Microsoft Internet Messaging API: Heap-based buffer overflow
Summary
A heap-based buffer overflow (CWE-122) in the Microsoft Internet Messaging API allows code execution. An input buffer is copied to an output buffer without checking that the input fits, which can corrupt memory. The attacker needs local access and low privileges, and no user interaction is required. Microsoft's title says Remote Code Execution, but the NVD vector (AV:L) describes a local attack; the wording should be clarified with Microsoft.
Affected
- Windows 10 Version 1607 (builds below 10.0.14393.9612; 32-bit builds to be confirmed in the advisory)
- Windows Server 2016 (builds below 10.0.14393.9612)
- Windows 10 Version 1809 (builds below 10.0.17763.9376)
- Windows 10 Version 21H2 (builds below 10.0.19044.7807)
- Windows 10 Version 22H2 (builds below 10.0.19045.7807)
- Windows 11 Version 23H2 (builds below 10.0.22631.7652)
- Windows 11 Version 24H2 (builds below 10.0.26100.9711)
- Windows Server 2012 and 2012 R2 (see advisory for builds)
Fixed in
- Windows 10 Version 1607 and Server 2016: 10.0.14393.9612 (x64; 32-bit build not confirmed)
- Windows 10 Version 1809: 10.0.17763.9376
- Windows 10 Version 21H2: 10.0.19044.7807
- Windows 10 Version 22H2: 10.0.19045.7807
- Windows 11 Version 23H2: 10.0.22631.7652
- Windows 11 Version 24H2: 10.0.26100.9711
- Windows Server 2012: 6.2.9200.26434
Workaround
Unknown. No vendor-documented workaround was found. Patching is the only confirmed fix.
Exploitation
- Microsoft states that no publicly available exploit code exists or that an exploit is theoretical; Exploited: No.
- CISA SSVC: exploitation none, not automatable.
- EPSS below 1 percent (very low).
- No public proof of concept found in the research; CISA KEV should be re-checked.
Our recommendation
- Install the October 2026 security update on all affected Windows hosts, prioritising Windows 10 1607 and Server 2016 systems used by multiple users.
- Verify the 32-bit fixed build for Windows 10 1607 in the Microsoft advisory before rollout.
- Check whether Windows 10 1607 is still supported; if not, plan an upgrade.
- Review local accounts, RDS/terminal servers and shared hosts, and monitor for privilege escalation.
- Re-check CISA KEV and the advisory for updates.
Information from Microsoft (Patch Tuesday October 2026)
- Severity (Microsoft)
- Important
- Impact
- Remote code execution – Microsoft Internet Messaging API
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Windows, Windows ESU (Extended Security Updates)
- Updates (KB)
- KB5126651, KB5126655, KB5126657, KB5126646, KB5127648, KB5126653
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-83919 msrc.microsoft.com
- Vulners: CVE-2026-83919 (CNA data) vulners.com
- OpenCVE: CVE-2026-83919 (NVD data) app.opencve.io
- VulDB: CVE-2026-83919 vuldb.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 2 Nov 2026, 02:34.
Original description (NVD)
Heap-based buffer overflow in Microsoft Internet Messaging API allows an authorized attacker to execute code locally.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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