multipleAlert } from '../../mocks';
 import { CaseCommentModel } from './case_with_comments';
+import { MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES } from '../../../common/constants';
+import {
+  commentExternalReference,
+  commentFileExternalReference,
+  commentPersistableState,
+} from '../../client/cases/mock';
 
 describe('CaseCommentModel', () => {
   const theCase = mockCases[0];
@@ -267,6 +273,52 @@ describe('CaseCommentModel', () => {
 
       expect(clientArgs.services.attachmentService.create).not.toHaveBeenCalled();
     });
+
+    describe('validation', () => {
+      clientArgs.services.attachmentService.countPersistableStateAndExternalReferenceAttachments.mockResolvedValue(
+        MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES
+      );
+
+      afterAll(() => {
+        jest.clearAllMocks();
+      });
+
+      it('throws if limit is reached when creating persistable state attachment', async () => {
+        await expect(
+          model.createComment({
+            id: 'comment-1',
+            commentReq: commentPersistableState,
+            createdDate,
+          })
+        ).rejects.toThrow(
+          `Case has reached the maximum allowed number (${MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES}) of attached persistable state and external reference attachments.`
+        );
+      });
+
+      it('throws if limit is reached when creating external reference', async () => {
+        await expect(
+          model.createComment({
+            id: 'comment-1',
+            commentReq: commentExternalReference,
+            createdDate,
+          })
+        ).rejects.toThrow(
+          `Case has reached the maximum allowed number (${MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES}) of attached persistable state and external reference attachments.`
+        );
+      });
+
+      it('does not throw if creating a file external reference and the limit is reached', async () => {
+        clientArgs.fileService.find.mockResolvedValue({ total: 0, files: [] });
+
+        await expect(
+          model.createComment({
+            id: 'comment-1',
+            commentReq: commentFileExternalReference,
+            createdDate,
+          })
+        ).resolves.not.toThrow();
+      });
+    });
   });
 
   describe('bulkCreate', () => {
@@ -526,5 +578,45 @@ describe('CaseCommentModel', () => {
       expect(multipleAlertsCall.attributes.alertId).toEqual(['test-id-3', 'test-id-5']);
       expect(multipleAlertsCall.attributes.index).toEqual(['test-index-3', 'test-index-5']);
     });
+
+    describe('validation', () => {
+      clientArgs.services.attachmentService.countPersistableStateAndExternalReferenceAttachments.mockResolvedValue(
+        MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES
+      );
+
+      afterAll(() => {
+        jest.clearAllMocks();
+      });
+
+      it('throws if limit is reached when creating persistable state attachment', async () => {
+        await expect(
+          model.bulkCreate({
+            attachments: [commentPersistableState],
+          })
+        ).rejects.toThrow(
+          `Case has reached the maximum allowed number (${MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES}) of attached persistable state and external reference attachments.`
+        );
+      });
+
+      it('throws if limit is reached when creating external reference', async () => {
+        await expect(
+          model.bulkCreate({
+            attachments: [commentExternalReference],
+          })
+        ).rejects.toThrow(
+          `Case has reached the maximum allowed number (${MAX_PERSISTABLE_STATE_AND_EXTERNAL_REFERENCES}) of attached persistable state and external reference attachments.`
+        );
+      });
+
+      it('does not throw if creating a file external reference and the limit is reached', async () => {
+        clientArgs.fileService.find.mockResolvedValue({ total: 0, files: [] });
+
+        await expect(
+          model.bulkCreate({
+            attachments: [commentFileExternalReference],
+          })
+        ).resolves.not.toThrow();
+      });
+    });
   });
 });
diff --git a/x-pack/plugins/cases/server/common/utils.test.ts b/x-pack/plugins/cases/server/common/utils.test.ts
index 1c42d75be05d1..ce470b66080ca 100644
--- a/x-pack/plugins/cases/server/common/utils.test.ts
+++ b/x-pack/plugins/cases/server/common/utils.test.ts
@@ -33,6 +33,7 @@ import {
   getCaseViewPath,
   isSOError,
   countUserAttachments,
+  isPersistableStateOrExternalReference,
 } from './utils';
 import { newCase } from '../routes/api/__mocks__/request_responses';
 import { CASE_VIEW_PAGE_TABS } from '../../common/types';
@@ -40,6 +41,12 @@ import { mockCases, mockCaseComments } from '../mocks';
 import { createAlertAttachment, createUserAttachment } from '../services/attachments/test_utils';
 import type { CaseConnector } from '../../common/types/domain';
 import { ConnectorTypes } from '../../common/types/domain';
+import {
+  createAlertRequests,
+  createExternalReferenceRequests,
+  createPersistableStateRequests,
+  createUserRequests,
+} from './limiter_checker/test_utils';
 
 interface CommentReference {
   ids: string[];
@@ -1353,4 +1360,25 @@ describe('common utils', () => {
       expect(countUserAttachments(attachments)).toBe(0);
     });
   });
+
+  describe('isPersistableStateOrExternalReference', () => {
+    it('returns true for persistable state request', () => {
+      expect(isPersistableStateOrExternalReference(createPersistableStateRequests(1)[0])).toBe(
+        true
+      );
+    });
+
+    it('returns true for external reference request', () => {
+      expect(isPersistableStateOrExternalReference(createExternalReferenceRequests(1)[0])).toBe(
+        true
+      );
+    });
+
+    it('returns false for other request types', () => {
+      expect(isPersistableStateOrExternalReference(createUserRequests(1)[0])).toBe(false);
+      expect(isPersistableStateOrExternalReference(createAlertRequests(1, 'alert-id')[0])).toBe(
+        false
+      );
+    });
+  });
 });
diff --git a/x-pack/plugins/cases/server/common/utils.ts b/x-pack/plugins/cases/server/common/utils.ts
index 78a4ffeb7593f..64e25eb78ac7b 100644
--- a/x-pack/plugins/cases/server/common/utils.ts
+++ b/x-pack/plugins/cases/server/common/utils.ts
@@ -254,6 +254,16 @@ export const isCommentRequestTypeAlert = (
   return context.type === CommentType.alert;
 };
 
+/**
+ * Returns true if a Comment Request is trying to create either a persistableState or an
+ * externalReference attachment.
+ */
+export const isPersistableStateOrExternalReference = (context: CommentRequest): boolean => {
+  return (
+    context.type === CommentType.persistableState || context.type === CommentType.externalReference
+  );
+};
+
 /**
  * A type narrowing function for file attachments.
  */
diff --git a/x-pack/plugins/cases/server/services/attachments/index.test.ts b/x-pack/plugins/cases/server/services/attachments/index.test.ts
index 54a48f0a53c67..610ce7df5bb9d 100644
--- a/x-pack/plugins/cases/server/services/attachments/index.test.ts
+++ b/x-pack/plugins/cases/server/services/attachments/index.test.ts
@@ -537,4 +537,111 @@ describe('AttachmentService', () => {
       });
     });
   });
+
+  describe('countPersistableStateAndExternalReferenceAttachments', () => {
+    it('does not throw and calls unsecuredSavedObjectsClient.find with the right parameters', async () => {
+      unsecuredSavedObjectsClient.find.mockResolvedValue(
+        createSOFindResponse([{ ...createUserAttachment(), score: 0 }])
+      );
+
+      await expect(
+        service.countPersistableStateAndExternalReferenceAttachments({ caseId: 'test-id' })
+      ).resolves.not.toThrow();
+
+      expect(unsecuredSavedObjectsClient.find.mock.calls[0][0]).toMatchInlineSnapshot(`
+        Object {
+          "filter": Object {
+            "arguments": Array [
+              Object {
+                "arguments": Array [
+                  Object {
+                    "arguments": Array [
+                      Object {
+                        "isQuoted": false,
+                        "type": "literal",
+                        "value": "cases-comments.attributes.type",
+                      },
+                      Object {
+                        "isQuoted": false,
+                        "type": "literal",
+                        "value": "persistableState",
+                      },
+                    ],
+                    "function": "is",
+                    "type": "function",
+                  },
+                  Object {
+                    "arguments": Array [
+                      Object {
+                        "isQuoted": false,
+                        "type": "literal",
+                        "value": "cases-comments.attributes.type",
+                      },
+                      Object {
+                        "isQuoted": false,
+                        "type": "literal",
+                        "value": "externalReference",
+                      },
+                    ],
+                    "function": "is",
+                    "type": "function",
+                  },
+                ],
+                "function": "or",
+                "type": "function",
+              },
+              Object {
+                "arguments": Array [
+                  Object {
+                    "arguments": Array [
+                      Object {
+                        "isQuoted": false,
+                        "type": "literal",
+                        "value": "cases-comments.attributes.externalReferenceAttachmentTypeId",
+                      },
+                      Object {
+                        "isQuoted": false,
+                        "type": "literal",
+                        "value": ".files",
+                      },
+                    ],
+                    "function": "is",
+                    "type": "function",
+                  },
+                ],
+                "function": "not",
+                "type": "function",
+              },
+            ],
+            "function": "and",
+            "type": "function",
+          },
+          "hasReference": Object {
+            "id": "test-id",
+            "type": "cases",
+          },
+          "page": 1,
+          "perPage": 1,
+          "sortField": "created_at",
+          "type": "cases-comments",
+        }
+      `);
+    });
+
+    it('returns the expected total', async () => {
+      const total = 3;
+
+      unsecuredSavedObjectsClient.find.mockResolvedValue(
+        createSOFindResponse(
+          Array(total).fill({ ...createUserAttachment({ foo: 'bar' }), score: 0 })
+        )
+      );
+
+      const res = await service.countPersistableStateAndExternalReferenceAttachments({
+        caseId: 'test-id',
+      });
+
+      expect(res).toBe(total);
+    });
+  });
 });
diff --git a/x-pack/plugins/cases/server/services/attachments/index.ts b/x-pack/plugins/cases/server/services/attachments/index.ts
index b0d282e00aa96..8f5c9ee69c07c 100644
--- a/x-pack/plugins/cases/server/services/attachments/index.ts
+++ b/x-pack/plugins/cases/server/services/attachments/index.ts
@@ -14,8 +14,13 @@ import type {
 } from '@kbn/core/server';
 
 import type * as estypes from '@elastic/elasticsearch/lib/api/typesWithBodyKey';
+import { fromKueryExpression } from '@kbn/es-query';
 import { CommentAttributesRt, CommentType, decodeOrThrow } from '../../../common/api';
-import { CASE_COMMENT_SAVED_OBJECT, CASE_SAVED_OBJECT } from '../../../common/constants';
+import {
+  CASE_COMMENT_SAVED_OBJECT,
+  CASE_SAVED_OBJECT,
+  FILE_ATTACHMENT_TYPE,
+} from '../../../common/constants';
 import { buildFilter, combineFilters } from '../../client/utils';
 import { defaultSortField, isSOError } from '../../common/utils';
 import type { AggregationResponse } from '../../client/metrics/types';
@@ -124,6 +129,50 @@ export class AttachmentService {
     }
   }
 
+  /**
+   * Counts the persistableState and externalReference attachments that are not .files
+   */
+  public async countPersistableStateAndExternalReferenceAttachments({
+    caseId,
+  }: {
+    caseId: string;
+  }): Promise<number> {
+    try {
+      this.context.log.debug(
+        `Attempting to count persistableState and externalReference attachments for case id ${caseId}`
+      );
+
+      const typeFilter = buildFilter({
+        filters: [CommentType.persistableState, CommentType.externalReference],
+        field: 'type',
+        operator: 'or',
+        type: CASE_COMMENT_SAVED_OBJECT,
+      });
+
+      const excludeFilesFilter = fromKueryExpression(
+        `not ${CASE_COMMENT_SAVED_OBJECT}.attributes.externalReferenceAttachmentTypeId: ${FILE_ATTACHMENT_TYPE}`
+      );
+
+      const combinedFilter = combineFilters([typeFilter, excludeFilesFilter]);
+
+      const response = await this.context.unsecuredSavedObjectsClient.find<{ total: number }>({
+        type: CASE_COMMENT_SAVED_OBJECT,
+        hasReference: { type: CASE_SAVED_OBJECT, id: caseId },
+        page: 1,
+        perPage: 1,
+        sortField: defaultSortField,
+        filter: combinedFilter,
+      });
+
+      return response.total;
+    } catch (error) {
+      this.context.log.error(
+        `Error while attempting to count persistableState and externalReference attachments for case id ${caseId}: ${error}`
+      );
+      throw error;
+    }
+  }
+
   /**
    * Executes the aggregations against the actions attached to a case.
    */
diff --git a/x-pack/plugins/cases/server/services/mocks.ts b/x-pack/plugins/cases/server/services/mocks.ts
index e9cb492e39458..b43a3c226b1e4 100644
--- a/x-pack/plugins/cases/server/services/mocks.ts
+++ b/x-pack/plugins/cases/server/services/mocks.ts
@@ -177,6 +177,7 @@ export const createAttachmentServiceMock = (): AttachmentServiceMock => {
     countAlertsAttachedToCase: jest.fn(),
     executeCaseActionsAggregations: jest.fn(),
     executeCaseAggregations: jest.fn(),
+    countPersistableStateAndExternalReferenceAttachments: jest.fn(),
   };
 
   // the cast here is required because jest.Mocked tries to include private members and would throw an error
diff --git a/x-pack/test/cases_api_integration/security_and_spaces/tests/common/comments/post_comment.ts b/x-pack/test/cases_api_integration/security_and_spaces/tests/common/comments/post_comment.ts
index 8851a95c6ebc3..0bcde310a224b 100644
--- a/x-pack/test/cases_api_integration/security_and_spaces/tests/common/comments/post_comment.ts
+++ b/x-pack/test/cases_api_integration/security_and_spaces/tests/common/comments/post_comment.ts
@@ -16,6 +16,7 @@ import {
   CaseStatuses,
   CommentRequestExternalReferenceSOType,
   CommentRequestAlertType,
+  ExternalReferenceStorageType,
 } from '@kbn/cases-plugin/common/api';
 import { FtrProviderContext } from '../../../../common/ftr_provider_context';
 import {
@@ -42,6 +43,7 @@ import {
   getCaseUserActions,
   removeServerGeneratedPropertiesFromUserAction,
   getAllComments,
+  bulkCreateAttachments,
 } from '../../../../common/lib/api';
 import {
   createSignalsIndex,
@@ -468,6 +470,76 @@ export default ({ getService }: FtrProviderContext): void => {
           expectedHttpCode: 400,
         });
       });
+
+      it('400s when attempting to add a persistable state to a case that already has 100', async () => {
+        const postedCase = await createCase(supertest, postCaseReq);
+
+        const attachments = Array(100).fill({
+          type: CommentType.externalReference as const,
+          owner: 'securitySolutionFixture',
+          externalReferenceAttachmentTypeId: '.test',
+          externalReferenceId: 'so-id',
+          externalReferenceMetadata: {},
+          externalReferenceStorage: {
+            soType: 'external-ref',
+            type: ExternalReferenceStorageType.savedObject as const,
+          },
+        });
+
+        await bulkCreateAttachments({
+          supertest,
+          caseId: postedCase.id,
+          params: attachments,
+          expectedHttpCode: 200,
+        });
+
+        await createComment({
+          supertest,
+          caseId: postedCase.id,
+          params: {
+            persistableStateAttachmentTypeId: '.test',
+            persistableStateAttachmentState: {},
+            type: CommentType.persistableState as const,
+            owner: 'securitySolutionFixture',
+          },
+          expectedHttpCode: 400,
+        });
+      });
+
+      it('400s when attempting to add an external reference to a case that already has 100', async () => {
+        const postedCase = await createCase(supertest, postCaseReq);
+
+        const attachments = Array(100).fill({
+          persistableStateAttachmentTypeId: '.test',
+          persistableStateAttachmentState: {},
+          type: CommentType.persistableState as const,
+          owner: 'securitySolutionFixture',
+        });
+
+        await bulkCreateAttachments({
+          supertest,
+          caseId: postedCase.id,
+          params: attachments,
+          expectedHttpCode: 200,
+        });
+
+        await createComment({
+          supertest,
+          caseId: postedCase.id,
+          params: {
+            type: CommentType.externalReference as const,
+            owner: 'securitySolutionFixture',
+            externalReferenceAttachmentTypeId: '.test',
+            externalReferenceId: 'so-id',
+            externalReferenceMetadata: {},
+            externalReferenceStorage: {
+              soType: 'external-ref',
+              type: ExternalReferenceStorageType.savedObject as const,
+            },
+          },
+          expectedHttpCode: 400,
+        });
+      });
     });
 
     describe('alerts', () => {
diff --git a/x-pack/test/cases_api_integration/security_and_spaces/tests/common/internal/bulk_create_attachments.ts b/x-pack/test/cases_api_integration/security_and_spaces/tests/common/internal/bulk_create_attachments.ts
index a0b626482f473..e5eaedb6dae8c 100644
--- a/x-pack/test/cases_api_integration/security_and_spaces/tests/common/internal/bulk_create_attachments.ts
+++ b/x-pack/test/cases_api_integration/security_and_spaces/tests/common/internal/bulk_create_attachments.ts
@@ -15,6 +15,7 @@ import {
   CaseStatuses,
   CommentRequestExternalReferenceSOType,
   CommentType,
+  ExternalReferenceStorageType,
 } from '@kbn/cases-plugin/common/api';
 import { FtrProviderContext } from '../../../../common/ftr_provider_context';
 import {
@@ -42,6 +43,7 @@ import {
   createAndUploadFile,
   deleteAllFiles,
   getAllComments,
+  createComment,
 } from '../../../../common/lib/api';
 import {
   createSignalsIndex,
@@ -619,102 +621,174 @@ export default ({ getService }: FtrProviderContext): void => {
         await createCaseAndBulkCreateAttachments({ supertest, expectedHttpCode: 400 });
       });
 
-      it('400s when attempting to add more than 1K alerts to a case', async () => {
-        const alerts = [...Array(1001).keys()].map((num) => `test-${num}`);
-        const postedCase = await createCase(supertest, postCaseReq);
-        await bulkCreateAttachments({
-          supertest,
-          caseId: postedCase.id,
-          params: [
-            {
-              ...postCommentAlertReq,
-              alertId: alerts,
-              index: alerts,
-            },
-          ],
-          expectedHttpCode: 400,
+      describe('validation', () => {
+        it('400s when attempting to add more than 1K alerts to a case', async () => {
+          const alerts = [...Array(1001).keys()].map((num) => `test-${num}`);
+          const postedCase = await createCase(supertest, postCaseReq);
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: [
+              {
+                ...postCommentAlertReq,
+                alertId: alerts,
+                index: alerts,
+              },
+            ],
+            expectedHttpCode: 400,
+          });
         });
-      });
 
-      it('400s when attempting to add more than 1K alerts to a case in the same request', async () => {
-        const alerts = [...Array(1001).keys()].map((num) => `test-${num}`);
-        const postedCase = await createCase(supertest, postCaseReq);
-        await bulkCreateAttachments({
-          supertest,
-          caseId: postedCase.id,
-          params: [
-            {
-              ...postCommentAlertReq,
-              alertId: alerts.slice(0, 500),
-              index: alerts.slice(0, 500),
-            },
-            {
-              ...postCommentAlertReq,
-              alertId: alerts.slice(500, alerts.length),
-              index: alerts.slice(500, alerts.length),
-            },
-            postCommentAlertReq,
-          ],
-          expectedHttpCode: 400,
+        it('400s when attempting to add more than 1K alerts to a case in the same request', async () => {
+          const alerts = [...Array(1001).keys()].map((num) => `test-${num}`);
+          const postedCase = await createCase(supertest, postCaseReq);
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: [
+              {
+                ...postCommentAlertReq,
+                alertId: alerts.slice(0, 500),
+                index: alerts.slice(0, 500),
+              },
+              {
+                ...postCommentAlertReq,
+                alertId: alerts.slice(500, alerts.length),
+                index: alerts.slice(500, alerts.length),
+              },
+              postCommentAlertReq,
+            ],
+            expectedHttpCode: 400,
+          });
         });
-      });
 
-      it('400s when attempting to add an alert to a case that already has 1K alerts', async () => {
-        const alerts = [...Array(1000).keys()].map((num) => `test-${num}`);
-        const postedCase = await createCase(supertest, postCaseReq);
-        await bulkCreateAttachments({
-          supertest,
-          caseId: postedCase.id,
-          params: [
-            {
-              ...postCommentAlertReq,
-              alertId: alerts,
-              index: alerts,
-            },
-          ],
+        it('400s when attempting to add an alert to a case that already has 1K alerts', async () => {
+          const alerts = [...Array(1000).keys()].map((num) => `test-${num}`);
+          const postedCase = await createCase(supertest, postCaseReq);
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: [
+              {
+                ...postCommentAlertReq,
+                alertId: alerts,
+                index: alerts,
+              },
+            ],
+          });
+
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: [
+              {
+                ...postCommentAlertReq,
+                alertId: 'test-id',
+                index: 'test-index',
+              },
+            ],
+            expectedHttpCode: 400,
+          });
         });
 
-        await bulkCreateAttachments({
-          supertest,
-          caseId: postedCase.id,
-          params: [
-            {
-              ...postCommentAlertReq,
-              alertId: 'test-id',
-              index: 'test-index',
-            },
-          ],
-          expectedHttpCode: 400,
+        it('400s when the case already has alerts and the sum of existing and new alerts exceed 1k', async () => {
+          const alerts = [...Array(1200).keys()].map((num) => `test-${num}`);
+          const postedCase = await createCase(supertest, postCaseReq);
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: [
+              {
+                ...postCommentAlertReq,
+                alertId: alerts.slice(0, 500),
+                index: alerts.slice(0, 500),
+              },
+            ],
+          });
+
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: [
+              {
+                ...postCommentAlertReq,
+                alertId: alerts.slice(500),
+                index: alerts.slice(500),
+              },
+              postCommentAlertReq,
+            ],
+            expectedHttpCode: 400,
+          });
         });
-      });
 
-      it('400s when the case already has alerts and the sum of existing and new alerts exceed 1k', async () => {
-        const alerts = [...Array(1200).keys()].map((num) => `test-${num}`);
-        const postedCase = await createCase(supertest, postCaseReq);
-        await bulkCreateAttachments({
-          supertest,
-          caseId: postedCase.id,
-          params: [
-            {
-              ...postCommentAlertReq,
-              alertId: alerts.slice(0, 500),
-              index: alerts.slice(0, 500),
+        it('400s when attempting to bulk create persistable state attachments reaching the 100 limit', async () => {
+          const postedCase = await createCase(supertest, postCaseReq);
+
+          await createComment({
+            supertest,
+            caseId: postedCase.id,
+            params: {
+              type: CommentType.externalReference as const,
+              owner: 'securitySolutionFixture',
+              externalReferenceAttachmentTypeId: '.test',
+              externalReferenceId: 'so-id',
+              externalReferenceMetadata: {},
+              externalReferenceStorage: {
+                soType: 'external-ref',
+                type: ExternalReferenceStorageType.savedObject as const,
+              },
             },
-          ],
+            expectedHttpCode: 200,
+          });
+
+          const persistableStateAttachments = Array(100).fill({
+            persistableStateAttachmentTypeId: '.test',
+            persistableStateAttachmentState: {},
+            type: CommentType.persistableState as const,
+            owner: 'securitySolutionFixture',
+          });
+
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: persistableStateAttachments,
+            expectedHttpCode: 400,
+          });
         });
 
-        await bulkCreateAttachments({
-          supertest,
-          caseId: postedCase.id,
-          params: [
-            {
-              ...postCommentAlertReq,
-              alertId: alerts.slice(500),
-              index: alerts.slice(500),
+        it('400s when attempting to bulk create >100 external reference attachments reaching the 100 limit', async () => {
+          const postedCase = await createCase(supertest, postCaseReq);
+
+          await createComment({
+            supertest,
+            caseId: postedCase.id,
+            params: {
+              persistableStateAttachmentTypeId: '.test',
+              persistableStateAttachmentState: {},
+              type: CommentType.persistableState as const,
+              owner: 'securitySolutionFixture',
             },
-            postCommentAlertReq,
-          ],
-          expectedHttpCode: 400,
+            expectedHttpCode: 200,
+          });
+
+          const externalRequestAttachments = Array(100).fill({
+            type: CommentType.externalReference as const,
+            owner: 'securitySolutionFixture',
+            externalReferenceAttachmentTypeId: '.test',
+            externalReferenceId: 'so-id',
+            externalReferenceMetadata: {},
+            externalReferenceStorage: {
+              soType: 'external-ref',
+              type: ExternalReferenceStorageType.savedObject as const,
+            },
+          });
+
+          await bulkCreateAttachments({
+            supertest,
+            caseId: postedCase.id,
+            params: externalRequestAttachments,
+            expectedHttpCode: 400,
+          });
         });
       });
     });