GeezerVison is showing github.com — OOM a potential denial of service in the CGI server on Windows · Issue #119452 · python/cpython
OOM a potential denial of service in the CGI server on Windows #119452DescriptionWhen http.server.CGIHTTPRequestHandler on Windows (and other platforms without fork()) handles the POST request, it reads the whole body of the POST request in memory before sending it to the subprocess running the script. The underlying SocketIO allocates the amount of memory specified in the Content-Length header before actual reading the data, so a small request with incorrect Content-Length can cause consumption of the large amount of memory and CPU time and can be used in the DOS attack on the server.
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Activitypicnixz commented on May 23, 2024 Correct me if I'm wrong, but the incriminated lines are the following right:
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if self.command.lower() == "post" and nbytes > 0: |
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data = self.rfile.read(nbytes) |
If so, could I perhaps take on this one? (I never directly contributed to CPython so I think I can take this one to setup everything that's needed, unless you are already working on it). serhiy-storchaka commented on May 23, 2024 Thank you for volunteering @picnixz, but I already have a solution. I have been somewhat delayed in publishing it because I discovered other problem: the large body was truncated on Windows, because SocketIO.read() is unbuffered and can return a partial data. picnixz commented on May 23, 2024 No worries! I think you are much faster than me on that since I'm not really familiar with the http and IO-related codebase. cmaloney commented on Oct 26, 2025 Given this code has been removed for 3.15 (gh-133810), do we need to fix this on older versions or can we close this issue? encukou commented on Jan 26, 2026 All the backports are merged now. vstinner commented on May 13, 2026 The test is a little bit fragile. Example of failure on "AMD64 FreeBSD Refleaks 3.13" buildbot: https://buildbot.python.org/#/builders/1615/builds/1722
ERROR: test_large_content_length_truncated (test.test_httpservers.CGIHTTPServerTestCase.test_large_content_length_truncated)
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Traceback (most recent call last):
File "/buildbot/buildarea/3.13.ware-freebsd.refleak/build/Lib/test/test_httpservers.py", line 1028, in test_large_content_length_truncated
res = self.request('/cgi-bin/file1.py', 'POST', b'x', headers)
File "/buildbot/buildarea/3.13.ware-freebsd.refleak/build/Lib/test/test_httpservers.py", line 89, in request
return self.connection.getresponse()
~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/buildbot/buildarea/3.13.ware-freebsd.refleak/build/Lib/http/client.py", line 1450, in getresponse
response.begin()
~~~~~~~~~~~~~~^^
File "/buildbot/buildarea/3.13.ware-freebsd.refleak/build/Lib/http/client.py", line 336, in begin
version, status, reason = self._read_status()
~~~~~~~~~~~~~~~~~^^
File "/buildbot/buildarea/3.13.ware-freebsd.refleak/build/Lib/http/client.py", line 297, in _read_status
line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
File "/buildbot/buildarea/3.13.ware-freebsd.refleak/build/Lib/socket.py", line 723, in readinto
return self._sock.recv_into(b)
~~~~~~~~~~~~~~~~~~~~^^^
ConnectionResetError: [Errno 54] Connection reset by peer
One explanation is the hardcoded timeout of 1 ms in the test:
with support.swap_attr(self.request_handler, 'timeout', 0.001): This timeout is too low if the machine running the test has high system load.
For example, ./python -m test test_httpservers -v -m test_large_content_length_truncated -F -j10 fails quickly.
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