The Fix
Fix WebSocket reader with fragmented masked messages to handle large payloads correctly.
Based on closed aio-libs/aiohttp issue #10763 · PR/commit linked
@@ -0,0 +1,3 @@
@@ -0,0 +1,3 @@
+Fixed reading fragmented WebSocket messages when the payload was masked -- by :user:`bdraco`.
+
+The problem first appeared in 3.11.17
repro.py
from aiohttp import web
async def Handler(request: web.Request):
try:
ws = web.WebSocketResponse(max_msg_size=100*1024*1024)
await ws.prepare(request)
while True:
rawMsg = await ws.receive_bytes()
print(len(rawMsg))
await ws.send_bytes(rawMsg)
except BaseException as e:
print(e)
if __name__ == "__main__":
app = web.Application()
app.add_routes([web.get("/get_task", Handler)])
web.run_app(app, path="./test_websocket.socket", handle_signals=False)
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
fix.md
Option A — Apply the official fix\nFix WebSocket reader with fragmented masked messages to handle large payloads correctly.\nWhen NOT to use: This fix should not be applied if the application relies on the previous behavior of message handling.\n\n
Why This Fix Works in Production
- Trigger: ```python-traceback
- Mechanism: The issue was caused by a refactoring oversight in handling fragmented masked WebSocket messages
Production impact:
- If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).
Why This Breaks in Prod
- The issue was caused by a refactoring oversight in handling fragmented masked WebSocket messages
- Production symptom (often without a traceback): ```python-traceback
Proof / Evidence
- GitHub issue: #10763
- Fix PR: https://github.com/aio-libs/aiohttp/pull/10764
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.80
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.59
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Version 3.11.18, which includes the fix, is currently being published. This update addresses the regression. Thank you for your patience and understanding.”
“Thanks for the report. I'll take a look in the morning when I get up”
“confirmed the reproducer. yanked 3.11.17 Will fix it in the morning. I have a good idea whats wrong”
“On the bright side, I’ve identified the issue. Unfortunately, it stemmed from a straightforward refactoring oversight that I should have caught earlier.”
Failure Signature (Search String)
- ```python-traceback
- No exception, but error message.
Copy-friendly signature
signature.txt
Failure Signature
-----------------
```python-traceback
No exception, but error message.
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
```python-traceback
No exception, but error message.
Minimal Reproduction
repro.py
from aiohttp import web
async def Handler(request: web.Request):
try:
ws = web.WebSocketResponse(max_msg_size=100*1024*1024)
await ws.prepare(request)
while True:
rawMsg = await ws.receive_bytes()
print(len(rawMsg))
await ws.send_bytes(rawMsg)
except BaseException as e:
print(e)
if __name__ == "__main__":
app = web.Application()
app.add_routes([web.get("/get_task", Handler)])
web.run_app(app, path="./test_websocket.socket", handle_signals=False)
What Broke
Clients sending large messages receive zero-length responses from the server.
Why It Broke
The issue was caused by a refactoring oversight in handling fragmented masked WebSocket messages
Fix Options (Details)
Option A — Apply the official fix
Fix WebSocket reader with fragmented masked messages to handle large payloads correctly.
When NOT to use: This fix should not be applied if the application relies on the previous behavior of message handling.
Fix reference: https://github.com/aio-libs/aiohttp/pull/10764
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if the application relies on the previous behavior of message handling.
Verify Fix
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.