diff --git a/src/nene2/middleware/request_id.py b/src/nene2/middleware/request_id.py
index 11b7e6e..6dafa21 100644
--- a/src/nene2/middleware/request_id.py
+++ b/src/nene2/middleware/request_id.py
@@ -4,6 +4,7 @@
 Uses contextvars so downstream code (e.g. structlog) can read the ID.
 """
 
+import re
 import uuid
 from contextvars import ContextVar
 
@@ -13,14 +14,31 @@
 
 _REQUEST_ID_HEADER = "X-Request-Id"
 
+# UUID v4 canonical form — 8-4-4-4-12 hex, version=4, variant=8/9/a/b
+_UUID_V4_RE = re.compile(
+    r"^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$",
+    re.IGNORECASE,
+)
+
 request_id_var: ContextVar[str] = ContextVar("request_id", default="")
 
 
+def _validated_request_id(value: str | None) -> str:
+    """Return value if it is a valid UUID v4, otherwise generate a fresh one."""
+    if value and _UUID_V4_RE.match(value):
+        return value.lower()
+    return str(uuid.uuid4())
+
+
 class RequestIdMiddleware(BaseHTTPMiddleware):
-    """Generate or forward X-Request-Id and expose it via contextvars."""
+    """Generate or forward X-Request-Id and expose it via contextvars.
+
+    Client-supplied X-Request-Id is accepted only when it matches UUID v4
+    format, preventing log injection via arbitrary header values.
+    """
 
     async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
-        request_id = request.headers.get(_REQUEST_ID_HEADER) or str(uuid.uuid4())
+        request_id = _validated_request_id(request.headers.get(_REQUEST_ID_HEADER))
         request_id_var.set(request_id)
         response = await call_next(request)
         response.headers[_REQUEST_ID_HEADER] = request_id
diff --git a/tests/nene2/middleware/test_request_id.py b/tests/nene2/middleware/test_request_id.py
index 6c6f0e0..889392d 100644
--- a/tests/nene2/middleware/test_request_id.py
+++ b/tests/nene2/middleware/test_request_id.py
@@ -1,11 +1,17 @@
 """Tests for RequestIdMiddleware."""
 
+import re
+
 from fastapi import FastAPI
 from fastapi.responses import JSONResponse
 from fastapi.testclient import TestClient
 
 from nene2.middleware import RequestIdMiddleware, request_id_var
 
+_UUID_V4_RE = re.compile(
+    r"^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$"
+)
+
 
 def _make_app() -> FastAPI:
     app = FastAPI()
@@ -24,13 +30,32 @@ def test_response_has_x_request_id() -> None:
     assert response.status_code == 200
     assert "X-Request-Id" in response.headers
     rid = response.headers["X-Request-Id"]
-    assert len(rid) == 36  # UUID v4 format
+    assert len(rid) == 36
+
+
+def test_forwards_valid_uuid_v4_request_id() -> None:
+    valid_id = "550e8400-e29b-41d4-a716-446655440000"
+    client = TestClient(_make_app())
+    response = client.get("/ping", headers={"X-Request-Id": valid_id})
+    assert response.headers["X-Request-Id"] == valid_id
 
 
-def test_forwards_provided_request_id() -> None:
+def test_invalid_request_id_is_replaced_with_new_uuid() -> None:
+    """Non-UUID values must not be forwarded to prevent log injection."""
     client = TestClient(_make_app())
     response = client.get("/ping", headers={"X-Request-Id": "my-trace-id-123"})
-    assert response.headers["X-Request-Id"] == "my-trace-id-123"
+    rid = response.headers["X-Request-Id"]
+    assert rid != "my-trace-id-123"
+    assert _UUID_V4_RE.match(rid), f"Expected UUID v4, got {rid!r}"
+
+
+def test_newline_in_request_id_is_rejected() -> None:
+    """Newlines in X-Request-Id must be rejected to prevent log injection."""
+    client = TestClient(_make_app())
+    response = client.get("/ping", headers={"X-Request-Id": "abc\nERROR injected"})
+    rid = response.headers["X-Request-Id"]
+    assert "\n" not in rid
+    assert _UUID_V4_RE.match(rid)
 
 
 def test_request_id_available_in_contextvars() -> None: