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CVE-2024-30251 — Denial of service when trying to parse malformed POST requests in aiohttp

2026-09-29T10:32:50.886932+00:00

Affected: aio-libs aiohttp, aio-libs_project aiohttp

aiohttp is an asynchronous HTTP client/server framework for asyncio and Python. In affected versions an attacker can send a specially crafted POST (multipart/form-data) request. When the aiohttp server processes it, the server will enter an infinite loop and be unable to process any further requests. An attacker can stop the application from serving requests after sending a single request. This issue has been addressed in version 3.9.4. Users are advised to upgrade. Users unable to upgrade may manually apply a patch to their systems. Please see the linked GHSA for instructions.

GHSA-5m98-qgg9-wh84 — aiohttp vulnerable to Denial of Service when trying to parse malformed POST requests

2026-09-29T10:32:50.887030+00:00

Affected: PyPI: aiohttp

### Summary An attacker can send a specially crafted POST (multipart/form-data) request. When the aiohttp server processes it, the server will enter an infinite loop and be unable to process any further requests.

### Impact An attacker can stop the application from serving requests after sending a single request.

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For anyone needing to patch older versions of aiohttp, the minimum diff needed to resolve the issue is (located in `_read_chunk_from_length()`):

```diff diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py index 227be605c..71fc2654a 100644 --- a/aiohttp/multipart.py +++ b/aiohttp/multipart.py @@ -338,6 +338,8 @@ class BodyPartReader: assert self._length is not None, "Content-Length required for chunked read" chunk_size = min(size, self._length - self._read_bytes) chunk = await self._content.read(chunk_size) + if self._content.at_eof(): + self._at_eof = True return chunk async def _read_chunk_from_stream(self, size: int) -> bytes: ```

This does however introduce some very minor issues with handling form data. So, if possible, it would be recommended to also backport the changes in: https://github.com/aio-libs/aiohttp/commit/cebe526b9c34dc3a3da9140409db63014bc4cf19 https://github.com/aio-libs/aiohttp/commit/7eecdff163ccf029fbb1ddc9de4169d4aaeb6597 https://github.com/aio-libs/aiohttp/commit/f21c6f2ca512a026ce7f0f6c6311f62d6a638866

PYSEC-2026-1098 — aiohttp vulnerable to Denial of Service when trying to parse malformed POST requests

2026-09-29T10:32:50.887104+00:00

Affected: PyPI: aiohttp

### Impact An attacker can stop the application from serving requests after sending a single request.

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For anyone needing to patch older versions of aiohttp, the minimum diff needed to resolve the issue is (located in `_read_chunk_from_length()`):