Unbounded memory buffering in SelectorSocketTransport.writelines()

Vulnerability Description

Starting in Python 3.12.0, the asyncio._SelectorSocketTransport.writelines() method would not "pause" writing and signal to the Protocol to drain the buffer to the wire once the write buffer reached the "high-water mark". Because of this, Protocols would not periodically drain the write buffer potentially leading to memory exhaustion. This vulnerability likely impacts a small number of users, you must be using Python 3.12.0 or later, on macOS or Linux, using the asyncio module with protocols, and using .writelines() method which had new zero-copy-on-write behavior in Python 3.12.0 and later. If not all of these factors are true then your usage of Python is unaffected.

References

• https://github.com/python/cpython/issues/127655
• https://github.com/python/cpython/pull/127656
• https://mail.python.org/archives/list/security-announce@python.org/thread/H4O3UBAOAQQXGT4RE3E4XQYR5XLROORB/
• https://github.com/python/cpython/commit/71e8429ac8e2adc10084ab5ec29a62f4b6671a82
• https://github.com/python/cpython/commit/9aa0deb2eef2655a1029ba228527b152353135b5
• https://github.com/python/cpython/commit/e991ac8f2037d78140e417cc9a9486223eb3e786