diff --git a/api/app/settings/common.py b/api/app/settings/common.py
index d948e14b5a9d..4f3690288b85 100644
--- a/api/app/settings/common.py
+++ b/api/app/settings/common.py
@@ -383,6 +383,9 @@
     "whitenoise.middleware.WhiteNoiseMiddleware",
     "django.contrib.sessions.middleware.SessionMiddleware",
     "corsheaders.middleware.CorsMiddleware",
+    # Must come after CorsMiddleware: it can short-circuit with a response of
+    # its own, and CorsMiddleware needs to wrap it to add CORS headers to that.
+    "core.middleware.query_params.RejectNulByteQueryParamsMiddleware",
     "django.middleware.common.CommonMiddleware",
     "django.middleware.csrf.CsrfViewMiddleware",
     "django.contrib.auth.middleware.AuthenticationMiddleware",
diff --git a/api/core/middleware/query_params.py b/api/core/middleware/query_params.py
new file mode 100644
index 000000000000..8eafa65f679f
--- /dev/null
+++ b/api/core/middleware/query_params.py
@@ -0,0 +1,29 @@
+from collections.abc import Callable
+
+from django.http import HttpRequest, HttpResponse, HttpResponseBadRequest
+
+
+class RejectNulByteQueryParamsMiddleware:
+    """
+    Reject requests whose query parameters contain a NUL (0x00) character.
+
+    Passing one through to a query against the string field of a Postgres
+    row raises an unhandled `ValueError: A string literal cannot contain
+    NUL (0x00) characters`, so reject it here, before any view can pass it
+    to the ORM.
+    """
+
+    def __init__(self, get_response: Callable[[HttpRequest], HttpResponse]) -> None:
+        self.get_response = get_response
+
+    def __call__(self, request: HttpRequest) -> HttpResponse:
+        # `.values()` only yields the last value per key: a repeated key
+        # (`?a=x&a=y`) would let a NUL byte in an earlier value slip through.
+        # `.lists()` yields every value for every key.
+        if any(
+            "\x00" in value for _, values in request.GET.lists() for value in values
+        ):
+            return HttpResponseBadRequest(
+                "Query parameters must not contain NUL characters."
+            )
+        return self.get_response(request)
diff --git a/api/tests/unit/core/middleware/test_unit_core_middleware_query_params.py b/api/tests/unit/core/middleware/test_unit_core_middleware_query_params.py
new file mode 100644
index 000000000000..a663d21289d1
--- /dev/null
+++ b/api/tests/unit/core/middleware/test_unit_core_middleware_query_params.py
@@ -0,0 +1,63 @@
+from django.http import HttpResponse
+from django.test import RequestFactory
+
+from core.middleware.query_params import RejectNulByteQueryParamsMiddleware
+
+
+def test_reject_nul_byte_query_params_middleware__nul_byte_in_query_param__returns_bad_request(  # type: ignore[no-untyped-def]  # noqa: E501
+    mocker, rf: RequestFactory
+):
+    # Given
+    mocked_get_response = mocker.MagicMock()
+    request = rf.get(
+        "/api/v1/environments/some-key/identities/", {"identifier": "foo\x00bar"}
+    )
+
+    middleware = RejectNulByteQueryParamsMiddleware(mocked_get_response)
+
+    # When
+    response = middleware(request)
+
+    # Then
+    assert response.status_code == 400
+    mocked_get_response.assert_not_called()
+
+
+def test_reject_nul_byte_query_params_middleware__no_nul_byte__calls_get_response(  # type: ignore[no-untyped-def]  # noqa: E501
+    mocker, rf: RequestFactory
+):
+    # Given
+    a_response = HttpResponse()
+    mocked_get_response = mocker.MagicMock(return_value=a_response)
+    request = rf.get(
+        "/api/v1/environments/some-key/identities/", {"identifier": "foobar"}
+    )
+
+    middleware = RejectNulByteQueryParamsMiddleware(mocked_get_response)
+
+    # When
+    response = middleware(request)
+
+    # Then
+    assert response is a_response
+    mocked_get_response.assert_called_once_with(request)
+
+
+def test_reject_nul_byte_query_params_middleware__nul_byte_in_repeated_key__returns_bad_request(  # type: ignore[no-untyped-def]  # noqa: E501
+    mocker, rf: RequestFactory
+):
+    # Given - `identifier` is repeated; `QueryDict.values()` would only see
+    # the last ("foobar"), silently missing the NUL byte in the first.
+    mocked_get_response = mocker.MagicMock()
+    request = rf.get(
+        "/api/v1/environments/some-key/identities/?identifier=foo%00bar&identifier=foobar"
+    )
+
+    middleware = RejectNulByteQueryParamsMiddleware(mocked_get_response)
+
+    # When
+    response = middleware(request)
+
+    # Then
+    assert response.status_code == 400
+    mocked_get_response.assert_not_called()