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.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.58%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
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.
- CVSS 4.0
- 9.1 CRITICALCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
- EPSS
- 0.58% probability · 47th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-416, CWE-787, CWE-825
- Source
- cna@python.org
EPSS 0.6% since 26 October 2026; no change recorded yet. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
References
- https://github.com/python/cpython/commit/47128e64f98c3a20271138a98c2922bea2a3ee0e
- https://github.com/python/cpython/commit/6a5f79c8d7bbf22b083b240910c7a8781a59437d
- https://github.com/python/cpython/commit/8fc66aef6d7b3ae58f43f5c66f9366cc8cbbfcd2
- https://github.com/python/cpython/commit/c3cf71c3366fe49acb776a639405c0eea6169c20
- https://github.com/python/cpython/commit/e20c6c9667c99ecaab96e1a2b3767082841ffc8b
- https://github.com/python/cpython/commit/ea8d735eb084cf8cc021df1a30e90d10a8f052e3
- https://github.com/python/cpython/issues/148395
- https://github.com/python/cpython/pull/148396
- https://mail.python.org/archives/list/security-announce@python.org/thread/HTWB2Z6KT5QQX4RYEZAFININDHNOSIF3/
- http://www.openwall.com/lists/oss-security/2026/04/13/10
- https://access.redhat.com/errata/RHSA-2026:10117
- https://access.redhat.com/errata/RHSA-2026:10140
- https://access.redhat.com/errata/RHSA-2026:10141
- https://access.redhat.com/errata/RHSA-2026:10711
- https://access.redhat.com/errata/RHSA-2026:10745
- https://access.redhat.com/errata/RHSA-2026:10774
- https://access.redhat.com/errata/RHSA-2026:10949
- https://access.redhat.com/errata/RHSA-2026:10950
- https://access.redhat.com/errata/RHSA-2026:11062
- https://access.redhat.com/errata/RHSA-2026:11077
- https://access.redhat.com/errata/RHSA-2026:11768
- https://access.redhat.com/errata/RHSA-2026:13692
- https://access.redhat.com/errata/RHSA-2026:13812
- https://access.redhat.com/errata/RHSA-2026:14652
- https://access.redhat.com/errata/RHSA-2026:14653
- https://access.redhat.com/errata/RHSA-2026:14656
- https://access.redhat.com/errata/RHSA-2026:16699
- https://access.redhat.com/errata/RHSA-2026:17525
- https://access.redhat.com/errata/RHSA-2026:17619
- https://access.redhat.com/errata/RHSA-2026:19019
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.