Python CPython up to 3.14.x Decompression Call use after free

Summaryinfo

A vulnerability classified as critical has been found in Python CPython up to 3.14.x. The affected element is the function lzma.LZMADecompressor/bz2.BZ2Decompressor/gzip.GzipFile of the component Decompression Call Handler. The manipulation leads to use after free. This vulnerability is traded as CVE-2026-6100. It is possible to initiate the attack remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in Python CPython up to 3.14.x. It has been rated as critical. This issue affects the function lzma.LZMADecompressor/bz2.BZ2Decompressor/gzip.GzipFile of the component Decompression Call Handler. The manipulation with an unknown input leads to a use after free vulnerability. Using CWE to declare the problem leads to CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

The weakness was shared by Stan Ulbrych and Ryan Hileman as 148395. The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-6100 since 04/10/2026. The exploitation is known to be difficult. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available.

The vulnerability scanner Nessus provides a plugin with the ID 307390 (Fedora 45 : python3.12 (2026-2dfcf9d705)), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.15.0 eliminates this vulnerability. Applying the patch 6a5f79c8d7bbf22b083b240910c7a8781a59437d is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (307390) and CERT Bund (WID-SEC-2026-1087). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Affected

• Debian Linux
• Red Hat Enterprise Linux
• Fedora Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• IBM SPSS
• SUSE openSUSE
• RESF Rocky Linux
• Open Source Python

Productinfo

Type

• Programming Language Software

Vendor

• Python

Name

• CPython

Version

• 3.0
• 3.1
• 3.2
• 3.3
• 3.4
• 3.5
• 3.6
• 3.7
• 3.8
• 3.9
• 3.10
• 3.11
• 3.12
• 3.13
• 3.14

License

• open-source

Website

• Product: https://github.com/python/cpython/

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Entryinfo

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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