Advisory · CVE-2026-69639
Heap overflow in Windows Libarchive allows network code execution
The Libarchive component bundled with Windows has a heap-based buffer overflow an unauthenticated attacker can trigger over the network to execute code. Microsoft rates it CVSS 3.1 9.8.
- Vendor
- Microsoft
- Product
- Windows Libarchive
- Identifier / CWE
- CVE-2026-69639
CWE-122 - Action timing
- Immediate
Explain it like I’m five
The tool that unpacks archives for the system can be handed a file that overflows its memory and unpacks an attacker instead.
- 01Reach the component
An unauthenticated attacker sends crafted network traffic to a Windows system running Windows Libarchive.
- 02Memory flaw triggers
The component mishandles the input and corrupts or reuses memory outside its intended bounds.
- 03Code runs
The attacker gains code execution in the component's context on the target system.
What happened
Microsoft disclosed CVE-2026-69639 in its October 2026 update guide: a heap-based buffer overflow (CWE-122) in Windows Libarchive lets an unauthorized attacker execute code over a network.
Microsoft lists the affected Windows releases in its update guide. Apply the October 2026 security updates for your release rather than relying on a version guess here.
What to do
- Inventory every Windows Libarchive deployment, including test, training, and individually managed instances.
- Apply the vendor fix or upgrade exactly as directed in the linked vendor bulletin or advisory.
- Restrict network access to Windows Libarchive while remediation is underway, especially for internet-facing instances.
- Review application and server logs for archive processing errors, crashes in tools that extract downloaded files, and unexpected child processes after archive handling.
- Check for accounts, content, or configuration created outside normal change processes, and rotate exposed credentials where access cannot be ruled out.
- Verify the deployed version or patch level after remediation.
Management note
Archive parsing is a classic delivery path. Keep the October updates on schedule and be suspicious of archives from new sources until they land.