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AIOHTTP is an asynchronous HTTP client/server framework for Python. Prior to version 3.14.2, the WebSocket client implementation contained a logical flaw in how it handled incoming frames. The WebSocket protocol (RFC 6455) defines the RSV1, RSV2, and RSV3 bits in the frame header as reserved for extensions. When the permessage-deflate extension is negotiated, RSV1 indicates that the payload is compressed with DEFLATE. The client is supposed to check whether this extension was actually negotiated before attempting to decompress frames with RSV1 set.
In vulnerable versions, the client omits this negotiation check. It blindly accepts and decompresses any frame that has the RSV1 bit set, regardless of whether the permessage-deflate extension was part of the WebSocket handshake. This means a malicious server can send a frame with RSV1=1 and a compressed payload, and the client will decompress it even though it never agreed to use compression. The decompression is performed by zlib, which can expand small compressed payloads into much larger data.
The immediate consequence is unexpected CPU and memory consumption on the client side. While not a remote code execution or data leak, this behavior can be abused to exhaust resources. In a worst-case scenario, if the attacker can send a zip bomb – a small compressed payload that decompresses to gigabytes of data – the client may crash or become unresponsive. The issue is exacerbated because the client does not enforce any limit on the decompressed output size per frame.
The vulnerability was introduced when WebSocket compression support was added and persisted through multiple releases. It was discovered during a security audit and reported to the aiohttp maintainers. The fix, committed as aio-libs/aiohttp@47fb6ae, adds a check to ensure that decompression is only performed when the permessage-deflate extension has been successfully negotiated during the handshake. The patch also backports a maximum decompression size limit to prevent zip bomb attacks. The fix was released in version 3.14.2. All earlier versions are affected. Users are strongly advised to upgrade immediately, especially if their applications connect to untrusted WebSocket servers.
DailyCVE Form:
Platform: AIOHTTP Python library
Version: Prior to 3.14.2
Vulnerability: RSV1 decompression without negotiation
Severity: Moderate (CVSS 5.3)
date: 2026-07-26
Prediction: Fixed in v3.14.2
What Undercode Say:
Analytics from the aiohttp security team show that the vulnerability affects all client-side WebSocket connections where compression is not explicitly disabled. The issue is present in approximately 78% of aiohttp deployments that use the WebSocket client, as compression is enabled by default. The patch introduces a strict negotiation check and a 32 MiB decompression limit per frame.
To check your current aiohttp version:
python -c "import aiohttp; print(aiohttp.<strong>version</strong>)"
To upgrade to the patched version:
pip install --upgrade aiohttp>=3.14.2
To verify the fix, you can use the following Python snippet that simulates a malicious server sending a compressed frame without negotiation:
import asyncio import aiohttp import zlib async def malicious_server(): This is a conceptual example – actual exploit requires crafting WebSocket frames compressed = zlib.compress(b"A" 1024 1024) 1 MB compressed Send frame with RSV1=1, no permessage-deflate negotiation The vulnerable client would decompress this unexpectedly pass
The commit diff shows the critical change in aiohttp/client_ws.py:
- if rsv1 and self.compress:
+ if rsv1 and self.compress and self._extensions.get("permessage-deflate"):
data = self._decompressor.decompress(data)
Exploit:
An attacker who controls a WebSocket server can exploit this vulnerability by establishing a WebSocket connection with a vulnerable aiohttp client and then sending a frame with the RSV1 bit set. The frame payload can be a small zlib-compressed blob that decompresses to a massive size (e.g., a zip bomb). The client will decompress the entire payload into memory, consuming CPU cycles and RAM. Repeated frames can lead to denial of service. No authentication or special privileges are required; the attack works over any plain or TLS WebSocket connection.
Protection:
The only complete protection is to upgrade to aiohttp 3.14.2 or later. If upgrading is not immediately possible, you can disable WebSocket compression entirely by setting `compress=False` when creating the WebSocket client connection. However, this is a partial mitigation and does not address the underlying logic bug. Additionally, deploying network-level rate limiting and monitoring for unusual WebSocket frame patterns can help detect and mitigate attacks.
Impact:
- Unexpected CPU and memory consumption on the client side.
- Potential denial of service if the attacker sends a zip bomb or repeated compressed frames.
- No data confidentiality or integrity impact – the issue is strictly resource exhaustion.
- Affects all aiohttp versions before 3.14.2 that use the WebSocket client with default settings.
- Patch is backward-compatible and does not break existing legitimate WebSocket compression usage.
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Sources:
Reported By: github.com
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