diff --git a/plugins/platforms/line/adapter.py b/plugins/platforms/line/adapter.py
index 447cf5fb0c800..751f1243363d5 100644
--- a/plugins/platforms/line/adapter.py
+++ b/plugins/platforms/line/adapter.py
@@ -276,6 +276,17 @@ def verify_line_signature(body: bytes, signature: str, channel_secret: str) -> b
     return hmac.compare_digest(expected, signature)
 
 
+async def _read_limited_request_body(request: Any, max_bytes: int) -> bytes:
+    """Read at most ``max_bytes`` from an aiohttp request body."""
+    try:
+        body = await request.content.readexactly(max_bytes + 1)
+    except asyncio.IncompleteReadError as exc:
+        body = exc.partial
+    if len(body) > max_bytes:
+        raise ValueError("payload too large")
+    return body
+
+
 # ---------------------------------------------------------------------------
 # Cache state machine — slow-LLM postback flow
 # ---------------------------------------------------------------------------
@@ -869,12 +880,12 @@ async def _handle_webhook(self, request) -> Any:
         # Body cap defends against memory-exhaustion via crafted Content-Length
         # (aiohttp's client_max_size only applies to certain body modes).
         try:
-            body = await request.read()
+            body = await _read_limited_request_body(request, WEBHOOK_BODY_MAX_BYTES)
+        except ValueError:
+            return web.Response(status=413, text="payload too large")
         except Exception as exc:
             logger.debug("LINE: read failed: %s", exc)
             return web.Response(status=400, text="bad request")
-        if len(body) > WEBHOOK_BODY_MAX_BYTES:
-            return web.Response(status=413, text="payload too large")
 
         signature = request.headers.get("X-Line-Signature", "")
         if not verify_line_signature(body, signature, self.channel_secret):
diff --git a/tests/plugins/test_line_body_limit.py b/tests/plugins/test_line_body_limit.py
new file mode 100644
index 0000000000000..73caeb3f401a5
--- /dev/null
+++ b/tests/plugins/test_line_body_limit.py
@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+import asyncio
+
+import pytest
+
+from plugins.platforms.line.adapter import _read_limited_request_body
+
+
+class _Content:
+    def __init__(self, body: bytes):
+        self.body = body
+        self.read_size = None
+
+    async def readexactly(self, size: int) -> bytes:
+        self.read_size = size
+        if len(self.body) < size:
+            raise asyncio.IncompleteReadError(self.body, size)
+        return self.body[:size]
+
+
+class _Request:
+    def __init__(self, body: bytes):
+        self.content = _Content(body)
+
+
+def test_read_limited_request_body_returns_short_body():
+    request = _Request(b'{"events":[]}')
+
+    body = asyncio.run(_read_limited_request_body(request, 32))
+
+    assert body == b'{"events":[]}'
+    assert request.content.read_size == 33
+
+
+def test_read_limited_request_body_rejects_oversized_body():
+    request = _Request(b"x" * 33)
+
+    with pytest.raises(ValueError, match="payload too large"):
+        asyncio.run(_read_limited_request_body(request, 32))
+
+    assert request.content.read_size == 33