diff --git a/api_schemas/ad_schema.py b/api_schemas/ad_schema.py
index 1b52ef96..1875b1e9 100644
--- a/api_schemas/ad_schema.py
+++ b/api_schemas/ad_schema.py
@@ -3,6 +3,7 @@
 from api_schemas.base_schema import BaseSchema
 from helpers.constants import MAX_BOOK_AUTHOR, MAX_BOOK_TITLE
 
+
 class AdRead(BaseSchema):
     ad_id: int
     title: str
@@ -12,17 +13,17 @@ class AdRead(BaseSchema):
     user_id: int
     selling: bool
     condition: int
-    
-    
+
+
 class AdCreate(BaseSchema):
     title: Annotated[str, StringConstraints(max_length=MAX_BOOK_TITLE)]
     author: Annotated[str, StringConstraints(max_length=MAX_BOOK_AUTHOR)] | None
     price: int | None
     course: Annotated[str, StringConstraints(max_length=MAX_BOOK_TITLE)] | None
-    user_id: int
     selling: bool
     condition: int
-    
+
+
 class AdUpdate(BaseSchema):
     title: Annotated[str, StringConstraints(max_length=MAX_BOOK_TITLE)] | None = None
     author: Annotated[str, StringConstraints(max_length=MAX_BOOK_AUTHOR)] | None = None
diff --git a/helpers/csv_response_factory.py b/helpers/csv_response_factory.py
index 6d397e12..3b94b0e2 100644
--- a/helpers/csv_response_factory.py
+++ b/helpers/csv_response_factory.py
@@ -6,9 +6,29 @@
 
 from api_schemas.csv_schemas.base_csv_schema import BaseCsvSchema
 
-
 T = TypeVar("T", bound=BaseCsvSchema)
 
+# Characters which make Excel/LibreOffice treat a cell as a formula instead of text.
+# Leading tab/CR are included because spreadsheet apps strip them before parsing.
+FORMULA_TRIGGERS = ("=", "+", "-", "@", "\t", "\r")
+
+
+def escape_csv_value(value: str) -> str:
+    """
+    Neutralize spreadsheet formula injection (CWE-1236).
+
+    Much of what we export is free text written by ordinary members (names,
+    food preferences, motivations). Without this, a member can set their food
+    preference to something like `=cmd|'/c calc'!A1` and get it executed on the
+    machine of whichever admin opens the exported file.
+
+    Prefixing with a single quote makes the spreadsheet render the literal text.
+    This is pretty standard, as per CWE-1236. Only the first char matters.
+    """
+    if value.startswith(FORMULA_TRIGGERS):
+        return f"'{value}"
+    return value
+
 
 class CsvResponseFactory(Generic[T]):
     def __init__(self, none_str: str = "") -> None:
@@ -22,7 +42,7 @@ def append(self, row: T) -> None:
 
         dump = row.model_dump(by_alias=True)
         for k in self.__columns.keys():
-            self.__columns[k].append(str(dump.get(k) or self.none_str))
+            self.__columns[k].append(escape_csv_value(str(dump.get(k) or self.none_str)))
 
     def __initialize_headers(self, row: T) -> None:
         model_fields = {
diff --git a/routes/access_serve_router.py b/routes/access_serve_router.py
index 1b1c1737..509c4e64 100644
--- a/routes/access_serve_router.py
+++ b/routes/access_serve_router.py
@@ -6,7 +6,6 @@
 from typing import get_args
 import datetime
 
-
 access_serve_router = APIRouter()
 
 
@@ -78,9 +77,6 @@ def get_all_access_ids(door: str, db: DB_dependency) -> list[str]:
     all_access_ids = sorted(set(direct_access_ids + post_access_ids))
 
     # Remove all stil-ids which are not alphanumeric with dashes,
-    # just a failsafe if stil_id is not set properly since we will be putting these in html
-    for stil_id in all_access_ids:
-        if not stil_id or not stil_id.replace("-", "").isalnum():
-            all_access_ids.remove(stil_id)
-
-    return all_access_ids
+    # just a failsafe if stil_id is not set properly since we will be putting these in html.
+    # Build a new list to avoid bugs related to skipping elements.
+    return [stil_id for stil_id in all_access_ids if stil_id and stil_id.replace("-", "").isalnum()]
diff --git a/routes/ad_router.py b/routes/ad_router.py
index 04799829..8579f0f5 100644
--- a/routes/ad_router.py
+++ b/routes/ad_router.py
@@ -17,12 +17,12 @@ def get_all_ads(db: DB_dependency):
 
 
 @ad_router.post("/", response_model=AdRead)
-def create_ad(data: AdCreate, db: DB_dependency):
+def create_ad(data: AdCreate, current_user: Annotated[User_DB, Permission.member()], db: DB_dependency):
     ad = BookAd_DB(
         title=data.title,
         course=data.course,
         author=data.author,
-        user_id=data.user_id,
+        user_id=current_user.id,
         selling=data.selling,
         condition=data.condition,
         price=data.price,
diff --git a/routes/post_router.py b/routes/post_router.py
index 333182ee..a3a98ec5 100644
--- a/routes/post_router.py
+++ b/routes/post_router.py
@@ -124,7 +124,7 @@ def get_post(post_id: int, db: DB_dependency):
     return post
 
 
-@post_router.get("/users/{post_id}", response_model=list[SimpleUserRead])
+@post_router.get("/users/{post_id}", response_model=list[SimpleUserRead], dependencies=[Permission.member()])
 def get_all_users_with_post(post_id: int, db: DB_dependency):
     posts = db.query(Post_DB).filter_by(id=post_id).one_or_none()
     if posts is None:
diff --git a/tests/test_security_regressions.py b/tests/test_security_regressions.py
new file mode 100644
index 00000000..ec1af548
--- /dev/null
+++ b/tests/test_security_regressions.py
@@ -0,0 +1,192 @@
+# type: ignore
+"""
+Regression tests for security fixes.
+"""
+
+import datetime
+
+import pytest
+
+from db_models.council_model import Council_DB
+from db_models.event_model import Event_DB
+from db_models.user_door_access_model import UserDoorAccess_DB
+
+from .basic_factories import auth_headers
+
+
+@pytest.fixture()
+def simple_event(db_session):
+    """A minimal event with signups unconfirmed."""
+    council = Council_DB(
+        name_sv="EventUtskott",
+        description_sv="beskrivning",
+        name_en="EventCouncil",
+        description_en="description",
+    )
+    db_session.add(council)
+    db_session.commit()
+
+    now = datetime.datetime.now(datetime.timezone.utc)
+    event = Event_DB(
+        council_id=council.id,
+        starts_at=now + datetime.timedelta(days=2),
+        ends_at=now + datetime.timedelta(days=3),
+        signup_start=now - datetime.timedelta(days=1),
+        signup_end=now + datetime.timedelta(days=1),
+        title_sv="Testevent",
+        title_en="Test event",
+        description_sv="beskrivning",
+        description_en="description",
+        location="LC",
+        dress_code="Cool",
+        price=0,
+    )
+    db_session.add(event)
+    db_session.commit()
+    db_session.refresh(event)
+    return event
+
+
+####################################################################
+# Posts: the member roster of a post was world-readable
+####################################################################
+
+
+def test_post_users_requires_authentication(client, member_post):
+    """
+    Listing who holds a post discloses guild members' names. The sibling route
+    GET /posts/ already requires membership; this one must match.
+    """
+    res = client.get(f"/posts/users/{member_post.id}")
+
+    assert res.status_code == 401, res.text
+
+
+def test_post_users_allowed_for_member(client, member_token, member_post, membered_user):
+    """Members must still be able to read a post's roster."""
+    res = client.get(
+        f"/posts/users/{member_post.id}",
+        headers=auth_headers(member_token),
+    )
+
+    assert res.status_code == 200, res.text
+    assert membered_user.id in [user["id"] for user in res.json()]
+
+
+####################################################################
+# Door access serving: malformed stil-ids must never be served
+####################################################################
+
+
+def _make_user_with_door_access(db_session, client, email, stil_id, door="Arkivet"):
+    from .basic_factories import create_membered_user
+
+    user = create_membered_user(client, db_session, email=email, first_name="Door", last_name="User")
+    user.stil_id = stil_id
+    db_session.commit()
+
+    now = datetime.datetime.now(datetime.timezone.utc)
+    access = UserDoorAccess_DB(
+        user_id=user.id,
+        door=door,
+        starttime=now - datetime.timedelta(days=1),
+        endtime=now + datetime.timedelta(days=1),
+    )
+    db_session.add(access)
+    db_session.commit()
+    return user
+
+
+def test_access_serve_filters_all_malformed_stil_ids(client, db_session):
+    """
+    The endpoint's output is interpolated into HTML by the frontend (after more html removal there),
+    so the non-alphanumeric failsafe is a security control. It previously mutated the
+    list while iterating over it, which silently skipped entries: with two
+    adjacent bad ids, the second one survived and was served.
+    """
+    # Chosen so that, sorted, the two malformed ids land next to each other.
+    _make_user_with_door_access(db_session, client, "door_a@example.com", "aaa<script>")
+    _make_user_with_door_access(db_session, client, "door_b@example.com", "aab<img/onerror=x>")
+    _make_user_with_door_access(db_session, client, "door_c@example.com", "abc-1234")
+
+    res = client.get("/access-serve/Arkivet")
+
+    assert res.status_code == 200, res.text
+    served = res.json()
+
+    assert served == ["abc-1234"], f"malformed stil-ids leaked into output: {served}"
+    for stil_id in served:
+        assert stil_id.replace("-", "").isalnum()
+
+
+####################################################################
+# CSV exports: member-controlled text must not become a formula
+####################################################################
+
+
+@pytest.mark.parametrize(
+    "payload",
+    [
+        "=cmd|'/c calc'!A1",
+        "+1+1",
+        "-1+1",
+        "@SUM(1+1)",
+        "\t=1+1",
+        "\r=1+1",
+    ],
+)
+def test_escape_csv_value_neutralizes_formula_triggers(payload):
+    from helpers.csv_response_factory import escape_csv_value
+
+    escaped = escape_csv_value(payload)
+
+    assert not escaped.startswith(("=", "+", "-", "@", "\t", "\r"))
+
+
+def test_escape_csv_value_leaves_ordinary_text_alone():
+    from helpers.csv_response_factory import escape_csv_value
+
+    for value in ["Anna", "Vegetarian, Gluten", "", "abc-1234", "1+1"]:
+        assert escape_csv_value(value) == value
+
+
+def test_event_csv_export_escapes_member_supplied_text(client, db_session, admin_token, membered_user, simple_event):
+    """
+    End to end: a member sets a spreadsheet formula as their free-text food
+    preference, an admin exports the signup CSV. The formula must arrive
+    quoted, not live, since the admin is the one who opens the file.
+    """
+    from db_models.event_user_model import EventUser_DB
+
+    membered_user.other_food_preferences = "=cmd|'/c calc'!A1"
+    membered_user.first_name = '=HYPERLINK("http://evil.example","click")'
+    db_session.commit()
+
+    signup = EventUser_DB(
+        user=membered_user,
+        user_id=membered_user.id,
+        event=simple_event,
+        event_id=simple_event.id,
+    )
+    signup.confirmed_status = True
+    db_session.add(signup)
+    db_session.commit()
+
+    res = client.get(
+        f"/events/event-signups/confirmed/{simple_event.id}/csv",
+        headers=auth_headers(admin_token),
+    )
+
+    assert res.status_code == 200, res.text
+    body = res.text
+
+    assert "=cmd|" in body, "the value should still be present, just neutralized"
+    # No cell may begin with a formula trigger.
+    import csv as csv_module
+    import io
+
+    for row in csv_module.reader(io.StringIO(body)):
+        for cell in row:
+            assert not cell.startswith(
+                ("=", "+", "-", "@", "\t", "\r")
+            ), f"cell would be evaluated as a formula: {cell!r}"