EUVD-2026-56705
GHSA-3r88-gh6h-7xh6
Severity by source
AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Local file-parse trigger requires user to open file (UI:R, AV:L); no attacker privileges needed (PR:N); full code execution yields C:H/I:H/A:H with no scope change.
Primary rating from Vendor (microsoft).
CVSS VectorVendor: microsoft
Lifecycle Timeline
3DescriptionCVE.org
Integer overflow or wraparound in Microsoft Office allows an unauthorized attacker to execute code locally.
AnalysisAI
Local code execution in Microsoft Office stems from an integer overflow that cascades into a heap-based buffer overflow (CWE-122), affecting Office 2016 through LTSC 2024 on both Windows and macOS. An unauthenticated attacker who can convince a target user to open a crafted Office document gains full code execution with the victim's privilege level - achieving high confidentiality, integrity, and availability impact per the CVSS:3.1 AV:L/C:H/I:H/A:H rating. No public exploit code or CISA KEV listing was identified at time of analysis, but the patch is vendor-confirmed and available via Microsoft's Security Update Guide.
Technical ContextAI
The vulnerability is rooted in CWE-122 (Heap-based Buffer Overflow), with Microsoft's MSRC tagging it additionally as an integer overflow/wraparound - the likely mechanism being that arithmetic on a size or length value wraps around to a smaller integer, causing an undersized heap allocation, which is then overflowed when actual data is written into it. This class of flaw is common in document-parsing code where untrusted input dimensions control buffer sizes. The affected component exists across Microsoft Office's Windows SKUs (Office 2016 build 16.0.x, Office 2019, LTSC 2021, LTSC 2024, Microsoft 365 Apps for Enterprise) and macOS SKUs (Office 365 for Mac, LTSC for Mac 2021 and 2024), indicating the vulnerable parsing logic is shared across platforms. The local attack vector (AV:L) reflects that exploitation occurs within the Office process context when the malicious file is parsed, not via a network-facing service.
RemediationAI
Apply the vendor-released patch immediately. For Windows builds, update Microsoft Office 2016 to version 16.0.5565.1001 or later; all other Windows SKUs (Office 2019, LTSC 2021, LTSC 2024, Microsoft 365 Apps for Enterprise) should be updated via the Microsoft Update channel - consult https://aka.ms/OfficeSecurityReleases for exact build numbers. For macOS, update Office 365 for Mac, LTSC for Mac 2021, and LTSC for Mac 2024 to build 16.112.26081010 or later via Microsoft AutoUpdate or the Mac App Store. If patching is immediately infeasible, consider blocking delivery of unsolicited Office documents via email gateway policies, enabling Protected View for all externally sourced documents (which prevents automatic execution of embedded content upon open), and restricting macro execution via Group Policy. Note that Protected View mitigation trades off usability for users who routinely edit email attachments. The canonical advisory and patching instructions are at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-64903.
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Same weakness CWE-122 – Heap-based Buffer Overflow
View allSame technique Buffer Overflow
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External POC / Exploit Code
Leaving vuln.today