diff --git a/source/app/blueprints/case/case_assets_routes.py b/source/app/blueprints/case/case_assets_routes.py
index e7409e113..3f83244f3 100644
--- a/source/app/blueprints/case/case_assets_routes.py
+++ b/source/app/blueprints/case/case_assets_routes.py
@@ -68,6 +68,7 @@
 from app.util import ac_case_requires
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 
 case_assets_blueprint = Blueprint('case_assets',
                                   __name__,
@@ -203,7 +204,8 @@ def add_asset_modal(caseid):
     ioc = get_iocs(caseid)
     attributes = get_default_custom_attributes('asset')
 
-    return render_template("modal_add_case_multi_asset.html", form=form, asset=None, ioc=ioc, attributes=attributes)
+    return render_template("modal_add_case_multi_asset.html", form=form, asset=None, ioc=ioc, attributes=attributes,
+                           page_uid=generate_page_uid())
 
 
 @case_assets_blueprint.route('/case/assets/add', methods=['POST'])
@@ -390,7 +392,8 @@ def asset_view_modal(cur_id, caseid, url_redir):
     comments_map = get_case_assets_comments_count([cur_id])
 
     return render_template("modal_add_case_asset.html", form=form, asset=asset, map={}, ioc=case_iocs,
-                           ioc_prefill=ioc_prefill, attributes=asset.custom_attributes, comments_map=comments_map)
+                           ioc_prefill=ioc_prefill, attributes=asset.custom_attributes, comments_map=comments_map,
+                           page_uid=generate_page_uid())
 
 
 @case_assets_blueprint.route('/case/assets/update/<int:cur_id>', methods=['POST'])
diff --git a/source/app/blueprints/case/case_ioc_routes.py b/source/app/blueprints/case/case_ioc_routes.py
index 51834f464..9816ade3b 100644
--- a/source/app/blueprints/case/case_ioc_routes.py
+++ b/source/app/blueprints/case/case_ioc_routes.py
@@ -61,6 +61,7 @@
 from app.util import ac_case_requires
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 from app.business.iocs import create
 from app.business.iocs import update
 from app.business.iocs import delete
@@ -241,7 +242,8 @@ def case_add_ioc_modal(caseid):
 
     attributes = get_default_custom_attributes('ioc')
 
-    return render_template("modal_add_case_ioc.html", form=form, ioc=Ioc(), attributes=attributes)
+    return render_template("modal_add_case_ioc.html", form=form, ioc=Ioc(), attributes=attributes,
+                           page_uid=generate_page_uid())
 
 
 @case_ioc_blueprint.route('/case/ioc/delete/<int:cur_id>', methods=['POST'])
@@ -277,7 +279,7 @@ def case_view_ioc_modal(cur_id, caseid, url_redir):
     comments_map = get_case_iocs_comments_count([cur_id])
 
     return render_template("modal_add_case_ioc.html", form=form, ioc=ioc, attributes=ioc.custom_attributes,
-                           comments_map=comments_map)
+                           comments_map=comments_map, page_uid=generate_page_uid())
 
 
 @case_ioc_blueprint.route('/case/ioc/<int:cur_id>', methods=['GET'])
diff --git a/source/app/blueprints/case/case_rfiles_routes.py b/source/app/blueprints/case/case_rfiles_routes.py
index 09308de45..16f301f28 100644
--- a/source/app/blueprints/case/case_rfiles_routes.py
+++ b/source/app/blueprints/case/case_rfiles_routes.py
@@ -52,6 +52,7 @@
 from app.util import ac_case_requires
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 
 case_rfiles_blueprint = Blueprint(
     'case_rfiles',
@@ -149,14 +150,15 @@ def case_edit_rfile_modal(cur_id, caseid, url_redir):
     comments_map = get_case_evidence_comments_count([cur_id])
 
     return render_template("modal_add_case_rfile.html", rfile=crf, attributes=crf.custom_attributes,
-                           comments_map=comments_map)
+                           comments_map=comments_map, page_uid=generate_page_uid())
 
 
 @case_rfiles_blueprint.route('/case/evidences/add/modal', methods=['GET'])
 @ac_api_case_requires(CaseAccessLevel.full_access)
 def case_add_rfile_modal(caseid):
 
-    return render_template("modal_add_case_rfile.html", rfile=None, attributes=get_default_custom_attributes('evidence'))
+    return render_template("modal_add_case_rfile.html", rfile=None, attributes=get_default_custom_attributes('evidence'),
+                           page_uid=generate_page_uid())
 
 
 @case_rfiles_blueprint.route('/case/evidences/update/<int:cur_id>', methods=['POST'])
diff --git a/source/app/blueprints/case/case_tasks_routes.py b/source/app/blueprints/case/case_tasks_routes.py
index 999500ce3..b64772b70 100644
--- a/source/app/blueprints/case/case_tasks_routes.py
+++ b/source/app/blueprints/case/case_tasks_routes.py
@@ -59,6 +59,7 @@
 from app.util import ac_case_requires
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 
 case_tasks_blueprint = Blueprint('case_tasks',
                                  __name__,
@@ -138,7 +139,7 @@ def case_add_task_modal(caseid):
     form.task_assignees_id.choices = []
 
     return render_template("modal_add_case_task.html", form=form, task=task, uid=current_user.id, user_name=None,
-                           attributes=task.custom_attributes)
+                           attributes=task.custom_attributes, page_uid=generate_page_uid())
 
 
 @case_tasks_blueprint.route('/case/tasks/add', methods=['POST'])
@@ -207,7 +208,7 @@ def case_task_view_modal(cur_id, caseid, url_redir):
     comments_map = get_case_tasks_comments_count([task.id])
 
     return render_template("modal_add_case_task.html", form=form, task=task, user_name=user_name,
-                           comments_map=comments_map, attributes=task.custom_attributes)
+                           comments_map=comments_map, attributes=task.custom_attributes, page_uid=generate_page_uid())
 
 
 @case_tasks_blueprint.route('/case/tasks/update/<int:cur_id>', methods=['POST'])
diff --git a/source/app/blueprints/case/case_timeline_routes.py b/source/app/blueprints/case/case_timeline_routes.py
index 9816bc38b..9c1ee0a67 100644
--- a/source/app/blueprints/case/case_timeline_routes.py
+++ b/source/app/blueprints/case/case_timeline_routes.py
@@ -81,6 +81,7 @@
 from app.util import add_obj_history_entry
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 
 
 event_tags = ["Network", "Server", "ActiveDirectory", "Computer", "Malware", "User Interaction"]
@@ -759,7 +760,8 @@ def event_view_modal(cur_id, caseid, url_redir):
     return render_template("modal_add_case_event.html", form=form, event=event, user_name=usr_name, tags=event_tags,
                            assets=assets, iocs=iocs, comments_map=comments_map,
                            assets_prefill=assets_prefill, iocs_prefill=iocs_prefill,
-                           category=event.category, attributes=event.custom_attributes)
+                           category=event.category, attributes=event.custom_attributes,
+                           page_uid=generate_page_uid())
 
 
 @case_timeline_blueprint.route('/case/timeline/events/update/<int:cur_id>', methods=["POST"])
@@ -837,7 +839,7 @@ def case_add_event_modal(caseid):
 
     return render_template("modal_add_case_event.html", form=form, event=event,
                            tags=event_tags, assets=assets, iocs=iocs, assets_prefill=None, category=def_cat,
-                           attributes=event.custom_attributes)
+                           attributes=event.custom_attributes, page_uid=generate_page_uid())
 
 
 @case_timeline_blueprint.route('/case/timeline/filter-help/modal', methods=['GET'])
diff --git a/source/app/blueprints/manage/manage_attributes_routes.py b/source/app/blueprints/manage/manage_attributes_routes.py
index bf8fcfd16..270da1cb3 100644
--- a/source/app/blueprints/manage/manage_attributes_routes.py
+++ b/source/app/blueprints/manage/manage_attributes_routes.py
@@ -34,6 +34,7 @@
 from app.util import ac_requires
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 
 manage_attributes_blueprint = Blueprint('manage_attributes', __name__, template_folder='templates')
 
@@ -104,7 +105,7 @@ def attributes_preview(caseid, url_redir):
     except Exception as e:
         return response_error("Invalid JSON", data=str(e))
 
-    templated = render_template("modal_preview_attribute.html", attributes=attribute)
+    templated = render_template("modal_preview_attribute.html", attributes=attribute, page_uid=generate_page_uid())
 
     return response_success(data=templated)
 
diff --git a/source/app/blueprints/manage/manage_cases_routes.py b/source/app/blueprints/manage/manage_cases_routes.py
index b0d6580ad..8ba750945 100644
--- a/source/app/blueprints/manage/manage_cases_routes.py
+++ b/source/app/blueprints/manage/manage_cases_routes.py
@@ -67,6 +67,7 @@
 from app.util import ac_requires
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 from app.business.cases import delete
 from app.business.cases import update
 from app.business.cases import create
@@ -124,7 +125,7 @@ def details_case(cur_id: int, caseid: int, url_redir: bool) -> Union[str, Respon
         print(res.get('status_name'))
         return render_template("modal_case_info_from_case.html", data=res, form=form, protagonists=protagonists,
                                case_classifications=case_classifications, case_states=case_states, customers=customers,
-                               severities=severities)
+                               severities=severities, page_uid=generate_page_uid())
 
     else:
         return response_error("Unknown case")
@@ -340,7 +341,7 @@ def add_case_modal():
 
     attributes = get_default_custom_attributes('case')
 
-    return render_template('modal_add_case.html', form=form, attributes=attributes)
+    return render_template('modal_add_case.html', form=form, attributes=attributes, page_uid=generate_page_uid())
 
 
 @manage_cases_blueprint.route('/manage/cases/add', methods=['POST'])
diff --git a/source/app/blueprints/manage/manage_customers_routes.py b/source/app/blueprints/manage/manage_customers_routes.py
index aff4c6d49..f996408a0 100644
--- a/source/app/blueprints/manage/manage_customers_routes.py
+++ b/source/app/blueprints/manage/manage_customers_routes.py
@@ -58,6 +58,7 @@
 from app.util import page_not_found
 from app.util import response_error
 from app.util import response_success
+from app.util import generate_page_uid
 
 manage_customers_blueprint = Blueprint(
     'manage_customers',
@@ -312,7 +313,7 @@ def view_customer_modal(client_id, caseid, url_redir):
     form.customer_sla.data = customer.sla
 
     return render_template("modal_add_customer.html", form=form, customer=customer,
-                           attributes=customer.custom_attributes)
+                           attributes=customer.custom_attributes, page_uid=generate_page_uid())
 
 
 @manage_customers_blueprint.route('/manage/customers/update/<int:client_id>', methods=['POST'])
@@ -346,7 +347,8 @@ def add_customers_modal(caseid, url_redir):
         return redirect(url_for('manage_customers.manage_customers', cid=caseid))
     form = AddCustomerForm()
     attributes = get_default_custom_attributes('client')
-    return render_template("modal_add_customer.html", form=form, customer=None, attributes=attributes)
+    return render_template("modal_add_customer.html", form=form, customer=None, attributes=attributes,
+                           page_uid=generate_page_uid())
 
 
 @manage_customers_blueprint.route('/manage/customers/add', methods=['POST'])
diff --git a/source/app/util.py b/source/app/util.py
index 4bdff48fe..41d85b72d 100644
--- a/source/app/util.py
+++ b/source/app/util.py
@@ -504,6 +504,19 @@ def regenerate_session():
     session.modified = True
 
 
+def generate_page_uid() -> str:
+    """Generate a short, DOM-safe, per-render-unique id prefix.
+
+    Namespaces the ids `modal_attributes_tabs.html` / `modal_attributes_nav.html` generate for
+    custom-attribute tabs and fields (see dfir-iris/iris-web#1111). Bootstrap 4's `data-toggle="pill"`
+    resolves its target pane via a document-global CSS id lookup on the link's `href`, not scoped to
+    the surrounding `.tab-content` — so every render must get its own unique prefix or two
+    simultaneously-rendered attribute-tab blocks collide and tab-switching breaks.
+    """
+    return f'pu{uuid.uuid4().hex[:8]}_'
+
+
+
 def api_login_required(f):
     @wraps(f)
     def wrap(*args, **kwargs):
diff --git a/tests/iris.py b/tests/iris.py
index 5f377b40d..bef41d0cc 100644
--- a/tests/iris.py
+++ b/tests/iris.py
@@ -17,16 +17,24 @@
 #  Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
 from pathlib import Path
+import re
 import shutil
 import time
+import requests
 from docker_compose import DockerCompose
 from rest_api import RestApi
 from server_timeout_error import ServerTimeoutError
 
 API_URL = 'http://127.0.0.1:8000'
+# Cookie-session routes need this, not API_URL: SESSION_COOKIE_SECURE=True (source/app/configuration.py)
+# means Flask's session cookie is never sent back over plain HTTP, only through nginx/HTTPS.
+HTTPS_URL = 'https://127.0.0.1'
 _API_KEY = 'B8BA5D730210B50F41C06941582D7965D57319D5685440587F98DFDC45A01594'
 _IRIS_PATH = Path('..')
 _TEST_DATA_PATH = Path('./data')
+# Matches IRIS_ADM_USERNAME (default) / IRIS_ADM_PASSWORD in tests/data/basic.env.
+_ADMIN_USERNAME = 'administrator'
+_ADMIN_PASSWORD = 'MySuperAdminPassword!'
 
 
 class Iris:
@@ -102,3 +110,26 @@ def get_cases(self):
 
     def get_cases_filter(self):
         return self._api.get('/manage/cases/filter')
+
+    def get_authenticated_session(self):
+        """Log in with a real cookie session (not the API key) for routes gated by @ac_requires(),
+        which read session['permissions'] directly and 500 if authenticated via API key only.
+        Must go through nginx/HTTPS (HTTPS_URL) -- SESSION_COOKIE_SECURE=True means the session
+        cookie is dropped by the client on a plain-HTTP round trip against API_URL."""
+        session = requests.Session()
+        session.verify = False
+        login_page = session.get(f'{HTTPS_URL}/login').text
+        csrf_match = re.search(r'name="csrf_token"[^>]*value="([^"]+)"', login_page)
+        if not csrf_match:
+            raise ServerTimeoutError('Could not find CSRF token on login page')
+        session.post(f'{HTTPS_URL}/login', data={
+            'csrf_token': csrf_match.group(1),
+            'username': _ADMIN_USERNAME,
+            'password': _ADMIN_PASSWORD,
+        })
+        return session
+
+    def get_html(self, session, path, case_id=None):
+        params = {'cid': case_id} if case_id is not None else None
+        response = session.get(f'{HTTPS_URL}{path}', params=params)
+        return response.status_code, response.text
diff --git a/tests/rest_api.py b/tests/rest_api.py
index 12e35a4c4..cc10c9188 100644
--- a/tests/rest_api.py
+++ b/tests/rest_api.py
@@ -37,6 +37,12 @@ def get(self, path, query_parameters=None):
         print(f'GET {url} => {response.status_code} {body}')
         return body
 
+    def get_raw(self, path, query_parameters=None):
+        url = self._build_url(path)
+        response = requests.get(url, headers=self._headers, params=query_parameters)
+        print(f'GET {url} => {response.status_code} ({len(response.text)} bytes, non-JSON)')
+        return response.status_code, response.text
+
     def post(self, path, payload, query_parameters=None):
         url = self._build_url(path)
         response = requests.post(url, headers=self._headers, params=query_parameters, json=payload)
diff --git a/tests/tests.py b/tests/tests.py
index 7c9f83659..90b6816f8 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -16,6 +16,8 @@
 #  along with this program; if not, write to the Free Software Foundation,
 #  Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 from unittest import TestCase
+import json
+import re
 from iris import Iris
 
 
@@ -67,4 +69,51 @@ def test_update_case_should_not_require_case_name_issue_358(self):
         case = self._subject.create_case()
         case_identifier = case['case_id']
         response = self._subject.update_case(case_identifier, {'case_tags': 'test,example'})
-        self.assertEqual('success', response['status'])
\ No newline at end of file
+        self.assertEqual('success', response['status'])
+
+    def test_custom_attribute_tabs_have_unique_ids_across_cases(self):
+        # Look up the 'case' attribute definition dynamically -- do not hardcode its id,
+        # the seeded attribute_id ordering is an implementation detail, not a contract.
+        attributes = self._subject._api.get('/manage/attributes/list')['data']
+        case_attribute = next(a for a in attributes if a['attribute_for'] == 'case')
+
+        schema = {
+            'Investigation Metadata': {
+                'MITRE ATT&CK Tactic': {'type': 'input_string', 'mandatory': False, 'value': ''},
+            }
+        }
+
+        # Create the cases BEFORE (re-)applying the attribute definition. New cases are not seeded
+        # with custom_attributes at creation time (that only happens via the "Add case" modal echoing
+        # its pre-fetched defaults back in the payload) -- but updating the attribute definition runs
+        # update_all_attributes(), which retroactively backfills every case whose custom_attributes is
+        # still None. Creating first and updating second reliably exercises that backfill path.
+        case_a = self._subject.create_case()
+        case_b = self._subject.create_case()
+
+        response = self._subject._api.post(
+            f'/manage/attributes/update/{case_attribute["attribute_id"]}',
+            {'attribute_content': json.dumps(schema), 'partial_overwrite': True, 'complete_overwrite': False}
+        )
+        self.assertEqual('success', response['status'])
+
+        session = self._subject.get_authenticated_session()
+        status_a, html_a = self._subject.get_html(session, f'/case/details/{case_a["case_id"]}', case_id=case_a['case_id'])
+        status_b, html_b = self._subject.get_html(session, f'/case/details/{case_b["case_id"]}', case_id=case_b['case_id'])
+
+        self.assertEqual(200, status_a)
+        self.assertEqual(200, status_b)
+
+        # "Info" must still be the default active tab in both renders.
+        self.assertIn('class="nav-link active show" id="pills-home-tab-nobd"', html_a)
+        self.assertIn('class="nav-link active show" id="pills-home-tab-nobd"', html_b)
+
+        # The generated tab id for the custom attribute pane must differ between two
+        # independently-rendered cases -- this is false on the unfixed code (both render
+        # "1_investigation_metadata" verbatim) and true once page_uid is a real per-render value.
+        id_pattern = re.compile(r'id="(\w*1_investigation_metadata)"')
+        id_a = id_pattern.search(html_a)
+        id_b = id_pattern.search(html_b)
+        self.assertIsNotNone(id_a, 'Investigation Metadata tab pane not found in case A details HTML')
+        self.assertIsNotNone(id_b, 'Investigation Metadata tab pane not found in case B details HTML')
+        self.assertNotEqual(id_a.group(1), id_b.group(1))
\ No newline at end of file