Use After Free in Python Software Foundation Cpython
CVE-2026-6100
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 used per call. If the decompressor instance is not re-used after an error condition, this usage is similarly not vulnerable.
Vulnerability class: Use-After-Free
Published · last modified .
CVSS v3 metric
CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.008 (53.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 54th percentile — 53.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.006 · newest: 0.008 · change: +0.002
Affected products
- Python Software Foundation Cpython — versions 0, 3.13.0, 3.11.0
Weakness classification (CWE)
References
- github.com/python/cpython/pull/148396 (patch)
- mail.python.org/archives/list/security-announce@python.org/thread/HTWB2Z6KT5QQX… (vendor-advisory)
- github.com/python/cpython/issues/148395 (issue-tracking)
- github.com/python/cpython/commit/6a5f79c8d7bbf22b083b240910c7a8781a59437d (patch)
- github.com/python/cpython/commit/8fc66aef6d7b3ae58f43f5c66f9366cc8cbbfcd2 (patch)
- github.com/python/cpython/commit/c3cf71c3366fe49acb776a639405c0eea6169c20 (patch)
- github.com/python/cpython/commit/47128e64f98c3a20271138a98c2922bea2a3ee0e (patch)
- github.com/python/cpython/commit/e20c6c9667c99ecaab96e1a2b3767082841ffc8b (patch)
- af854a3a-2127-422b-91ae-364da2661108
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Frequently asked questions
- What is CVE-2026-6100?
- CVE-2026-6100 is a high-severity vulnerability in Python Software Foundation Cpython, classified under Use After Free. CVSS score: 8.1/10. Published 2026-04-13.
- How severe is CVE-2026-6100?
- High severity. CVSS v3 base score is 8.1 out of 10.