There is a CRITICAL severity vulnerability affecting CPython.
Use-after-free (UAF) was possible in the lzma.LZMADecompressor,
bz2.BZ2Decompressor, and gzip.GzipFile when a memory allocation fails
with a MemoryError and the decompression instance is re-used. This
scenario can be triggered if the process is under memory pressure. The fix
cleans up the dangling pointer in this specific error condition.
The vulnerability is only present if the program re-uses decompressor
instances across multiple decompression calls even after a MemoryError is
raised during decompression. Using the helper functions to one-shot
decompress data such as lzma.decompress(), bz2.decompress(),
gzip.decompress(), and zlib.decompress() are not affected as a new
decompressor instance is created for each call. If the decompressor
instance is not re-used after an error condition, this usage is similarly
not vulnerable.