[oss-security][CVE-2026-6100] CPython: Use-after-free in lzma.LZMADecompressor, bz2.BZ2Decompressor, and gzip.GzipFile after re-use under memory pressure

[Alan Coopersmith <alan.coopersmith () oracle com>]

-------- Forwarded Message --------
Subject:        [Security-announce][CVE-2026-6100] Use-after-free in lzma.LZMADecompressor, bz2.BZ2Decompressor, and 
gzip.GzipFile after re-use under memory pressure
Date:   Mon, 13 Apr 2026 17:13:40 +0000
From:   Seth Larson <seth () python org>
Reply-To:       security-sig () python org
To:     security-announce () python org



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

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