python CVE-2026-6100

[Joe Locash]

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.

Please see the linked CVE ID for the latest information on affected
versions:

    https://www.cve.org/CVERecord?id=CVE-2026-6100
    https://github.com/python/cpython/pull/148396

Source: ​https://mail.python.org/archives/list/security-announce@python.org/thread/HTWB2Z6KT5QQX4RYEZAFININDHNOSIF3/

[Bruce Dubbs]

At first I thought this was for cython, but it is indeed a module in Python-3.14.4 (ticket #5907).

It first looked like the pull request could be implemented with a single sed across three files, but the changes are slightly different for each file. We can use:

sed -i '596 a\    bzs->next_in = NULL;'      Modules/_bz2module.c
sed -i '1122a\    lzs->next_in = NULL;'      Modules/_lzmamodule.c
sed -i '1677a\    self->zst.next_in = NULL;' Modules/zlibmodule.c

We need line numbers since there are multiple instances of '^{error:' in the files.}

Note that the line numbers are different from the pull request for Python-3.14.4.

[Joe Locash]

I'm attaching a patch that addresses this, and CVE-2026-1502 which was also fixed after the release of 3.14.4.