CVE-2026-6100: CWE-416 Use after free in Python Software Foundation CPython
CVE-2026-6100 is a critical use-after-free vulnerability in CPython affecting the lzma. LZMADecompressor, bz2. BZ2Decompressor, and gzip. GzipFile classes. It occurs when a memory allocation fails with a MemoryError during decompression and the same decompressor instance is reused afterward. This vulnerability does not affect one-shot decompression helper functions or cases where decompressor instances are not reused after an error. The flaw arises from a dangling pointer that is not cleaned up in this specific error condition.
AI Analysis
Technical Summary
This vulnerability involves a use-after-free condition in CPython's decompression classes (lzma.LZMADecompressor, bz2.BZ2Decompressor, gzip.GzipFile) triggered when a MemoryError occurs during decompression and the decompressor instance is reused. The issue is specific to scenarios where the same decompressor object is reused across multiple decompression calls after a memory allocation failure. One-shot decompression functions that create new decompressor instances per call are not affected. The root cause is a dangling pointer left uncleared after the MemoryError, which can lead to memory corruption.
Potential Impact
The vulnerability has a CVSS 4.0 score of 9.1 (critical), indicating a high impact. Exploitation could lead to memory corruption due to use-after-free, potentially causing crashes or arbitrary code execution. However, exploitation requires the program to reuse decompressor instances after a MemoryError, which is a specific and less common usage pattern. There are no known exploits in the wild at this time.
Mitigation Recommendations
Patch status is not yet confirmed — check the Python Software Foundation advisory for current remediation guidance. Until a fix is available, avoid reusing decompressor instances after a MemoryError is raised during decompression. Use the one-shot decompression helper functions (lzma.decompress(), bz2.decompress(), gzip.decompress(), zlib.decompress()) which are not affected by this issue.
CVE-2026-6100: CWE-416 Use after free in Python Software Foundation CPython
Description
CVE-2026-6100 is a critical use-after-free vulnerability in CPython affecting the lzma. LZMADecompressor, bz2. BZ2Decompressor, and gzip. GzipFile classes. It occurs when a memory allocation fails with a MemoryError during decompression and the same decompressor instance is reused afterward. This vulnerability does not affect one-shot decompression helper functions or cases where decompressor instances are not reused after an error. The flaw arises from a dangling pointer that is not cleaned up in this specific error condition.
CVSS v4.0
Score 9.1critical
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
This vulnerability involves a use-after-free condition in CPython's decompression classes (lzma.LZMADecompressor, bz2.BZ2Decompressor, gzip.GzipFile) triggered when a MemoryError occurs during decompression and the decompressor instance is reused. The issue is specific to scenarios where the same decompressor object is reused across multiple decompression calls after a memory allocation failure. One-shot decompression functions that create new decompressor instances per call are not affected. The root cause is a dangling pointer left uncleared after the MemoryError, which can lead to memory corruption.
Potential Impact
The vulnerability has a CVSS 4.0 score of 9.1 (critical), indicating a high impact. Exploitation could lead to memory corruption due to use-after-free, potentially causing crashes or arbitrary code execution. However, exploitation requires the program to reuse decompressor instances after a MemoryError, which is a specific and less common usage pattern. There are no known exploits in the wild at this time.
Mitigation Recommendations
Patch status is not yet confirmed — check the Python Software Foundation advisory for current remediation guidance. Until a fix is available, avoid reusing decompressor instances after a MemoryError is raised during decompression. Use the one-shot decompression helper functions (lzma.decompress(), bz2.decompress(), gzip.decompress(), zlib.decompress()) which are not affected by this issue.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- PSF
- Date Reserved
- 2026-04-10T21:13:45.428Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 69dd2f9182d89c981f2dce1c
Added to database: 4/13/2026, 6:01:53 PM
Last enriched: 4/21/2026, 6:20:56 AM
Last updated: 5/29/2026, 4:41:00 PM
Views: 1720
Community Reviews
0 reviewsCrowdsource mitigation strategies, share intel context, and vote on the most helpful responses. Sign in to add your voice and help keep defenders ahead.
Want to contribute mitigation steps or threat intel context? Sign in or create an account to join the community discussion.
Actions
Updates to AI analysis require Pro Console access. Upgrade inside Console → Billing.
Need more coverage?
Upgrade to Pro Console for AI refresh and higher limits.
For incident response and remediation, OffSeq services can help resolve threats faster.
Latest Threats
Check if your credentials are on the dark web
Instant breach scanning across billions of leaked records. Free tier available.