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gh-145244: Fix use-after-free on borrowed dict key in json encoder - #145245
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gpshead merged 3 commits intoApr 11, 2026
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In encoder_encode_key_value(), key is a borrowed reference from
PyDict_Next(). If the default callback mutates or clears the dict,
key becomes a dangling pointer. The error path then calls
_PyErr_FormatNote("%R", key) on freed memory.
Fix by holding strong references to key and value unconditionally
during encoding, not just in the free-threading build.
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Misc/NEWS.d/next/Library/2026-02-26-12-00-00.gh-issue-145244.Kj31cp.rst
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…j31cp.rst Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
# Conflicts: # Lib/test/test_json/test_dump.py
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Apr 11, 2026
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Modules/_json.c
| Py_INCREF(key); | ||
| Py_INCREF(value); | ||
| #endif | ||
| if (encoder_encode_key_value(s, writer, first, dct, key, value, |
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reviewer note for the future: I explored if we could deterministically understand if encoder_encode_key_value was going to call user code which could lead to the problem... it's too complicated. while a couple heuristics exist for a subset of cases, they'd be fragile.
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gpshead
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Sorry, @raminfp and @gpshead, I could not cleanly backport this to 3.14 due to a conflict. cherry_picker 8a466fa3d90a9e1f04d23c05ee2cf3f3c406ba30 3.14 |
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…der (pythonGH-145245) In encoder_encode_key_value(), key is a borrowed reference from PyDict_Next(). If the default callback mutates or clears the dict, key becomes a dangling pointer. The error path then calls _PyErr_FormatNote("%R", key) on freed memory. Fix by holding strong references to key and value unconditionally during encoding, not just in the free-threading build. Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
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Fix use-after-free in encoder_encode_key_value() where borrowed references to dict keys from PyDict_Next() can become dangling pointers if the default callback mutates the dictionary during encoding.