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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

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/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